6.3. ECPG — Embedded SQL in C
This chapter describes the embedded SQL package for PostgreSQL. It was written by Linus Tolke (linus@epact.se) and Michael Meskes (meskes@postgresql.org). Originally it was written to work with C. It also works with C++, but it does not recognize all C++ constructs yet.
This documentation is quite incomplete. But since this interface is standardized, additional information can be found in many resources about SQL.
6.3.1. The Concept
An embedded SQL program consists of code written in an ordinary programming language, in this case C, mixed with SQL commands in specially marked sections. To build the program, the source code (.pgc) is first passed through the embedded SQL preprocessor, which converts it to an ordinary C program (.c), and afterwards it can be processed by a C compiler. (For details about the compiling and linking see ecpg-process.) Converted ECPG applications call functions in the libpq library through the embedded SQL library (ecpglib), and communicate with the PostgreSQL server using the normal frontend-backend protocol.
Embedded SQL has advantages over other methods for handling SQL commands from C code. First, it takes care of the tedious passing of information to and from variables in your C program. Second, the SQL code in the program is checked at build time for syntactical correctness. Third, embedded SQL in C is specified in the SQL standard and supported by many other SQL database systems. The PostgreSQL implementation is designed to match this standard as much as possible, and it is usually possible to port embedded SQL programs written for other SQL databases to PostgreSQL with relative ease.
As already stated, programs written for the embedded SQL interface are normal C programs with special code inserted to perform database-related actions. This special code always has the form:
EXEC SQL ...;
These statements syntactically take the place of a C statement.
Depending on the particular statement, they can appear at the global level or within a function.
Embedded SQL statements follow the case-sensitivity rules of normal SQL code, and not those of C. Also they allow nested C-style comments as per the SQL standard. The C part of the program, however, follows the C standard of not accepting nested comments. Embedded SQL statements likewise use SQL rules, not C rules, for parsing quoted strings and identifiers. (See sql-syntax-strings and sql-syntax-identifiers respectively. Note that ECPG assumes that standard_conforming_strings is on.) Of course, the C part of the program follows C quoting rules.
The following sections explain all the embedded SQL statements.
6.3.2. Managing Database Connections
This section describes how to open, close, and switch database connections.
6.3.2.1. Connecting to the Database Server
One connects to a database using the following statement:
EXEC SQL CONNECT TO target AS connection-name USER user-name;
The target can be specified in the
following ways:
dbname@hostname:port
tcp:postgresql://hostname:port/dbname?options
unix:postgresql://localhost:port/dbname?options
an SQL string literal containing one of the above forms
a reference to a character variable containing one of the above forms (see examples)
DEFAULT
The connection target DEFAULT initiates a connection to the default database under the default user name. No separate user name or connection name can be specified in that case.
If you specify the connection target directly (that is, not as a string literal or variable reference), then the components of the target are passed through normal SQL parsing; this means that, for example, the hostname must look like one or more SQL identifiers separated by dots, and those identifiers will be case-folded unless double-quoted. Values of any options must be SQL identifiers, integers, or variable references. Of course, you can put nearly anything into an SQL identifier by double-quoting it. In practice, it is probably less error-prone to use a (single-quoted) string literal or a variable reference than to write the connection target directly.
There are also different ways to specify the user name:
username
username/password
username IDENTIFIED BY password
username USING password
As above, the parameters username and password can be an SQL identifier, an SQL string literal, or a reference to a character variable.
If the connection target includes any options, those consist of keyword=value specifications separated by ampersands (&). The allowed key words are the same ones recognized by libpq (see libpq-paramkeywords). Spaces are ignored before any keyword or value, though not within or after one. Note that there is no way to write & within a value.
Notice that when specifying a socket connection (with the unix: prefix), the host name must be exactly localhost. To select a non-default socket directory, write the directory’s pathname as the value of a host option in the options part of the target.
The connection-name is used to handle multiple connections in one program. It can be omitted if a program uses only one connection. The most recently opened connection becomes the current connection, which is used by default when an SQL statement is to be executed (see later in this chapter).
Here are some examples of CONNECT statements:
EXEC SQL CONNECT TO mydb@sql.mydomain.com;
EXEC SQL CONNECT TO tcp:postgresql://sql.mydomain.com/mydb AS myconnection USER john;
EXEC SQL BEGIN DECLARE SECTION;
const char *target = "[email protected]";
const char *user = "john";
const char *passwd = "secret";
EXEC SQL END DECLARE SECTION;
...
EXEC SQL CONNECT TO :target USER :user USING :passwd;
/* or EXEC SQL CONNECT TO :target USER :user/:passwd; */
The last example makes use of the feature referred to above as
character variable references. You will see in later sections how C variables can be used in SQL statements when you prefix them with a colon.
Be advised that the format of the connection target is not specified in the SQL standard. So if you want to develop portable applications, you might want to use something based on the last example above to encapsulate the connection target string somewhere.
- If untrusted users have access to a database that has not adopted a
linkend=»ddl-schemas-patterns»>secure schema usage pattern,
begin each session by removing publicly-writable schemas from search_path. For example, add options=-c search_path= to options, or issue EXEC SQL SELECT pg_catalog.set_config(„search_path“, „“, false); after connecting. This consideration is not specific to ECPG; it applies to every interface for executing arbitrary SQL commands.
6.3.2.2. Choosing a Connection
SQL statements in embedded SQL programs are by default executed on the current connection, that is, the most recently opened one. If an application needs to manage multiple connections, then there are three ways to handle this.
The first option is to explicitly choose a connection for each SQL statement, for example:
EXEC SQL AT connection-name SELECT ...;
This option is particularly suitable if the application needs to
use several connections in mixed order.
If your application uses multiple threads of execution, they cannot share a connection concurrently. You must either explicitly control access to the connection (using mutexes) or use a connection for each thread.
The second option is to execute a statement to switch the current connection. That statement is:
EXEC SQL SET CONNECTION connection-name;
This option is particularly convenient if many statements are to be
executed on the same connection.
Here is an example program managing multiple database connections:
#include <stdio.h>
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
int
main()
{
EXEC SQL CONNECT TO testdb1 AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL CONNECT TO testdb2 AS con2 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL CONNECT TO testdb3 AS con3 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
/* This query would be executed in the last opened database "testdb3". */
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb3)\n", dbname);
/* Using "AT" to run a query in "testdb2" */
EXEC SQL AT con2 SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb2)\n", dbname);
/* Switch the current connection to "testdb1". */
EXEC SQL SET CONNECTION con1;
EXEC SQL SELECT current_database() INTO :dbname;
printf("current=%s (should be testdb1)\n", dbname);
EXEC SQL DISCONNECT ALL;
return 0;
}
This example would produce this output:
current=testdb3 (should be testdb3)
current=testdb2 (should be testdb2)
current=testdb1 (should be testdb1)
The third option is to declare an SQL identifier linked to the connection, for example:
EXEC SQL AT connection-name DECLARE statement-name STATEMENT;
EXEC SQL PREPARE statement-name FROM :dyn-string;
Once you link an SQL identifier to a connection, you execute dynamic SQL
without an AT clause. Note that this option behaves like preprocessor directives, therefore the link is enabled only in the file.
Here is an example program using this option:
#include <stdio.h>
EXEC SQL BEGIN DECLARE SECTION;
char dbname[128];
char *dyn_sql = "SELECT current_database()";
EXEC SQL END DECLARE SECTION;
int main(){
EXEC SQL CONNECT TO postgres AS con1;
EXEC SQL CONNECT TO testdb AS con2;
EXEC SQL AT con1 DECLARE stmt STATEMENT;
EXEC SQL PREPARE stmt FROM :dyn_sql;
EXEC SQL EXECUTE stmt INTO :dbname;
printf("%s\n", dbname);
EXEC SQL DISCONNECT ALL;
return 0;
}
This example would produce this output, even if the default connection is testdb:
postgres
6.3.2.3. Closing a Connection
To close a connection, use the following statement:
EXEC SQL DISCONNECT connection;
The connection can be specified
in the following ways:
connection-name
DEFAULT
CURRENT
ALL
If no connection name is specified, the current connection is closed.
It is good style that an application always explicitly disconnect from every connection it opened.
6.3.3. Running SQL Commands
Any SQL command can be run from within an embedded SQL application. Below are some examples of how to do that.
6.3.3.1. Executing SQL Statements
Creating a table:
EXEC SQL CREATE TABLE foo (number integer, ascii char(16));
EXEC SQL CREATE UNIQUE INDEX num1 ON foo(number);
EXEC SQL COMMIT;
Inserting rows:
EXEC SQL INSERT INTO foo (number, ascii) VALUES (9999, 'doodad');
EXEC SQL COMMIT;
Deleting rows:
EXEC SQL DELETE FROM foo WHERE number = 9999;
EXEC SQL COMMIT;
Updates:
EXEC SQL UPDATE foo
SET ascii = 'foobar'
WHERE number = 9999;
EXEC SQL COMMIT;
SELECT statements that return a single result row can also be executed using EXEC SQL directly. To handle result sets with multiple rows, an application has to use a cursor; see ecpg-cursors below. (As a special case, an application can fetch multiple rows at once into an array host variable; see ecpg-variables-arrays.)
Single-row select:
EXEC SQL SELECT foo INTO :FooBar FROM table1 WHERE ascii = 'doodad';
Also, a configuration parameter can be retrieved with the SHOW command:
EXEC SQL SHOW search_path INTO :var;
The tokens of the form :something are host variables, that is, they refer to variables in the C program. They are explained in ecpg-variables.
6.3.3.2. Using Cursors
To retrieve a result set holding multiple rows, an application has to declare a cursor and fetch each row from the cursor. The steps to use a cursor are the following: declare a cursor, open it, fetch a row from the cursor, repeat, and finally close it.
Select using cursors:
EXEC SQL DECLARE foo_bar CURSOR FOR
SELECT number, ascii FROM foo
ORDER BY ascii;
EXEC SQL OPEN foo_bar;
EXEC SQL FETCH foo_bar INTO :FooBar, DooDad;
...
EXEC SQL CLOSE foo_bar;
EXEC SQL COMMIT;
For more details about declaring a cursor, see ecpg-sql-declare; for more details about fetching rows from a cursor, see sql-fetch.
Примечание
The ECPG DECLARE command does not actually cause a statement to be sent to the PostgreSQL backend. The cursor is opened in the backend (using the backend’s DECLARE command) at the point when the OPEN command is executed.
6.3.3.3. Managing Transactions
In the default mode, statements are committed only when EXEC SQL COMMIT is issued. The embedded SQL interface also supports autocommit of transactions (similar to psql’s default behavior) via the -t command-line option to ecpg (see app-ecpg) or via the EXEC SQL SET AUTOCOMMIT TO ON statement. In autocommit mode, each command is automatically committed unless it is inside an explicit transaction block. This mode can be explicitly turned off using EXEC SQL SET AUTOCOMMIT TO OFF.
The following transaction management commands are available:
Commit an in-progress transaction.
Roll back an in-progress transaction.
Prepare the current transaction for two-phase commit.
Commit a transaction that is in prepared state.
Roll back a transaction that is in prepared state.
Enable autocommit mode.
Disable autocommit mode. This is the default.
6.3.3.4. Prepared Statements
When the values to be passed to an SQL statement are not known at compile time, or the same statement is going to be used many times, then prepared statements can be useful.
The statement is prepared using the command PREPARE. For the values that are not known yet, use the placeholder ?:
EXEC SQL PREPARE stmt1 FROM "SELECT oid, datname FROM pg_database WHERE oid = ?";
If a statement returns a single row, the application can call EXECUTE after PREPARE to execute the statement, supplying the actual values for the placeholders with a USING clause:
EXEC SQL EXECUTE stmt1 INTO :dboid, :dbname USING 1;
If a statement returns multiple rows, the application can use a cursor declared based on the prepared statement. To bind input parameters, the cursor must be opened with a USING clause:
EXEC SQL PREPARE stmt1 FROM "SELECT oid,datname FROM pg_database WHERE oid > ?";
EXEC SQL DECLARE foo_bar CURSOR FOR stmt1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
EXEC SQL OPEN foo_bar USING 100;
...
while (1)
{
EXEC SQL FETCH NEXT FROM foo_bar INTO :dboid, :dbname;
...
}
EXEC SQL CLOSE foo_bar;
When you don’t need the prepared statement anymore, you should deallocate it:
EXEC SQL DEALLOCATE PREPARE name;
For more details about PREPARE, see ecpg-sql-prepare. Also see ecpg-dynamic for more details about using placeholders and input parameters.
6.3.4. Using Host Variables
In ecpg-commands you saw how you can execute SQL statements from an embedded SQL program. Some of those statements only used fixed values and did not provide a way to insert user-supplied values into statements or have the program process the values returned by the query. Those kinds of statements are not really useful in real applications. This section explains in detail how you can pass data between your C program and the embedded SQL statements using a simple mechanism called host variables. In an embedded SQL program we consider the SQL statements to be guests in the C program code which is the host language. Therefore the variables of the C program are called host variables.
Another way to exchange values between PostgreSQL backends and ECPG applications is the use of SQL descriptors, described in ecpg-descriptors.
6.3.4.1. Overview
Passing data between the C program and the SQL statements is particularly simple in embedded SQL. Instead of having the program paste the data into the statement, which entails various complications, such as properly quoting the value, you can simply write the name of a C variable into the SQL statement, prefixed by a colon. For example:
EXEC SQL INSERT INTO sometable VALUES (:v1, 'foo', :v2);
This statement refers to two C variables named
v1 and v2 and also uses a regular SQL string literal, to illustrate that you are not restricted to use one kind of data or the other.
This style of inserting C variables in SQL statements works anywhere a value expression is expected in an SQL statement.
6.3.4.2. Declare Sections
To pass data from the program to the database, for example as parameters in a query, or to pass data from the database back to the program, the C variables that are intended to contain this data need to be declared in specially marked sections, so the embedded SQL preprocessor is made aware of them.
This section starts with:
EXEC SQL BEGIN DECLARE SECTION;
and ends with:
EXEC SQL END DECLARE SECTION;
Between those lines, there must be normal C variable declarations,
such as:
int x = 4;
char foo[16], bar[16];
As you can see, you can optionally assign an initial value to the variable.
The variable’s scope is determined by the location of its declaring section within the program. You can also declare variables with the following syntax which implicitly creates a declare section:
EXEC SQL int i = 4;
You can have as many declare sections in a program as you like.
The declarations are also echoed to the output file as normal C variables, so there’s no need to declare them again. Variables that are not intended to be used in SQL commands can be declared normally outside these special sections.
The definition of a structure or union also must be listed inside a DECLARE section. Otherwise the preprocessor cannot handle these types since it does not know the definition.
6.3.4.3. Retrieving Query Results
Now you should be able to pass data generated by your program into an SQL command. But how do you retrieve the results of a query? For that purpose, embedded SQL provides special variants of the usual commands SELECT and FETCH. These commands have a special INTO clause that specifies which host variables the retrieved values are to be stored in. SELECT is used for a query that returns only single row, and FETCH is used for a query that returns multiple rows, using a cursor.
Here is an example:
/*
* assume this table:
* CREATE TABLE test1 (a int, b varchar(50));
*/
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL SELECT a, b INTO :v1, :v2 FROM test;
So the **INTO** clause appears between the select
list and the FROM clause. The number of elements in the select list and the list after INTO (also called the target list) must be equal.
Here is an example using the command FETCH:
EXEC SQL BEGIN DECLARE SECTION;
int v1;
VARCHAR v2;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL DECLARE foo CURSOR FOR SELECT a, b FROM test;
...
do
{
...
EXEC SQL FETCH NEXT FROM foo INTO :v1, :v2;
...
} while (...);
Here the **INTO** clause appears after all the
normal clauses.
6.3.4.4. Type Mapping
When ECPG applications exchange values between the PostgreSQL server and the C application, such as when retrieving query results from the server or executing SQL statements with input parameters, the values need to be converted between PostgreSQL data types and host language variable types (C language data types, concretely). One of the main points of ECPG is that it takes care of this automatically in most cases.
In this respect, there are two kinds of data types: Some simple PostgreSQL data types, such as integer and text, can be read and written by the application directly. Other PostgreSQL data types, such as timestamp and numeric can only be accessed through special library functions; see ecpg-special-types.
ecpg-datatype-hostvars-table shows which PostgreSQL data types correspond to which C data types. When you wish to send or receive a value of a given PostgreSQL data type, you should declare a C variable of the corresponding C data type in the declare section.
Mapping Between PostgreSQL Data Types and C Variable Types
6.3.4.4.1. Handling Character Strings
To handle SQL character string data types, such as varchar and text, there are two possible ways to declare the host variables.
One way is using char[], an array of char, which is the most common way to handle character data in C.
EXEC SQL BEGIN DECLARE SECTION;
char str[50];
EXEC SQL END DECLARE SECTION;
Note that you have to take care of the length yourself. If you
use this host variable as the target variable of a query which returns a string with more than 49 characters, a buffer overflow occurs.
The other way is using the VARCHAR type, which is a special type provided by ECPG. The definition on an array of type VARCHAR is converted into a named struct for every variable. A declaration like:
VARCHAR var[180];
is converted into:
struct varchar_var { int len; char arr[180]; } var;
The member **arr** hosts the string
including a terminating zero byte. Thus, to store a string in a VARCHAR host variable, the host variable has to be declared with the length including the zero byte terminator. The member len holds the length of the string stored in the arr without the terminating zero byte. When a host variable is used as input for a query, if strlen(arr) and len are different, the shorter one is used.
VARCHAR can be written in upper or lower case, but not in mixed case.
char and VARCHAR host variables can also hold values of other SQL types, which will be stored in their string forms.
6.3.4.4.2. Accessing Special Data Types
ECPG contains some special types that help you to interact easily with some special data types from the PostgreSQL server. In particular, it has implemented support for the numeric, decimal, date, timestamp, and interval types. These data types cannot usefully be mapped to primitive host variable types (such as int, long long int, or char[]), because they have a complex internal structure. Applications deal with these types by declaring host variables in special types and accessing them using functions in the pgtypes library. The pgtypes library, described in detail in ecpg-pgtypes contains basic functions to deal with those types, such that you do not need to send a query to the SQL server just for adding an interval to a time stamp for example.
The follow subsections describe these special data types. For more details about pgtypes library functions, see ecpg-pgtypes.
6.3.4.4.2.1. timestamp, date
Here is a pattern for handling timestamp variables in the ECPG host application.
First, the program has to include the header file for the timestamp type:
#include <pgtypes_timestamp.h>
Next, declare a host variable as type timestamp in the declare section:
EXEC SQL BEGIN DECLARE SECTION;
timestamp ts;
EXEC SQL END DECLARE SECTION;
And after reading a value into the host variable, process it using pgtypes library functions. In following example, the timestamp value is converted into text (ASCII) form with the PGTYPEStimestamp_to_asc() function:
EXEC SQL SELECT now()::timestamp INTO :ts;
printf("ts = %s\n", PGTYPEStimestamp_to_asc(ts));
This example will show some result like following:
ts = 2010-06-27 18:03:56.949343
In addition, the DATE type can be handled in the same way. The program has to include pgtypes_date.h, declare a host variable as the date type and convert a DATE value into a text form using PGTYPESdate_to_asc() function. For more details about the pgtypes library functions, see ecpg-pgtypes.
6.3.4.4.2.2. interval
The handling of the interval type is also similar to the timestamp and date types. It is required, however, to allocate memory for an interval type value explicitly. In other words, the memory space for the variable has to be allocated in the heap memory, not in the stack memory.
Here is an example program:
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_interval.h>
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
interval *in;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
in = PGTYPESinterval_new();
EXEC SQL SELECT '1 min'::interval INTO :in;
printf("interval = %s\n", PGTYPESinterval_to_asc(in));
PGTYPESinterval_free(in);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
6.3.4.4.2.3. numeric, decimal
The handling of the numeric and decimal types is similar to the interval type: It requires defining a pointer, allocating some memory space on the heap, and accessing the variable using the pgtypes library functions. For more details about the pgtypes library functions, see ecpg-pgtypes.
No functions are provided specifically for the decimal type. An application has to convert it to a numeric variable using a pgtypes library function to do further processing.
Here is an example program handling numeric and decimal type variables.
#include <stdio.h>
#include <stdlib.h>
#include <pgtypes_numeric.h>
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
numeric *num;
numeric *num2;
decimal *dec;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
num = PGTYPESnumeric_new();
dec = PGTYPESdecimal_new();
EXEC SQL SELECT 12.345::numeric(4,2), 23.456::decimal(4,2) INTO :num, :dec;
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 0));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 1));
printf("numeric = %s\n", PGTYPESnumeric_to_asc(num, 2));
/* Convert decimal to numeric to show a decimal value. */
num2 = PGTYPESnumeric_new();
PGTYPESnumeric_from_decimal(dec, num2);
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 0));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 1));
printf("decimal = %s\n", PGTYPESnumeric_to_asc(num2, 2));
PGTYPESnumeric_free(num2);
PGTYPESdecimal_free(dec);
PGTYPESnumeric_free(num);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
6.3.4.4.2.4. bytea
The handling of the bytea type is similar to that of VARCHAR. The definition on an array of type bytea is converted into a named struct for every variable. A declaration like:
bytea var[180];
is converted into:
struct bytea_var { int len; char arr[180]; } var;
The member **arr** hosts binary format
data. It can also handle „0“ as part of data, unlike VARCHAR. The data is converted from/to hex format and sent/received by ecpglib.
Примечание
bytea variable can be used only when guc-bytea-output is set to hex.
6.3.4.4.3. Host Variables with Nonprimitive Types
As a host variable you can also use arrays, typedefs, structs, and pointers.
6.3.4.4.3.1. Arrays
There are two use cases for arrays as host variables. The first is a way to store some text string in char[] or VARCHAR[], as explained in ecpg-char. The second use case is to retrieve multiple rows from a query result without using a cursor. Without an array, to process a query result consisting of multiple rows, it is required to use a cursor and the FETCH command. But with array host variables, multiple rows can be received at once. The length of the array has to be defined to be able to accommodate all rows, otherwise a buffer overflow will likely occur.
Following example scans the pg_database system table and shows all OIDs and names of the available databases:
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
int dbid[8];
char dbname[8][16];
int i;
EXEC SQL END DECLARE SECTION;
memset(dbname, 0, sizeof(char)* 16 * 8);
memset(dbid, 0, sizeof(int) * 8);
EXEC SQL CONNECT TO testdb;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
/* Retrieve multiple rows into arrays at once. */
EXEC SQL SELECT oid,datname INTO :dbid, :dbname FROM pg_database;
for (i = 0; i < 8; i++)
printf("oid=%d, dbname=%s\n", dbid[i], dbname[i]);
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
This example shows following result. (The exact values depend on local circumstances.)
oid=1, dbname=template1
oid=11510, dbname=template0
oid=11511, dbname=postgres
oid=313780, dbname=testdb
oid=0, dbname=
oid=0, dbname=
oid=0, dbname=
6.3.4.4.3.2. Structures
A structure whose member names match the column names of a query result, can be used to retrieve multiple columns at once. The structure enables handling multiple column values in a single host variable.
The following example retrieves OIDs, names, and sizes of the available databases from the pg_database system table and using the pg_database_size() function. In this example, a structure variable dbinfo_t with members whose names match each column in the SELECT result is used to retrieve one result row without putting multiple host variables in the FETCH statement.
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
long long int size;
} dbinfo_t;
dbinfo_t dbval;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, dbval.size);
}
EXEC SQL CLOSE cur1;
This example shows following result. (The exact values depend on local circumstances.)
oid=1, datname=template1, size=4324580
oid=11510, datname=template0, size=4243460
oid=11511, datname=postgres, size=4324580
oid=313780, datname=testdb, size=8183012
Structure host variables absorb as many columns as the structure as fields. Additional columns can be assigned to other host variables. For example, the above program could also be restructured like this, with the size variable outside the structure:
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int oid;
char datname[65];
} dbinfo_t;
dbinfo_t dbval;
long long int size;
EXEC SQL END DECLARE SECTION;
memset(&dbval, 0, sizeof(dbinfo_t));
EXEC SQL DECLARE cur1 CURSOR FOR SELECT oid, datname, pg_database_size(oid) AS size FROM pg_database;
EXEC SQL OPEN cur1;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch multiple columns into one structure. */
EXEC SQL FETCH FROM cur1 INTO :dbval, :size;
/* Print members of the structure. */
printf("oid=%d, datname=%s, size=%lld\n", dbval.oid, dbval.datname, size);
}
EXEC SQL CLOSE cur1;
6.3.4.4.3.3. Typedefs
Use the typedef keyword to map new types to already existing types.
EXEC SQL BEGIN DECLARE SECTION;
typedef char mychartype[40];
typedef long serial_t;
EXEC SQL END DECLARE SECTION;
Note that you could also use:
EXEC SQL TYPE serial_t IS long;
This declaration does not need to be part of a declare section.
6.3.4.4.3.4. Pointers
You can declare pointers to the most common types. Note however that you cannot use pointers as target variables of queries without auto-allocation. See ecpg-descriptors for more information on auto-allocation.
EXEC SQL BEGIN DECLARE SECTION;
int *intp;
char **charp;
EXEC SQL END DECLARE SECTION;
6.3.4.5. Handling Nonprimitive SQL Data Types
This section contains information on how to handle nonscalar and user-defined SQL-level data types in ECPG applications. Note that this is distinct from the handling of host variables of nonprimitive types, described in the previous section.
6.3.4.5.1. Arrays
Multi-dimensional SQL-level arrays are not directly supported in ECPG. One-dimensional SQL-level arrays can be mapped into C array host variables and vice-versa. However, when creating a statement ecpg does not know the types of the columns, so that it cannot check if a C array is input into a corresponding SQL-level array. When processing the output of an SQL statement, ecpg has the necessary information and thus checks if both are arrays.
If a query accesses elements of an array separately, then this avoids the use of arrays in ECPG. Then, a host variable with a type that can be mapped to the element type should be used. For example, if a column type is array of integer, a host variable of type int can be used. Also if the element type is varchar or text, a host variable of type char[] or VARCHAR[] can be used.
Here is an example. Assume the following table:
CREATE TABLE t3 (
ii integer[]
);
testdb=> SELECT * FROM t3;
ii
-------------
{1,2,3,4,5}
(1 row)
The following example program retrieves the 4th element of the array and stores it into a host variable of type int:
EXEC SQL BEGIN DECLARE SECTION;
int ii;
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii ;
printf("ii=%d\n", ii);
}
EXEC SQL CLOSE cur1;
This example shows the following result:
ii=4
To map multiple array elements to the multiple elements in an array type host variables each element of array column and each element of the host variable array have to be managed separately, for example:
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii[1], ii[2], ii[3], ii[4] FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :ii_a[0], :ii_a[1], :ii_a[2], :ii_a[3];
...
}
Note again that
EXEC SQL BEGIN DECLARE SECTION;
int ii_a[8];
EXEC SQL END DECLARE SECTION;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT ii FROM t3;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* WRONG */
EXEC SQL FETCH FROM cur1 INTO :ii_a;
...
}
would not work correctly in this case, because you cannot map an
array type column to an array host variable directly.
Another workaround is to store arrays in their external string representation in host variables of type char[] or VARCHAR[]. For more details about this representation, see arrays-input. Note that this means that the array cannot be accessed naturally as an array in the host program (without further processing that parses the text representation).
6.3.4.5.2. Composite Types
Composite types are not directly supported in ECPG, but an easy workaround is possible. The available workarounds are similar to the ones described for arrays above: Either access each attribute separately or use the external string representation.
For the following examples, assume the following type and table:
CREATE TYPE comp_t AS (intval integer, textval varchar(32));
CREATE TABLE t4 (compval comp_t);
INSERT INTO t4 VALUES ( (256, 'PostgreSQL') );
The most obvious solution is to access each attribute separately. The following program retrieves data from the example table by selecting each attribute of the type comp_t separately:
EXEC SQL BEGIN DECLARE SECTION;
int intval;
varchar textval[33];
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Fetch each element of the composite type column into host variables. */
EXEC SQL FETCH FROM cur1 INTO :intval, :textval;
printf("intval=%d, textval=%s\n", intval, textval.arr);
}
EXEC SQL CLOSE cur1;
To enhance this example, the host variables to store values in the FETCH command can be gathered into one structure. For more details about the host variable in the structure form, see ecpg-variables-struct. To switch to the structure, the example can be modified as below. The two host variables, intval and textval, become members of the comp_t structure, and the structure is specified on the FETCH command.
EXEC SQL BEGIN DECLARE SECTION;
typedef struct
{
int intval;
varchar textval[33];
} comp_t;
comp_t compval;
EXEC SQL END DECLARE SECTION;
/* Put each element of the composite type column in the SELECT list. */
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).intval, (compval).textval FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
EXEC SQL CLOSE cur1;
Although a structure is used in the FETCH command, the attribute names in the SELECT clause are specified one by one. This can be enhanced by using a * to ask for all attributes of the composite type value.
...
EXEC SQL DECLARE cur1 CURSOR FOR SELECT (compval).* FROM t4;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
/* Put all values in the SELECT list into one structure. */
EXEC SQL FETCH FROM cur1 INTO :compval;
printf("intval=%d, textval=%s\n", compval.intval, compval.textval.arr);
}
...
This way, composite types can be mapped into structures almost
seamlessly, even though ECPG does not understand the composite type itself.
Finally, it is also possible to store composite type values in their external string representation in host variables of type char[] or VARCHAR[]. But that way, it is not easily possible to access the fields of the value from the host program.
6.3.4.5.3. User-Defined Base Types
New user-defined base types are not directly supported by ECPG. You can use the external string representation and host variables of type char[] or VARCHAR[], and this solution is indeed appropriate and sufficient for many types.
Here is an example using the data type complex from the example in xtypes. The external string representation of that type is (%f,%f), which is defined in the functions complex_in() and complex_out() functions in xtypes. The following example inserts the complex type values (1,1) and (3,3) into the columns a and b, and select them from the table after that.
EXEC SQL BEGIN DECLARE SECTION;
varchar a[64];
varchar b[64];
EXEC SQL END DECLARE SECTION;
EXEC SQL INSERT INTO test_complex VALUES ('(1,1)', '(3,3)');
EXEC SQL DECLARE cur1 CURSOR FOR SELECT a, b FROM test_complex;
EXEC SQL OPEN cur1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH FROM cur1 INTO :a, :b;
printf("a=%s, b=%s\n", a.arr, b.arr);
}
EXEC SQL CLOSE cur1;
This example shows following result:
a=(1,1), b=(3,3)
Another workaround is avoiding the direct use of the user-defined types in ECPG and instead create a function or cast that converts between the user-defined type and a primitive type that ECPG can handle. Note, however, that type casts, especially implicit ones, should be introduced into the type system very carefully.
For example,
CREATE FUNCTION create_complex(r double, i double) RETURNS complex
LANGUAGE SQL
IMMUTABLE
AS $$ SELECT $1 * complex '(1,0')' + $2 * complex '(0,1)' $$;
After this definition, the following
EXEC SQL BEGIN DECLARE SECTION;
double a, b, c, d;
EXEC SQL END DECLARE SECTION;
a = 1;
b = 2;
c = 3;
d = 4;
EXEC SQL INSERT INTO test_complex VALUES (create_complex(:a, :b), create_complex(:c, :d));
has the same effect as
EXEC SQL INSERT INTO test_complex VALUES ('(1,2)', '(3,4)');
6.3.4.6. Indicators
The examples above do not handle null values. In fact, the retrieval examples will raise an error if they fetch a null value from the database. To be able to pass null values to the database or retrieve null values from the database, you need to append a second host variable specification to each host variable that contains data. This second host variable is called the indicator and contains a flag that tells whether the datum is null, in which case the value of the real host variable is ignored. Here is an example that handles the retrieval of null values correctly:
EXEC SQL BEGIN DECLARE SECTION;
VARCHAR val;
int val_ind;
EXEC SQL END DECLARE SECTION:
...
EXEC SQL SELECT b INTO :val :val_ind FROM test1;
The indicator variable **val_ind** will be zero if
the value was not null, and it will be negative if the value was null. (See ecpg-oracle-compat to enable Oracle-specific behavior.)
The indicator has another function: if the indicator value is positive, it means that the value is not null, but it was truncated when it was stored in the host variable.
If the argument -r no_indicator is passed to the preprocessor ecpg, it works in no-indicator mode. In no-indicator mode, if no indicator variable is specified, null values are signaled (on input and output) for character string types as empty string and for integer types as the lowest possible value for type (for example, INT_MIN for int).
6.3.5. Dynamic SQL
In many cases, the particular SQL statements that an application has to execute are known at the time the application is written. In some cases, however, the SQL statements are composed at run time or provided by an external source. In these cases you cannot embed the SQL statements directly into the C source code, but there is a facility that allows you to call arbitrary SQL statements that you provide in a string variable.
6.3.5.1. Executing Statements without a Result Set
The simplest way to execute an arbitrary SQL statement is to use the command EXECUTE IMMEDIATE. For example:
EXEC SQL BEGIN DECLARE SECTION;
const char *stmt = "CREATE TABLE test1 (...);";
EXEC SQL END DECLARE SECTION;
EXEC SQL EXECUTE IMMEDIATE :stmt;
**EXECUTE IMMEDIATE** can be used for SQL
statements that do not return a result set (e.g., DDL, INSERT, UPDATE, DELETE). You cannot execute statements that retrieve data (e.g., SELECT) this way. The next section describes how to do that.
6.3.5.2. Executing a Statement with Input Parameters
A more powerful way to execute arbitrary SQL statements is to prepare them once and execute the prepared statement as often as you like. It is also possible to prepare a generalized version of a statement and then execute specific versions of it by substituting parameters. When preparing the statement, write question marks where you want to substitute parameters later. For example:
EXEC SQL BEGIN DECLARE SECTION;
const char *stmt = "INSERT INTO test1 VALUES(?, ?);";
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE mystmt FROM :stmt;
...
EXEC SQL EXECUTE mystmt USING 42, 'foobar';
When you don’t need the prepared statement anymore, you should deallocate it:
EXEC SQL DEALLOCATE PREPARE name;
6.3.5.3. Executing a Statement with a Result Set
To execute an SQL statement with a single result row, EXECUTE can be used. To save the result, add an INTO clause.
EXEC SQL BEGIN DECLARE SECTION;
const char *stmt = "SELECT a, b, c FROM test1 WHERE a > ?";
int v1, v2;
VARCHAR v3[50];
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE mystmt FROM :stmt;
...
EXEC SQL EXECUTE mystmt INTO :v1, :v2, :v3 USING 37;
An **EXECUTE** command can have an
INTO clause, a USING clause, both, or neither.
If a query is expected to return more than one result row, a cursor should be used, as in the following example. (See ecpg-cursors for more details about the cursor.)
EXEC SQL BEGIN DECLARE SECTION;
char dbaname[128];
char datname[128];
char *stmt = "SELECT u.usename as dbaname, d.datname "
" FROM pg_database d, pg_user u "
" WHERE d.datdba = u.usesysid";
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL PREPARE stmt1 FROM :stmt;
EXEC SQL DECLARE cursor1 CURSOR FOR stmt1;
EXEC SQL OPEN cursor1;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH cursor1 INTO :dbaname,:datname;
printf("dbaname=%s, datname=%s\n", dbaname, datname);
}
EXEC SQL CLOSE cursor1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
6.3.6. pgtypes Library
The pgtypes library maps PostgreSQL database types to C equivalents that can be used in C programs. It also offers functions to do basic calculations with those types within C, i.e., without the help of the PostgreSQL server. See the following example:
EXEC SQL BEGIN DECLARE SECTION;
date date1;
timestamp ts1, tsout;
interval iv1;
char *out;
EXEC SQL END DECLARE SECTION;
PGTYPESdate_today(&date1);
EXEC SQL SELECT started, duration INTO :ts1, :iv1 FROM datetbl WHERE d=:date1;
PGTYPEStimestamp_add_interval(&ts1, &iv1, &tsout);
out = PGTYPEStimestamp_to_asc(&tsout);
printf("Started + duration: %s\n", out);
PGTYPESchar_free(out);
6.3.6.1. Character Strings
Some functions such as PGTYPESnumeric_to_asc return a pointer to a freshly allocated character string. These results should be freed with PGTYPESchar_free instead of free. (This is important only on Windows, where memory allocation and release sometimes need to be done by the same library.)
6.3.6.2. The numeric Type
The numeric type offers to do calculations with arbitrary precision. See datatype-numeric for the equivalent type in the PostgreSQL server. Because of the arbitrary precision this variable needs to be able to expand and shrink dynamically. That’s why you can only create numeric variables on the heap, by means of the PGTYPESnumeric_new and PGTYPESnumeric_free functions. The decimal type, which is similar but limited in precision, can be created on the stack as well as on the heap.
The following functions can be used to work with the numeric type:
Request a pointer to a newly allocated numeric variable.
numeric *PGTYPESnumeric_new(void);
Free a numeric type, release all of its memory.
void PGTYPESnumeric_free(numeric *var);
Parse a numeric type from its string notation.
numeric *PGTYPESnumeric_from_asc(char *str, char **endptr); Valid formats are for example:
-2, .794, +3.44, 592.49E07 or -32.84e-4. If the value could be parsed successfully, a valid pointer is returned, else the NULL pointer. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL.
Returns a pointer to a string allocated by malloc that contains the string representation of the numeric type num.
char *PGTYPESnumeric_to_asc(numeric *num, int dscale); The numeric value will be printed with **dscale** decimal
digits, with rounding applied if necessary. The result must be freed with PGTYPESchar_free().
Add two numeric variables into a third one.
int PGTYPESnumeric_add(numeric *var1, numeric *var2, numeric *result); The function adds the variables **var1** and
var2 into the result variable result. The function returns 0 on success and -1 in case of error.
Subtract two numeric variables and return the result in a third one.
int PGTYPESnumeric_sub(numeric *var1, numeric *var2, numeric *result); The function subtracts the variable **var2** from
the variable var1. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error.
Multiply two numeric variables and return the result in a third one.
int PGTYPESnumeric_mul(numeric *var1, numeric *var2, numeric *result); The function multiplies the variables **var1** and
var2. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error.
Divide two numeric variables and return the result in a third one.
int PGTYPESnumeric_div(numeric *var1, numeric *var2, numeric *result); The function divides the variables **var1** by
var2. The result of the operation is stored in the variable result. The function returns 0 on success and -1 in case of error.
Compare two numeric variables.
int PGTYPESnumeric_cmp(numeric *var1, numeric *var2) This function compares two numeric variables. In case of error,
INT_MAX is returned. On success, the function returns one of three possible results:
1, if var1 is bigger than var2
-1, if var1 is smaller than var2
0, if var1 and var2 are equal
Convert an int variable to a numeric variable.
int PGTYPESnumeric_from_int(signed int int_val, numeric *var); This function accepts a variable of type signed int and stores it
in the numeric variable var. Upon success, 0 is returned and -1 in case of a failure.
Convert a long int variable to a numeric variable.
int PGTYPESnumeric_from_long(signed long int long_val, numeric *var); This function accepts a variable of type signed long int and stores it
in the numeric variable var. Upon success, 0 is returned and -1 in case of a failure.
Copy over one numeric variable into another one.
int PGTYPESnumeric_copy(numeric *src, numeric *dst); This function copies over the value of the variable that
src points to into the variable that dst points to. It returns 0 on success and -1 if an error occurs.
Convert a variable of type double to a numeric.
int PGTYPESnumeric_from_double(double d, numeric *dst); This function accepts a variable of type double and stores the result
in the variable that dst points to. It returns 0 on success and -1 if an error occurs.
Convert a variable of type numeric to double.
int PGTYPESnumeric_to_double(numeric *nv, double *dp) The function converts the numeric value from the variable that
nv points to into the double variable that dp points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally.
Convert a variable of type numeric to int.
int PGTYPESnumeric_to_int(numeric *nv, int *ip); The function converts the numeric value from the variable that
nv points to into the integer variable that ip points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally.
Convert a variable of type numeric to long.
int PGTYPESnumeric_to_long(numeric *nv, long *lp); The function converts the numeric value from the variable that
nv points to into the long integer variable that lp points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally.
Convert a variable of type numeric to decimal.
int PGTYPESnumeric_to_decimal(numeric *src, decimal *dst); The function converts the numeric value from the variable that
src points to into the decimal variable that dst points to. It returns 0 on success and -1 if an error occurs, including overflow. On overflow, the global variable errno will be set to PGTYPES_NUM_OVERFLOW additionally.
Convert a variable of type decimal to numeric.
int PGTYPESnumeric_from_decimal(decimal *src, numeric *dst); The function converts the decimal value from the variable that
src points to into the numeric variable that dst points to. It returns 0 on success and -1 if an error occurs. Since the decimal type is implemented as a limited version of the numeric type, overflow cannot occur with this conversion.
6.3.6.3. The date Type
The date type in C enables your programs to deal with data of the SQL type date. See datatype-datetime for the equivalent type in the PostgreSQL server.
The following functions can be used to work with the date type:
Extract the date part from a timestamp.
date PGTYPESdate_from_timestamp(timestamp dt); The function receives a timestamp as its only argument and returns the
extracted date part from this timestamp.
Parse a date from its textual representation.
date PGTYPESdate_from_asc(char *str, char **endptr); The function receives a C char* string **str** and a pointer to
a C char* string endptr. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL.
Note that the function always assumes MDY-formatted dates and there is currently no variable to change that within ECPG.
ecpg-pgtypesdate-from-asc-table shows the allowed input formats.
Valid Input Formats for PGTYPESdate_from_asc
Return the textual representation of a date variable.
char *PGTYPESdate_to_asc(date dDate); The function receives the date **dDate** as its only parameter.
It will output the date in the form 1999-01-18, i.e., in the YYYY-MM-DD format. The result must be freed with PGTYPESchar_free().
Extract the values for the day, the month and the year from a variable of type date.
void PGTYPESdate_julmdy(date d, int *mdy);
The function receives the date d and a pointer to an array of 3 integer values mdy. The variable name indicates the sequential order: mdy[0] will be set to contain the number of the month, mdy[1] will be set to the value of the day and mdy[2] will contain the year.
Create a date value from an array of 3 integers that specify the day, the month and the year of the date.
void PGTYPESdate_mdyjul(int *mdy, date *jdate); The function receives the array of the 3 integers (**mdy**) as
its first argument and as its second argument a pointer to a variable of type date that should hold the result of the operation.
Return a number representing the day of the week for a date value.
int PGTYPESdate_dayofweek(date d); The function receives the date variable **d** as its only
argument and returns an integer that indicates the day of the week for this date.
0 - Sunday
1 - Monday
2 - Tuesday
3 - Wednesday
4 - Thursday
5 - Friday
6 - Saturday
Get the current date.
void PGTYPESdate_today(date *d); The function receives a pointer to a date variable (**d**)
that it sets to the current date.
Convert a variable of type date to its textual representation using a format mask.
int PGTYPESdate_fmt_asc(date dDate, char *fmtstring, char *outbuf); The function receives the date to convert (**dDate**), the
format mask (fmtstring) and the string that will hold the textual representation of the date (outbuf).
On success, 0 is returned and a negative value if an error occurred.
The following literals are the field specifiers you can use:
dd - The number of the day of the month.
mm - The number of the month of the year.
yy - The number of the year as a two digit number.
yyyy - The number of the year as a four digit number.
ddd - The name of the day (abbreviated).
- mmm - The name of the month (abbreviated).
All other characters are copied 1:1 to the output string.
ecpg-pgtypesdate-fmt-asc-example-table indicates a few possible formats. This will give you an idea of how to use this function. All output lines are based on the same date: November 23, 1959.
Valid Input Formats for PGTYPESdate_fmt_asc
Use a format mask to convert a C char* string to a value of type date.
int PGTYPESdate_defmt_asc(date *d, char *fmt, char *str);
The function receives a pointer to the date value that should hold the result of the operation (d), the format mask to use for parsing the date (fmt) and the C char* string containing the textual representation of the date (str). The textual representation is expected to match the format mask. However you do not need to have a 1:1 mapping of the string to the format mask. The function only analyzes the sequential order and looks for the literals yy or yyyy that indicate the position of the year, mm to indicate the position of the month and dd to indicate the position of the day.
ecpg-rdefmtdate-example-table indicates a few possible formats. This will give you an idea of how to use this function.
Valid Input Formats for rdefmtdate
6.3.6.4. The timestamp Type
The timestamp type in C enables your programs to deal with data of the SQL type timestamp. See datatype-datetime for the equivalent type in the PostgreSQL server.
The following functions can be used to work with the timestamp type:
Parse a timestamp from its textual representation into a timestamp variable.
timestamp PGTYPEStimestamp_from_asc(char *str, char **endptr); The function receives the string to parse (**str**) and a
pointer to a C char* (endptr). At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL.
The function returns the parsed timestamp on success. On error, PGTYPESInvalidTimestamp is returned and errno is set to PGTYPES_TS_BAD_TIMESTAMP. See pgtypesinvalidtimestamp for important notes on this value.
In general, the input string can contain any combination of an allowed date specification, a whitespace character and an allowed time specification. Note that time zones are not supported by ECPG. It can parse them but does not apply any calculation as the PostgreSQL server does for example. Timezone specifiers are silently discarded.
ecpg-pgtypestimestamp-from-asc-example-table contains a few examples for input strings.
Valid Input Formats for PGTYPEStimestamp_from_asc
Converts a date to a C char* string.
char *PGTYPEStimestamp_to_asc(timestamp tstamp); The function receives the timestamp **tstamp** as
its only argument and returns an allocated string that contains the textual representation of the timestamp. The result must be freed with PGTYPESchar_free().
Retrieve the current timestamp.
void PGTYPEStimestamp_current(timestamp *ts); The function retrieves the current timestamp and saves it into the
timestamp variable that ts points to.
Convert a timestamp variable to a C char* using a format mask.
int PGTYPEStimestamp_fmt_asc(timestamp *ts, char *output, int str_len, char *fmtstr); The function receives a pointer to the timestamp to convert as its
first argument (ts), a pointer to the output buffer (output), the maximal length that has been allocated for the output buffer (str_len) and the format mask to use for the conversion (fmtstr).
Upon success, the function returns 0 and a negative value if an error occurred.
You can use the following format specifiers for the format mask. The format specifiers are the same ones that are used in the strftime function in libc. Any non-format specifier will be copied into the output buffer.
%A - is replaced by national representation of the full weekday name.
%a - is replaced by national representation of the abbreviated weekday name.
%B - is replaced by national representation of the full month name.
%b - is replaced by national representation of the abbreviated month name.
%C - is replaced by (year / 100) as decimal number; single digits are preceded by a zero.
%c - is replaced by national representation of time and date.
%D - is equivalent to %m/%d/%y.
%d - is replaced by the day of the month as a decimal number (01–31).
%E* %O* - POSIX locale extensions. The sequences %Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH %OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy are supposed to provide alternative representations.
Additionally %OB implemented to represent alternative months names (used standalone, without day mentioned).
%e - is replaced by the day of month as a decimal number (1–31); single digits are preceded by a blank.
%F - is equivalent to %Y-%m-%d.
%G - is replaced by a year as a decimal number with century. This year is the one that contains the greater part of the week (Monday as the first day of the week).
%g - is replaced by the same year as in %G, but as a decimal number without century (00–99).
%H - is replaced by the hour (24-hour clock) as a decimal number (00–23).
%h - the same as %b.
%I - is replaced by the hour (12-hour clock) as a decimal number (01–12).
%j - is replaced by the day of the year as a decimal number (001–366).
%k - is replaced by the hour (24-hour clock) as a decimal number (0–23); single digits are preceded by a blank.
%l - is replaced by the hour (12-hour clock) as a decimal number (1–12); single digits are preceded by a blank.
%M - is replaced by the minute as a decimal number (00–59).
%m - is replaced by the month as a decimal number (01–12).
%n - is replaced by a newline.
%O* - the same as %E*.
%p - is replaced by national representation of either ante meridiem or post meridiem as appropriate.
%R - is equivalent to %H:%M.
%r - is equivalent to %I:%M:%S %p.
%S - is replaced by the second as a decimal number (00–60).
%s - is replaced by the number of seconds since the Epoch, UTC.
%T - is equivalent to %H:%M:%S
%t - is replaced by a tab.
%U - is replaced by the week number of the year (Sunday as the first day of the week) as a decimal number (00–53).
%u - is replaced by the weekday (Monday as the first day of the week) as a decimal number (1–7).
%V - is replaced by the week number of the year (Monday as the first day of the week) as a decimal number (01–53). If the week containing January 1 has four or more days in the new year, then it is week 1; otherwise it is the last week of the previous year, and the next week is week 1.
%v - is equivalent to %e-%b-%Y.
%W - is replaced by the week number of the year (Monday as the first day of the week) as a decimal number (00–53).
%w - is replaced by the weekday (Sunday as the first day of the week) as a decimal number (0–6).
%X - is replaced by national representation of the time.
%x - is replaced by national representation of the date.
%Y - is replaced by the year with century as a decimal number.
%y - is replaced by the year without century as a decimal number (00–99).
%Z - is replaced by the time zone name.
%z - is replaced by the time zone offset from UTC; a leading plus sign stands for east of UTC, a minus sign for west of UTC, hours and minutes follow with two digits each and no delimiter between them (common form for RFC 822 date headers).
%+ - is replaced by national representation of the date and time.
%-* - GNU libc extension. Do not do any padding when performing numerical outputs.
$_* - GNU libc extension. Explicitly specify space for padding.
%0* - GNU libc extension. Explicitly specify zero for padding.
%% - is replaced by %.
Subtract one timestamp from another one and save the result in a variable of type interval.
int PGTYPEStimestamp_sub(timestamp *ts1, timestamp *ts2, interval *iv); The function will subtract the timestamp variable that **ts2**
points to from the timestamp variable that ts1 points to and will store the result in the interval variable that iv points to.
Upon success, the function returns 0 and a negative value if an error occurred.
Parse a timestamp value from its textual representation using a formatting mask.
int PGTYPEStimestamp_defmt_asc(char *str, char *fmt, timestamp *d); The function receives the textual representation of a timestamp in the
variable str as well as the formatting mask to use in the variable fmt. The result will be stored in the variable that d points to.
If the formatting mask fmt is NULL, the function will fall back to the default formatting mask which is %Y-%m-%d %H:%M:%S.
This is the reverse function to pgtypestimestampfmtasc. See the documentation there in order to find out about the possible formatting mask entries.
Add an interval variable to a timestamp variable.
int PGTYPEStimestamp_add_interval(timestamp *tin, interval *span, timestamp *tout); The function receives a pointer to a timestamp variable **tin**
and a pointer to an interval variable span. It adds the interval to the timestamp and saves the resulting timestamp in the variable that tout points to.
Upon success, the function returns 0 and a negative value if an error occurred.
Subtract an interval variable from a timestamp variable.
int PGTYPEStimestamp_sub_interval(timestamp *tin, interval *span, timestamp *tout); The function subtracts the interval variable that **span**
points to from the timestamp variable that tin points to and saves the result into the variable that tout points to.
Upon success, the function returns 0 and a negative value if an error occurred.
6.3.6.5. The interval Type
The interval type in C enables your programs to deal with data of the SQL type interval. See datatype-datetime for the equivalent type in the PostgreSQL server.
The following functions can be used to work with the interval type:
Return a pointer to a newly allocated interval variable.
interval *PGTYPESinterval_new(void);
Release the memory of a previously allocated interval variable.
void PGTYPESinterval_free(interval *intvl);
Parse an interval from its textual representation.
interval *PGTYPESinterval_from_asc(char *str, char **endptr); The function parses the input string **str** and returns a
pointer to an allocated interval variable. At the moment ECPG always parses the complete string and so it currently does not support to store the address of the first invalid character in *endptr. You can safely set endptr to NULL.
Convert a variable of type interval to its textual representation.
char *PGTYPESinterval_to_asc(interval *span); The function converts the interval variable that **span**
points to into a C char*. The output looks like this example: @ 1 day 12 hours 59 mins 10 secs. The result must be freed with PGTYPESchar_free().
Copy a variable of type interval.
int PGTYPESinterval_copy(interval *intvlsrc, interval *intvldest); The function copies the interval variable that **intvlsrc**
points to into the variable that intvldest points to. Note that you need to allocate the memory for the destination variable before.
6.3.6.6. The decimal Type
The decimal type is similar to the numeric type. However it is limited to a maximum precision of 30 significant digits. In contrast to the numeric type which can be created on the heap only, the decimal type can be created either on the stack or on the heap (by means of the functions PGTYPESdecimal_new and PGTYPESdecimal_free). There are a lot of other functions that deal with the decimal type in the Informix compatibility mode described in ecpg-informix-compat.
The following functions can be used to work with the decimal type and are not only contained in the libcompat library.
Request a pointer to a newly allocated decimal variable.
decimal *PGTYPESdecimal_new(void);
Free a decimal type, release all of its memory.
void PGTYPESdecimal_free(decimal *var);
6.3.6.7. errno Values of pgtypeslib
An argument should contain a numeric variable (or point to a numeric variable) but in fact its in-memory representation was invalid.
An overflow occurred. Since the numeric type can deal with almost arbitrary precision, converting a numeric variable into other types might cause overflow.
An underflow occurred. Since the numeric type can deal with almost arbitrary precision, converting a numeric variable into other types might cause underflow.
A division by zero has been attempted.
An invalid date string was passed to the PGTYPESdate_from_asc function.
Invalid arguments were passed to the PGTYPESdate_defmt_asc function.
An invalid token in the input string was found by the PGTYPESdate_defmt_asc function.
An invalid interval string was passed to the PGTYPESinterval_from_asc function, or an invalid interval value was passed to the PGTYPESinterval_to_asc function.
There was a mismatch in the day/month/year assignment in the PGTYPESdate_defmt_asc function.
An invalid day of the month value was found by the PGTYPESdate_defmt_asc function.
An invalid month value was found by the PGTYPESdate_defmt_asc function.
An invalid timestamp string pass passed to the PGTYPEStimestamp_from_asc function, or an invalid timestamp value was passed to the PGTYPEStimestamp_to_asc function.
An infinite timestamp value was encountered in a context that cannot handle it.
6.3.6.8. Special Constants of pgtypeslib
A value of type timestamp representing an invalid time stamp. This is returned by the function PGTYPEStimestamp_from_asc on parse error. Note that due to the internal representation of the timestamp data type, PGTYPESInvalidTimestamp is also a valid timestamp at the same time. It is set to 1899-12-31 23:59:59. In order to detect errors, make sure that your application does not only test for PGTYPESInvalidTimestamp but also for errno != 0 after each call to PGTYPEStimestamp_from_asc.
6.3.7. Using Descriptor Areas
An SQL descriptor area is a more sophisticated method for processing the result of a SELECT, FETCH or a DESCRIBE statement. An SQL descriptor area groups the data of one row of data together with metadata items into one data structure. The metadata is particularly useful when executing dynamic SQL statements, where the nature of the result columns might not be known ahead of time. PostgreSQL provides two ways to use Descriptor Areas: the named SQL Descriptor Areas and the C-structure SQLDAs.
6.3.7.1. Named SQL Descriptor Areas
A named SQL descriptor area consists of a header, which contains information concerning the entire descriptor, and one or more item descriptor areas, which basically each describe one column in the result row.
Before you can use an SQL descriptor area, you need to allocate one:
EXEC SQL ALLOCATE DESCRIPTOR identifier;
The identifier serves as the variable name of the
descriptor area. When you don’t need the descriptor anymore, you should deallocate it:
EXEC SQL DEALLOCATE DESCRIPTOR identifier;
To use a descriptor area, specify it as the storage target in an INTO clause, instead of listing host variables:
EXEC SQL FETCH NEXT FROM mycursor INTO SQL DESCRIPTOR mydesc;
If the result set is empty, the Descriptor Area will still contain
the metadata from the query, i.e., the field names.
For not yet executed prepared queries, the DESCRIBE statement can be used to get the metadata of the result set:
EXEC SQL BEGIN DECLARE SECTION;
char *sql_stmt = "SELECT * FROM table1";
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE stmt1 FROM :sql_stmt;
EXEC SQL DESCRIBE stmt1 INTO SQL DESCRIPTOR mydesc;
Before PostgreSQL 9.0, the SQL keyword was optional, so using DESCRIPTOR and SQL DESCRIPTOR produced named SQL Descriptor Areas. Now it is mandatory, omitting the SQL keyword produces SQLDA Descriptor Areas, see ecpg-sqlda-descriptors.
In DESCRIBE and FETCH statements, the INTO and USING keywords can be used to similarly: they produce the result set and the metadata in a Descriptor Area.
Now how do you get the data out of the descriptor area? You can think of the descriptor area as a structure with named fields. To retrieve the value of a field from the header and store it into a host variable, use the following command:
EXEC SQL GET DESCRIPTOR name :hostvar = field;
Currently, there is only one header field defined:
COUNT, which tells how many item descriptor areas exist (that is, how many columns are contained in the result). The host variable needs to be of an integer type. To get a field from the item descriptor area, use the following command:
EXEC SQL GET DESCRIPTOR name VALUE num :hostvar = field;
num can be a literal integer or a host
variable containing an integer. Possible fields are:
number of rows in the result set
actual data item (therefore, the data type of this field depends on the query)
When TYPE is 9, DATETIME_INTERVAL_CODE will have a value of 1 for DATE, 2 for TIME, 3 for TIMESTAMP, 4 for TIME WITH TIME ZONE, or 5 for TIMESTAMP WITH TIME ZONE.
not implemented
the indicator (indicating a null value or a value truncation)
not implemented
length of the datum in characters
name of the column
not implemented
length of the character representation of the datum in bytes
precision (for type numeric)
length of the datum in characters
length of the character representation of the datum in bytes
scale (for type numeric)
numeric code of the data type of the column
In EXECUTE, DECLARE and OPEN statements, the effect of the INTO and USING keywords are different. A Descriptor Area can also be manually built to provide the input parameters for a query or a cursor and USING SQL DESCRIPTOR name is the way to pass the input parameters into a parameterized query. The statement to build a named SQL Descriptor Area is below:
EXEC SQL SET DESCRIPTOR name VALUE num field = :hostvar;
PostgreSQL supports retrieving more that one record in one FETCH statement and storing the data in host variables in this case assumes that the variable is an array. E.g.:
EXEC SQL BEGIN DECLARE SECTION;
int id[5];
EXEC SQL END DECLARE SECTION;
EXEC SQL FETCH 5 FROM mycursor INTO SQL DESCRIPTOR mydesc;
EXEC SQL GET DESCRIPTOR mydesc VALUE 1 :id = DATA;
6.3.7.2. SQLDA Descriptor Areas
An SQLDA Descriptor Area is a C language structure which can be also used to get the result set and the metadata of a query. One structure stores one record from the result set.
EXEC SQL include sqlda.h;
sqlda_t *mysqlda;
EXEC SQL FETCH 3 FROM mycursor INTO DESCRIPTOR mysqlda;
Note that the **SQL** keyword is omitted. The paragraphs about
the use cases of the INTO and USING keywords in ecpg-named-descriptors also apply here with an addition. In a DESCRIBE statement the DESCRIPTOR keyword can be completely omitted if the INTO keyword is used:
EXEC SQL DESCRIBE prepared_statement INTO mysqlda;
The general flow of a program that uses SQLDA is: #. Prepare a query, and declare a cursor for it.
Declare an SQLDA for the result rows.
Declare an SQLDA for the input parameters, and initialize them (memory allocation, parameter settings).
Open a cursor with the input SQLDA.
Fetch rows from the cursor, and store them into an output SQLDA.
Read values from the output SQLDA into the host variables (with conversion if necessary).
Close the cursor.
Free the memory area allocated for the input SQLDA.
6.3.7.2.1. SQLDA Data Structure
SQLDA uses three data structure types: sqlda_t, sqlvar_t, and struct sqlname.
Совет
PostgreSQL’s SQLDA has a similar data structure to the one in IBM DB2 Universal Database, so some technical information on DB2’s SQLDA could help understanding PostgreSQL’s one better.
6.3.7.2.1.1. sqlda_t Structure
The structure type sqlda_t is the type of the actual SQLDA. It holds one record. And two or more sqlda_t structures can be connected in a linked list with the pointer in the desc_next field, thus representing an ordered collection of rows. So, when two or more rows are fetched, the application can read them by following the desc_next pointer in each sqlda_t node.
The definition of sqlda_t is:
struct sqlda_struct
{
char sqldaid[8];
long sqldabc;
short sqln;
short sqld;
struct sqlda_struct *desc_next;
struct sqlvar_struct sqlvar[1];
};
typedef struct sqlda_struct sqlda_t;
The meaning of the fields is:
It contains the literal string «SQLDA «.
It contains the size of the allocated space in bytes.
It contains the number of input parameters for a parameterized query in case it’s passed into OPEN, DECLARE or EXECUTE statements using the USING keyword. In case it’s used as output of SELECT, EXECUTE or FETCH statements, its value is the same as sqld statement
It contains the number of fields in a result set.
If the query returns more than one record, multiple linked SQLDA structures are returned, and desc_next holds a pointer to the next entry in the list.
This is the array of the columns in the result set.
6.3.7.2.1.2. sqlvar_t Structure
The structure type sqlvar_t holds a column value and metadata such as type and length. The definition of the type is:
struct sqlvar_struct
{
short sqltype;
short sqllen;
char *sqldata;
short *sqlind;
struct sqlname sqlname;
};
typedef struct sqlvar_struct sqlvar_t;
The meaning of the fields is:
Contains the type identifier of the field. For values, see enum ECPGttype in ecpgtype.h.
Contains the binary length of the field. e.g., 4 bytes for ECPGt_int.
Points to the data. The format of the data is described in ecpg-variables-type-mapping.
Points to the null indicator. 0 means not null, -1 means null.
The name of the field.
6.3.7.2.1.3. struct sqlname Structure
A struct sqlname structure holds a column name. It is used as a member of the sqlvar_t structure. The definition of the structure is:
#define NAMEDATALEN 64
struct sqlname
{
short length;
char data[NAMEDATALEN];
};
The meaning of the fields is:
Contains the length of the field name.
Contains the actual field name.
6.3.7.2.2. Retrieving a Result Set Using an SQLDA
The general steps to retrieve a query result set through an SQLDA are: #. Declare an sqlda_t structure to receive the result set.
Execute FETCH/EXECUTE/DESCRIBE commands to process a query specifying the declared SQLDA.
Check the number of records in the result set by looking at sqln, a member of the sqlda_t structure.
Get the values of each column from sqlvar[0], sqlvar[1], etc., members of the sqlda_t structure.
Go to next row (sqlda_t structure) by following the desc_next pointer, a member of the sqlda_t structure.
Repeat above as you need.
Here is an example retrieving a result set through an SQLDA.
First, declare a sqlda_t structure to receive the result set.
sqlda_t *sqlda1;
Next, specify the SQLDA in a command. This is a FETCH command example.
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
Run a loop following the linked list to retrieve the rows.
sqlda_t *cur_sqlda;
for (cur_sqlda = sqlda1;
cur_sqlda != NULL;
cur_sqlda = cur_sqlda->desc_next)
{
...
}
Inside the loop, run another loop to retrieve each column data (sqlvar_t structure) of the row.
for (i = 0; i < cur_sqlda->sqld; i++)
{
sqlvar_t v = cur_sqlda->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
...
}
To get a column value, check the sqltype value, a member of the sqlvar_t structure. Then, switch to an appropriate way, depending on the column type, to copy data from the sqlvar field to a host variable.
char var_buf[1024];
switch (v.sqltype)
{
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf) <= sqllen ? sizeof(var_buf) - 1 : sqllen));
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
...
}
6.3.7.2.3. Passing Query Parameters Using an SQLDA
The general steps to use an SQLDA to pass input parameters to a prepared query are: #. Create a prepared query (prepared statement)
Declare an sqlda_t structure as an input SQLDA.
Allocate memory area (as sqlda_t structure) for the input SQLDA.
Set (copy) input values in the allocated memory.
Open a cursor with specifying the input SQLDA.
Here is an example.
First, create a prepared statement.
EXEC SQL BEGIN DECLARE SECTION;
char query[1024] = "SELECT d.oid, * FROM pg_database d, pg_stat_database s WHERE d.oid = s.datid AND (d.datname = ? OR d.oid = ?)";
EXEC SQL END DECLARE SECTION;
EXEC SQL PREPARE stmt1 FROM :query;
Next, allocate memory for an SQLDA, and set the number of input parameters in sqln, a member variable of the sqlda_t structure. When two or more input parameters are required for the prepared query, the application has to allocate additional memory space which is calculated by (nr. of params - 1) * sizeof(sqlvar_t). The example shown here allocates memory space for two input parameters.
sqlda_t *sqlda2;
sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* number of input variables */
After memory allocation, store the parameter values into the sqlvar[] array. (This is same array used for retrieving column values when the SQLDA is receiving a result set.) In this example, the input parameters are «postgres», having a string type, and 1, having an integer type.
sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
int intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *) &intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);
By opening a cursor and specifying the SQLDA that was set up beforehand, the input parameters are passed to the prepared statement.
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
Finally, after using input SQLDAs, the allocated memory space must be freed explicitly, unlike SQLDAs used for receiving query results.
free(sqlda2);
6.3.7.2.4. A Sample Application Using SQLDA
Here is an example program, which describes how to fetch access statistics of the databases, specified by the input parameters, from the system catalogs.
This application joins two system tables, pg_database and pg_stat_database on the database OID, and also fetches and shows the database statistics which are retrieved by two input parameters (a database postgres, and OID 1).
First, declare an SQLDA for input and an SQLDA for output.
EXEC SQL include sqlda.h;
sqlda_t *sqlda1; /* an output descriptor */
sqlda_t *sqlda2; /* an input descriptor */
Next, connect to the database, prepare a statement, and declare a cursor for the prepared statement.
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
char query[1024] = "SELECT d.oid,* FROM pg_database d, pg_stat_database s WHERE d.oid=s.datid AND ( d.datname=? OR d.oid=? )";
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
Next, put some values in the input SQLDA for the input parameters. Allocate memory for the input SQLDA, and set the number of input parameters to sqln. Store type, value, and value length into sqltype, sqldata, and sqllen in the sqlvar structure.
/* Create SQLDA structure for input parameters. */
sqlda2 = (sqlda_t *) malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* number of input variables */
sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *)&intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);
After setting up the input SQLDA, open a cursor with the input SQLDA.
/* Open a cursor with input parameters. */
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
Fetch rows into the output SQLDA from the opened cursor. (Generally, you have to call FETCH repeatedly in the loop, to fetch all rows in the result set.)
while (1)
{
sqlda_t *cur_sqlda;
/* Assign descriptor to the cursor */
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
Next, retrieve the fetched records from the SQLDA, by following the linked list of the sqlda_t structure.
for (cur_sqlda = sqlda1 ;
cur_sqlda != NULL ;
cur_sqlda = cur_sqlda->desc_next)
{
...
Read each columns in the first record. The number of columns is stored in sqld, the actual data of the first column is stored in sqlvar[0], both members of the sqlda_t structure.
/* Print every column in a row. */
for (i = 0; i < sqlda1->sqld; i++)
{
sqlvar_t v = sqlda1->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
strncpy(name_buf, v.sqlname.data, v.sqlname.length);
name_buf[v.sqlname.length] = '\0';
Now, the column data is stored in the variable v. Copy every datum into host variables, looking at v.sqltype for the type of the column.
switch (v.sqltype) {
int intval;
double doubleval;
unsigned long long int longlongval;
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf) <= sqllen ? sizeof(var_buf)-1 : sqllen));
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
...
default:
...
}
printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);
}
Close the cursor after processing all of records, and disconnect from the database.
EXEC SQL CLOSE cur1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
The whole program is shown in ecpg-sqlda-example-example.
#include <stdlib.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
EXEC SQL include sqlda.h;
sqlda_t *sqlda1; /* descriptor for output */
sqlda_t *sqlda2; /* descriptor for input */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
char query[1024] = "SELECT d.oid,* FROM pg_database d, pg_stat_database s WHERE d.oid=s.datid AND ( d.datname=? OR d.oid=? )";
int intval;
unsigned long long int longlongval;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO uptimedb AS con1 USER uptime;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL PREPARE stmt1 FROM :query;
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
/* Create an SQLDA structure for an input parameter */
sqlda2 = (sqlda_t *)malloc(sizeof(sqlda_t) + sizeof(sqlvar_t));
memset(sqlda2, 0, sizeof(sqlda_t) + sizeof(sqlvar_t));
sqlda2->sqln = 2; /* a number of input variables */
sqlda2->sqlvar[0].sqltype = ECPGt_char;
sqlda2->sqlvar[0].sqldata = "postgres";
sqlda2->sqlvar[0].sqllen = 8;
intval = 1;
sqlda2->sqlvar[1].sqltype = ECPGt_int;
sqlda2->sqlvar[1].sqldata = (char *) &intval;
sqlda2->sqlvar[1].sqllen = sizeof(intval);
/* Open a cursor with input parameters. */
EXEC SQL OPEN cur1 USING DESCRIPTOR sqlda2;
while (1)
{
sqlda_t *cur_sqlda;
/* Assign descriptor to the cursor */
EXEC SQL FETCH NEXT FROM cur1 INTO DESCRIPTOR sqlda1;
for (cur_sqlda = sqlda1 ;
cur_sqlda != NULL ;
cur_sqlda = cur_sqlda->desc_next)
{
int i;
char name_buf[1024];
char var_buf[1024];
/* Print every column in a row. */
for (i=0 ; i<cur_sqlda->sqld ; i++)
{
sqlvar_t v = cur_sqlda->sqlvar[i];
char *sqldata = v.sqldata;
short sqllen = v.sqllen;
strncpy(name_buf, v.sqlname.data, v.sqlname.length);
name_buf[v.sqlname.length] = '\0';
switch (v.sqltype)
{
case ECPGt_char:
memset(&var_buf, 0, sizeof(var_buf));
memcpy(&var_buf, sqldata, (sizeof(var_buf)<=sqllen ? sizeof(var_buf)-1 : sqllen) );
break;
case ECPGt_int: /* integer */
memcpy(&intval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%d", intval);
break;
case ECPGt_long_long: /* bigint */
memcpy(&longlongval, sqldata, sqllen);
snprintf(var_buf, sizeof(var_buf), "%lld", longlongval);
break;
default:
{
int i;
memset(var_buf, 0, sizeof(var_buf));
for (i = 0; i < sqllen; i++)
{
char tmpbuf[16];
snprintf(tmpbuf, sizeof(tmpbuf), "%02x ", (unsigned char) sqldata[i]);
strncat(var_buf, tmpbuf, sizeof(var_buf));
}
}
break;
}
printf("%s = %s (type: %d)\n", name_buf, var_buf, v.sqltype);
}
printf("\n");
}
}
EXEC SQL CLOSE cur1;
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
The output of this example should look something like the following (some numbers will vary).
oid = 1 (type: 1)
datname = template1 (type: 1)
datdba = 10 (type: 1)
encoding = 0 (type: 5)
datistemplate = t (type: 1)
datallowconn = t (type: 1)
datconnlimit = -1 (type: 5)
datfrozenxid = 379 (type: 1)
dattablespace = 1663 (type: 1)
datconfig = (type: 1)
datacl = {=c/uptime,uptime=CTc/uptime} (type: 1)
datid = 1 (type: 1)
datname = template1 (type: 1)
numbackends = 0 (type: 5)
xact_commit = 113606 (type: 9)
xact_rollback = 0 (type: 9)
blks_read = 130 (type: 9)
blks_hit = 7341714 (type: 9)
tup_returned = 38262679 (type: 9)
tup_fetched = 1836281 (type: 9)
tup_inserted = 0 (type: 9)
tup_updated = 0 (type: 9)
tup_deleted = 0 (type: 9)
oid = 11511 (type: 1)
datname = postgres (type: 1)
datdba = 10 (type: 1)
encoding = 0 (type: 5)
datistemplate = f (type: 1)
datallowconn = t (type: 1)
datconnlimit = -1 (type: 5)
datfrozenxid = 379 (type: 1)
dattablespace = 1663 (type: 1)
datconfig = (type: 1)
datacl = (type: 1)
datid = 11511 (type: 1)
datname = postgres (type: 1)
numbackends = 0 (type: 5)
xact_commit = 221069 (type: 9)
xact_rollback = 18 (type: 9)
blks_read = 1176 (type: 9)
blks_hit = 13943750 (type: 9)
tup_returned = 77410091 (type: 9)
tup_fetched = 3253694 (type: 9)
tup_inserted = 0 (type: 9)
tup_updated = 0 (type: 9)
tup_deleted = 0 (type: 9)
6.3.8. Error Handling
This section describes how you can handle exceptional conditions and warnings in an embedded SQL program. There are two nonexclusive facilities for this.
Callbacks can be configured to handle warning and error conditions using the WHENEVER command.
Detailed information about the error or warning can be obtained from the sqlca variable.
6.3.8.1. Setting Callbacks
One simple method to catch errors and warnings is to set a specific action to be executed whenever a particular condition occurs. In general:
EXEC SQL WHENEVER condition action;
condition can be one of the following:
The specified action is called whenever an error occurs during the execution of an SQL statement.
The specified action is called whenever a warning occurs during the execution of an SQL statement.
The specified action is called whenever an SQL statement retrieves or affects zero rows. (This condition is not an error, but you might be interested in handling it specially.)
action can be one of the following:
This effectively means that the condition is ignored. This is the default.
Jump to the specified label (using a C goto statement).
Print a message to standard error. This is useful for simple programs or during prototyping. The details of the message cannot be configured.
Call exit(1), which will terminate the program.
Execute the C statement break. This should only be used in loops or switch statements.
Execute the C statement continue. This should only be used in loops statements. if executed, will cause the flow of control to return to the top of the loop.
Call the specified C functions with the specified arguments. (This use is different from the meaning of CALL and DO in the normal PostgreSQL grammar.)
The SQL standard only provides for the actions CONTINUE and GOTO (and GO TO).
Here is an example that you might want to use in a simple program. It prints a simple message when a warning occurs and aborts the program when an error happens:
EXEC SQL WHENEVER SQLWARNING SQLPRINT;
EXEC SQL WHENEVER SQLERROR STOP;
The statement EXEC SQL WHENEVER is a directive of the SQL preprocessor, not a C statement. The error or warning actions that it sets apply to all embedded SQL statements that appear below the point where the handler is set, unless a different action was set for the same condition between the first EXEC SQL WHENEVER and the SQL statement causing the condition, regardless of the flow of control in the C program. So neither of the two following C program excerpts will have the desired effect:
/*
* WRONG
*/
int main(int argc, char *argv[])
{
...
if (verbose) {
EXEC SQL WHENEVER SQLWARNING SQLPRINT;
}
...
EXEC SQL SELECT ...;
...
}
/*
* WRONG
*/
int main(int argc, char *argv[])
{
...
set_error_handler();
...
EXEC SQL SELECT ...;
...
}
static void set_error_handler(void)
{
EXEC SQL WHENEVER SQLERROR STOP;
}
6.3.8.2. sqlca
For more powerful error handling, the embedded SQL interface provides a global variable with the name sqlca (SQL communication area) that has the following structure:
struct
{
char sqlcaid[8];
long sqlabc;
long sqlcode;
struct
{
int sqlerrml;
char sqlerrmc[SQLERRMC_LEN];
} sqlerrm;
char sqlerrp[8];
long sqlerrd[6];
char sqlwarn[8];
char sqlstate[5];
} sqlca;
(In a multithreaded program, every thread automatically gets its
own copy of sqlca. This works similarly to the handling of the standard C global variable errno.)
sqlca covers both warnings and errors. If multiple warnings or errors occur during the execution of a statement, then sqlca will only contain information about the last one.
If no error occurred in the last SQL statement, sqlca.sqlcode will be 0 and sqlca.sqlstate will be «00000». If a warning or error occurred, then sqlca.sqlcode will be negative and sqlca.sqlstate will be different from «00000». A positive sqlca.sqlcode indicates a harmless condition, such as that the last query returned zero rows. sqlcode and sqlstate are two different error code schemes; details appear below.
If the last SQL statement was successful, then sqlca.sqlerrd[1] contains the OID of the processed row, if applicable, and sqlca.sqlerrd[2] contains the number of processed or returned rows, if applicable to the command.
In case of an error or warning, sqlca.sqlerrm.sqlerrmc will contain a string that describes the error. The field sqlca.sqlerrm.sqlerrml contains the length of the error message that is stored in sqlca.sqlerrm.sqlerrmc (the result of strlen(), not really interesting for a C programmer). Note that some messages are too long to fit in the fixed-size sqlerrmc array; they will be truncated.
In case of a warning, sqlca.sqlwarn[2] is set to W. (In all other cases, it is set to something different from W.) If sqlca.sqlwarn[1] is set to W, then a value was truncated when it was stored in a host variable. sqlca.sqlwarn[0] is set to W if any of the other elements are set to indicate a warning.
The fields sqlcaid, sqlabc, sqlerrp, and the remaining elements of sqlerrd and sqlwarn currently contain no useful information.
The structure sqlca is not defined in the SQL standard, but is implemented in several other SQL database systems. The definitions are similar at the core, but if you want to write portable applications, then you should investigate the different implementations carefully.
Here is one example that combines the use of WHENEVER and sqlca, printing out the contents of sqlca when an error occurs. This is perhaps useful for debugging or prototyping applications, before installing a more user-friendly error handler.
EXEC SQL WHENEVER SQLERROR CALL print_sqlca();
void
print_sqlca()
{
fprintf(stderr, "==== sqlca ====\n");
fprintf(stderr, "sqlcode: %ld\n", sqlca.sqlcode);
fprintf(stderr, "sqlerrm.sqlerrml: %d\n", sqlca.sqlerrm.sqlerrml);
fprintf(stderr, "sqlerrm.sqlerrmc: %s\n", sqlca.sqlerrm.sqlerrmc);
fprintf(stderr, "sqlerrd: %ld %ld %ld %ld %ld %ld\n", sqlca.sqlerrd[0],sqlca.sqlerrd[1],sqlca.sqlerrd[2],
sqlca.sqlerrd[3],sqlca.sqlerrd[4],sqlca.sqlerrd[5]);
fprintf(stderr, "sqlwarn: %d %d %d %d %d %d %d %d\n", sqlca.sqlwarn[0], sqlca.sqlwarn[1], sqlca.sqlwarn[2],
sqlca.sqlwarn[3], sqlca.sqlwarn[4], sqlca.sqlwarn[5],
sqlca.sqlwarn[6], sqlca.sqlwarn[7]);
fprintf(stderr, "sqlstate: %5s\n", sqlca.sqlstate);
fprintf(stderr, "===============\n");
}
The result could look as follows (here an error due to a misspelled table name):
==== sqlca ====
sqlcode: -400
sqlerrm.sqlerrml: 49
sqlerrm.sqlerrmc: relation "pg_databasep" does not exist on line 38
sqlerrd: 0 0 0 0 0 0
sqlwarn: 0 0 0 0 0 0 0 0
sqlstate: 42P01
===============
6.3.8.3. SQLSTATE vs. SQLCODE
The fields sqlca.sqlstate and sqlca.sqlcode are two different schemes that provide error codes. Both are derived from the SQL standard, but SQLCODE has been marked deprecated in the SQL-92 edition of the standard and has been dropped in later editions. Therefore, new applications are strongly encouraged to use SQLSTATE.
SQLSTATE is a five-character array. The five characters contain digits or upper-case letters that represent codes of various error and warning conditions. SQLSTATE has a hierarchical scheme: the first two characters indicate the general class of the condition, the last three characters indicate a subclass of the general condition. A successful state is indicated by the code 00000. The SQLSTATE codes are for the most part defined in the SQL standard. The PostgreSQL server natively supports SQLSTATE error codes; therefore a high degree of consistency can be achieved by using this error code scheme throughout all applications. For further information see errcodes-appendix.
SQLCODE, the deprecated error code scheme, is a simple integer. A value of 0 indicates success, a positive value indicates success with additional information, a negative value indicates an error. The SQL standard only defines the positive value +100, which indicates that the last command returned or affected zero rows, and no specific negative values. Therefore, this scheme can only achieve poor portability and does not have a hierarchical code assignment. Historically, the embedded SQL processor for PostgreSQL has assigned some specific SQLCODE values for its use, which are listed below with their numeric value and their symbolic name. Remember that these are not portable to other SQL implementations. To simplify the porting of applications to the SQLSTATE scheme, the corresponding SQLSTATE is also listed. There is, however, no one-to-one or one-to-many mapping between the two schemes (indeed it is many-to-many), so you should consult the global SQLSTATE listing in errcodes-appendix in each case.
These are the assigned SQLCODE values:
Indicates no error. (SQLSTATE 00000)
This is a harmless condition indicating that the last command retrieved or processed zero rows, or that you are at the end of the cursor. (SQLSTATE 02000)
When processing a cursor in a loop, you could use this code as a way to detect when to abort the loop, like this:
while (1) { EXEC SQL FETCH ... ; if (sqlca.sqlcode == ECPG_NOT_FOUND) break; } But **WHENEVER NOT FOUND DO BREAK** effectively
does this internally, so there is usually no advantage in writing this out explicitly.
Indicates that your virtual memory is exhausted. The numeric value is defined as -ENOMEM. (SQLSTATE YE001)
Indicates the preprocessor has generated something that the library does not know about. Perhaps you are running incompatible versions of the preprocessor and the library. (SQLSTATE YE002)
This means that the command specified more host variables than the command expected. (SQLSTATE 07001 or 07002)
This means that the command specified fewer host variables than the command expected. (SQLSTATE 07001 or 07002)
This means a query has returned multiple rows but the statement was only prepared to store one result row (for example, because the specified variables are not arrays). (SQLSTATE 21000)
The host variable is of type int and the datum in the database is of a different type and contains a value that cannot be interpreted as an int. The library uses strtol() for this conversion. (SQLSTATE 42804)
The host variable is of type unsigned int and the datum in the database is of a different type and contains a value that cannot be interpreted as an unsigned int. The library uses strtoul() for this conversion. (SQLSTATE 42804)
The host variable is of type float and the datum in the database is of another type and contains a value that cannot be interpreted as a float. The library uses strtod() for this conversion. (SQLSTATE 42804)
The host variable is of type numeric and the datum in the database is of another type and contains a value that cannot be interpreted as a numeric value. (SQLSTATE 42804)
The host variable is of type interval and the datum in the database is of another type and contains a value that cannot be interpreted as an interval value. (SQLSTATE 42804)
The host variable is of type date and the datum in the database is of another type and contains a value that cannot be interpreted as a date value. (SQLSTATE 42804)
The host variable is of type timestamp and the datum in the database is of another type and contains a value that cannot be interpreted as a timestamp value. (SQLSTATE 42804)
This means the host variable is of type bool and the datum in the database is neither „t“ nor „f“. (SQLSTATE 42804)
The statement sent to the PostgreSQL server was empty. (This cannot normally happen in an embedded SQL program, so it might point to an internal error.) (SQLSTATE YE002)
A null value was returned and no null indicator variable was supplied. (SQLSTATE 22002)
An ordinary variable was used in a place that requires an array. (SQLSTATE 42804)
The database returned an ordinary variable in a place that requires array value. (SQLSTATE 42804)
The value could not be inserted into the array. (SQLSTATE 42804)
The program tried to access a connection that does not exist. (SQLSTATE 08003)
The program tried to access a connection that does exist but is not open. (This is an internal error.) (SQLSTATE YE002)
The statement you are trying to use has not been prepared. (SQLSTATE 26000)
Duplicate key error, violation of unique constraint (Informix compatibility mode). (SQLSTATE 23505)
The descriptor specified was not found. The statement you are trying to use has not been prepared. (SQLSTATE 33000)
The descriptor index specified was out of range. (SQLSTATE 07009)
An invalid descriptor item was requested. (This is an internal error.) (SQLSTATE YE002)
During the execution of a dynamic statement, the database returned a numeric value and the host variable was not numeric. (SQLSTATE 07006)
During the execution of a dynamic statement, the database returned a non-numeric value and the host variable was numeric. (SQLSTATE 07006)
A result of the subquery is not single row (Informix compatibility mode). (SQLSTATE 21000)
Some error caused by the PostgreSQL server. The message contains the error message from the PostgreSQL server.
The PostgreSQL server signaled that we cannot start, commit, or rollback the transaction. (SQLSTATE 08007)
The connection attempt to the database did not succeed. (SQLSTATE 08001)
Duplicate key error, violation of unique constraint. (SQLSTATE 23505)
A result for the subquery is not single row. (SQLSTATE 21000)
An invalid cursor name was specified. (SQLSTATE 34000)
Transaction is in progress. (SQLSTATE 25001)
There is no active (in-progress) transaction. (SQLSTATE 25P01)
An existing cursor name was specified. (SQLSTATE 42P03)
6.3.9. Preprocessor Directives
Several preprocessor directives are available that modify how the ecpg preprocessor parses and processes a file.
6.3.9.1. Including Files
To include an external file into your embedded SQL program, use:
EXEC SQL INCLUDE filename;
EXEC SQL INCLUDE <filename>;
EXEC SQL INCLUDE "filename";
The embedded SQL preprocessor will look for a file named
filename.h, preprocess it, and include it in the resulting C output. Thus, embedded SQL statements in the included file are handled correctly.
The ecpg preprocessor will search a file at several directories in following order:
current directory
/usr/local/include
PostgreSQL include directory, defined at build time (e.g., /usr/local/pgsql/include)
/usr/include
But when EXEC SQL INCLUDE «filename» is used, only the current directory is searched.
In each directory, the preprocessor will first look for the file name as given, and if not found will append .h to the file name and try again (unless the specified file name already has that suffix).
Note that EXEC SQL INCLUDE is not the same as:
#include <filename.h>
because this file would not be subject to SQL command preprocessing.
Naturally, you can continue to use the C #include directive to include other header files.
Примечание
The include file name is case-sensitive, even though the rest of the EXEC SQL INCLUDE command follows the normal SQL case-sensitivity rules.
6.3.9.2. The define and undef Directives
Similar to the directive #define that is known from C, embedded SQL has a similar concept:
EXEC SQL DEFINE name;
EXEC SQL DEFINE name value;
So you can define a name:
EXEC SQL DEFINE HAVE_FEATURE;
And you can also define constants:
EXEC SQL DEFINE MYNUMBER 12;
EXEC SQL DEFINE MYSTRING 'abc';
Use **undef** to remove a previous definition:
EXEC SQL UNDEF MYNUMBER;
Of course you can continue to use the C versions #define and #undef in your embedded SQL program. The difference is where your defined values get evaluated. If you use EXEC SQL DEFINE then the ecpg preprocessor evaluates the defines and substitutes the values. For example if you write:
EXEC SQL DEFINE MYNUMBER 12;
...
EXEC SQL UPDATE Tbl SET col = MYNUMBER;
then **ecpg** will already do the substitution and your C compiler will never
see any name or identifier MYNUMBER. Note that you cannot use #define for a constant that you are going to use in an embedded SQL query because in this case the embedded SQL precompiler is not able to see this declaration.
6.3.9.3. ifdef, ifndef, elif, else, and endif Directives
You can use the following directives to compile code sections conditionally:
Checks a name and processes subsequent lines if name has been defined via EXEC SQL define name.
Checks a name and processes subsequent lines if name has not been defined via EXEC SQL define name.
Begins an optional alternative section after an EXEC SQL ifdef name or EXEC SQL ifndef name directive. Any number of elif sections can appear. Lines following an elif will be processed if name has been defined and no previous section of the same ifdef/ifndef…**endif** construct has been processed.
Begins an optional, final alternative section after an EXEC SQL ifdef name or EXEC SQL ifndef name directive. Subsequent lines will be processed if no previous section of the same ifdef/ifndef…**endif** construct has been processed.
Ends an ifdef/ifndef…**endif** construct. Subsequent lines are processed normally.
ifdef/ifndef…**endif** constructs can be nested, up to 127 levels deep.
This example will compile exactly one of the three SET TIMEZONE commands:
EXEC SQL ifdef TZVAR;
EXEC SQL SET TIMEZONE TO TZVAR;
EXEC SQL elif TZNAME;
EXEC SQL SET TIMEZONE TO TZNAME;
EXEC SQL else;
EXEC SQL SET TIMEZONE TO 'GMT';
EXEC SQL endif;
6.3.10. Processing Embedded SQL Programs
Now that you have an idea how to form embedded SQL C programs, you probably want to know how to compile them. Before compiling you run the file through the embedded SQL C preprocessor, which converts the SQL statements you used to special function calls. After compiling, you must link with a special library that contains the needed functions. These functions fetch information from the arguments, perform the SQL command using the libpq interface, and put the result in the arguments specified for output.
The preprocessor program is called ecpg and is included in a normal PostgreSQL installation. Embedded SQL programs are typically named with an extension .pgc. If you have a program file called prog1.pgc, you can preprocess it by simply calling:
ecpg prog1.pgc
This will create a file called prog1.c. If
your input files do not follow the suggested naming pattern, you can specify the output file explicitly using the -o option.
The preprocessed file can be compiled normally, for example:
cc -c prog1.c
The generated C source files include header files from the
PostgreSQL installation, so if you installed PostgreSQL in a location that is not searched by default, you have to add an option such as -I/usr/local/pgsql/include to the compilation command line.
To link an embedded SQL program, you need to include the libecpg library, like so:
cc -o myprog prog1.o prog2.o ... -lecpg
Again, you might have to add an option like
-L/usr/local/pgsql/lib to that command line.
You can use pg_config or pkg-config**with package name **libecpg to get the paths for your installation.
If you manage the build process of a larger project using make, it might be convenient to include the following implicit rule to your makefiles:
ECPG = ecpg
%.c: %.pgc
$(ECPG) $<
The complete syntax of the ecpg command is detailed in app-ecpg.
The ecpg library is thread-safe by default. However, you might need to use some threading command-line options to compile your client code.
6.3.11. Library Functions
The libecpg library primarily contains hidden functions that are used to implement the functionality expressed by the embedded SQL commands. But there are some functions that can usefully be called directly. Note that this makes your code unportable.
ECPGdebug(int on, FILE *stream) turns on debug logging if called with the first argument non-zero. Debug logging is done on stream. The log contains all SQL statements with all the input variables inserted, and the results from the PostgreSQL server. This can be very useful when searching for errors in your SQL statements.
Примечание
On Windows, if the ecpg libraries and an application are compiled with different flags, this function call will crash the application because the internal representation of the FILE pointers differ. Specifically, multithreaded/single-threaded, release/debug, and static/dynamic flags should be the same for the library and all applications using that library.
ECPGget_PGconn(const char *connection_name) ** returns the library database connection handle identified by the given name. If connection_name is set to **NULL, the current connection handle is returned. If no connection handle can be identified, the function returns NULL. The returned connection handle can be used to call any other functions from libpq, if necessary.
Примечание
It is a bad idea to manipulate database connection handles made from ecpg directly with libpq routines.
ECPGtransactionStatus(const char *connection_name) returns the current transaction status of the given connection identified by connection_name. See libpq-status and libpq’s libpq-PQtransactionStatus for details about the returned status codes.
ECPGstatus(int lineno, const char* connection_name) returns true if you are connected to a database and false if not. connection_name can be NULL if a single connection is being used.
6.3.12. Large Objects
Large objects are not directly supported by ECPG, but ECPG application can manipulate large objects through the libpq large object functions, obtaining the necessary PGconn object by calling the ECPGget_PGconn() function. (However, use of the ECPGget_PGconn() function and touching PGconn objects directly should be done very carefully and ideally not mixed with other ECPG database access calls.)
For more details about the ECPGget_PGconn(), see ecpg-library. For information about the large object function interface, see largeobjects.
Large object functions have to be called in a transaction block, so when autocommit is off, BEGIN commands have to be issued explicitly.
ecpg-lo-example shows an example program that illustrates how to create, write, and read a large object in an ECPG application.
#include <stdio.h>
#include <stdlib.h>
#include <libpq-fe.h>
#include <libpq/libpq-fs.h>
EXEC SQL WHENEVER SQLERROR STOP;
int
main(void)
{
PGconn *conn;
Oid loid;
int fd;
char buf[256];
int buflen = 256;
char buf2[256];
int rc;
memset(buf, 1, buflen);
EXEC SQL CONNECT TO testdb AS con1;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
conn = ECPGget_PGconn("con1");
printf("conn = %p\n", conn);
/* create */
loid = lo_create(conn, 0);
if (loid < 0)
printf("lo_create() failed: %s", PQerrorMessage(conn));
printf("loid = %d\n", loid);
/* write test */
fd = lo_open(conn, loid, INV_READ|INV_WRITE);
if (fd < 0)
printf("lo_open() failed: %s", PQerrorMessage(conn));
printf("fd = %d\n", fd);
rc = lo_write(conn, fd, buf, buflen);
if (rc < 0)
printf("lo_write() failed\n");
rc = lo_close(conn, fd);
if (rc < 0)
printf("lo_close() failed: %s", PQerrorMessage(conn));
/* read test */
fd = lo_open(conn, loid, INV_READ);
if (fd < 0)
printf("lo_open() failed: %s", PQerrorMessage(conn));
printf("fd = %d\n", fd);
rc = lo_read(conn, fd, buf2, buflen);
if (rc < 0)
printf("lo_read() failed\n");
rc = lo_close(conn, fd);
if (rc < 0)
printf("lo_close() failed: %s", PQerrorMessage(conn));
/* check */
rc = memcmp(buf, buf2, buflen);
printf("memcmp() = %d\n", rc);
/* cleanup */
rc = lo_unlink(conn, loid);
if (rc < 0)
printf("lo_unlink() failed: %s", PQerrorMessage(conn));
EXEC SQL COMMIT;
EXEC SQL DISCONNECT ALL;
return 0;
}
6.3.13. C++ Applications
ECPG has some limited support for C++ applications. This section describes some caveats.
The ecpg preprocessor takes an input file written in C (or something like C) and embedded SQL commands, converts the embedded SQL commands into C language chunks, and finally generates a .c file. The header file declarations of the library functions used by the C language chunks that ecpg generates are wrapped in extern «C» { … } blocks when used under C++, so they should work seamlessly in C++.
In general, however, the ecpg preprocessor only understands C; it does not handle the special syntax and reserved words of the C++ language. So, some embedded SQL code written in C++ application code that uses complicated features specific to C++ might fail to be preprocessed correctly or might not work as expected.
A safe way to use the embedded SQL code in a C++ application is hiding the ECPG calls in a C module, which the C++ application code calls into to access the database, and linking that together with the rest of the C++ code. See ecpg-cpp-and-c about that.
6.3.13.1. Scope for Host Variables
The ecpg preprocessor understands the scope of variables in C. In the C language, this is rather simple because the scopes of variables is based on their code blocks. In C++, however, the class member variables are referenced in a different code block from the declared position, so the ecpg preprocessor will not understand the scope of the class member variables.
For example, in the following case, the ecpg preprocessor cannot find any declaration for the variable dbname in the test method, so an error will occur.
class TestCpp
{
EXEC SQL BEGIN DECLARE SECTION;
char dbname[1024];
EXEC SQL END DECLARE SECTION;
public:
TestCpp();
void test();
~TestCpp();
};
TestCpp::TestCpp()
{
EXEC SQL CONNECT TO testdb1;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
}
void Test::test()
{
EXEC SQL SELECT current_database() INTO :dbname;
printf("current_database = %s\n", dbname);
}
TestCpp::~TestCpp()
{
EXEC SQL DISCONNECT ALL;
}
This code will result in an error like this:
ecpg test_cpp.pgc
test_cpp.pgc:28: ERROR: variable "dbname" is not declared
To avoid this scope issue, the test method could be modified to use a local variable as intermediate storage. But this approach is only a poor workaround, because it uglifies the code and reduces performance.
void TestCpp::test()
{
EXEC SQL BEGIN DECLARE SECTION;
char tmp[1024];
EXEC SQL END DECLARE SECTION;
EXEC SQL SELECT current_database() INTO :tmp;
strlcpy(dbname, tmp, sizeof(tmp));
printf("current_database = %s\n", dbname);
}
6.3.13.2. C++ Application Development with External C Module
If you understand these technical limitations of the ecpg preprocessor in C++, you might come to the conclusion that linking C objects and C++ objects at the link stage to enable C++ applications to use ECPG features could be better than writing some embedded SQL commands in C++ code directly. This section describes a way to separate some embedded SQL commands from C++ application code with a simple example. In this example, the application is implemented in C++, while C and ECPG is used to connect to the PostgreSQL server.
Three kinds of files have to be created: a C file (*.pgc), a header file, and a C++ file:
A sub-routine module to execute SQL commands embedded in C. It is going to be converted into test_mod.c by the preprocessor.
#include "test_mod.h" #include <stdio.h> void db_connect() { EXEC SQL CONNECT TO testdb1; EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT; } void db_test() { EXEC SQL BEGIN DECLARE SECTION; char dbname[1024]; EXEC SQL END DECLARE SECTION; EXEC SQL SELECT current_database() INTO :dbname; printf("current_database = %s\n", dbname); } void db_disconnect() { EXEC SQL DISCONNECT ALL; }
A header file with declarations of the functions in the C module (test_mod.pgc). It is included by test_cpp.cpp. This file has to have an extern «C» block around the declarations, because it will be linked from the C++ module.
#ifdef __cplusplus extern "C" { #endif void db_connect(); void db_test(); void db_disconnect(); #ifdef __cplusplus } #endif
The main code for the application, including the main routine, and in this example a C++ class.
#include "test_mod.h" class TestCpp { public: TestCpp(); void test(); ~TestCpp(); }; TestCpp::TestCpp() { db_connect(); } void TestCpp::test() { db_test(); } TestCpp::~TestCpp() { db_disconnect(); } int main(void) { TestCpp *t = new TestCpp(); t->test(); return 0; }
To build the application, proceed as follows. Convert test_mod.pgc into test_mod.c by running ecpg, and generate test_mod.o by compiling test_mod.c with the C compiler:
ecpg -o test_mod.c test_mod.pgc
cc -c test_mod.c -o test_mod.o
Next, generate test_cpp.o by compiling test_cpp.cpp with the C++ compiler:
c++ -c test_cpp.cpp -o test_cpp.o
Finally, link these object files, test_cpp.o and test_mod.o, into one executable, using the C++ compiler driver:
c++ test_cpp.o test_mod.o -lecpg -o test_cpp
6.3.14. Embedded SQL Commands
This section describes all SQL commands that are specific to embedded SQL. Also refer to the SQL commands listed in sql-commands, which can also be used in embedded SQL, unless stated otherwise.
ALLOCATE DESCRIPTOR allocate an SQL descriptor area
ALLOCATE DESCRIPTOR name
6.3.15. Description
ALLOCATE DESCRIPTOR allocates a new named SQL descriptor area, which can be used to exchange data between the PostgreSQL server and the host program.
Descriptor areas should be freed after use using the DEALLOCATE DESCRIPTOR command.
6.3.16. Parameters
A name of SQL descriptor, case sensitive. This can be an SQL identifier or a host variable.
6.3.17. Examples
EXEC SQL ALLOCATE DESCRIPTOR mydesc;
6.3.18. Compatibility
ALLOCATE DESCRIPTOR is specified in the SQL standard.
6.3.19. See Also
ecpg-sql-deallocate-descriptor ecpg-sql-get-descriptor ecpg-sql-set-descriptor
CONNECT establish a database connection
CONNECT TO connection_target [ AS connection_name ] [ USER connection_user ]
CONNECT TO DEFAULT
CONNECT connection_user
DATABASE connection_target
6.3.20. Description
The CONNECT command establishes a connection between the client and the PostgreSQL server.
6.3.21. Parameters
connection_target specifies the target server of the connection on one of several forms.
Connect over TCP/IP
Connect over Unix-domain sockets
Connect over TCP/IP
containing a value in one of the above forms
host variable of type char[] or VARCHAR[] containing a value in one of the above forms
An optional identifier for the connection, so that it can be referred to in other commands. This can be an SQL identifier or a host variable.
The user name for the database connection.
This parameter can also specify user name and password, using one the forms user_name/password, user_name IDENTIFIED BY password, or user_name USING password.
User name and password can be SQL identifiers, string constants, or host variables.
Use all default connection parameters, as defined by libpq.
6.3.22. Examples
Here a several variants for specifying connection parameters:
EXEC SQL CONNECT TO "connectdb" AS main;
EXEC SQL CONNECT TO "connectdb" AS second;
EXEC SQL CONNECT TO "unix:postgresql://200.46.204.71/connectdb" AS main USER connectuser;
EXEC SQL CONNECT TO "unix:postgresql://localhost/connectdb" AS main USER connectuser;
EXEC SQL CONNECT TO 'connectdb' AS main;
EXEC SQL CONNECT TO 'unix:postgresql://localhost/connectdb' AS main USER :user;
EXEC SQL CONNECT TO :db AS :id;
EXEC SQL CONNECT TO :db USER connectuser USING :pw;
EXEC SQL CONNECT TO @localhost AS main USER connectdb;
EXEC SQL CONNECT TO REGRESSDB1 as main;
EXEC SQL CONNECT TO AS main USER connectdb;
EXEC SQL CONNECT TO connectdb AS :id;
EXEC SQL CONNECT TO connectdb AS main USER connectuser/connectdb;
EXEC SQL CONNECT TO connectdb AS main;
EXEC SQL CONNECT TO connectdb@localhost AS main;
EXEC SQL CONNECT TO tcp:postgresql://localhost/ USER connectdb;
EXEC SQL CONNECT TO tcp:postgresql://localhost/connectdb USER connectuser IDENTIFIED BY connectpw;
EXEC SQL CONNECT TO tcp:postgresql://localhost:20/connectdb USER connectuser IDENTIFIED BY connectpw;
EXEC SQL CONNECT TO unix:postgresql://localhost/ AS main USER connectdb;
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb AS main USER connectuser;
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb USER connectuser IDENTIFIED BY "connectpw";
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb USER connectuser USING "connectpw";
EXEC SQL CONNECT TO unix:postgresql://localhost/connectdb?connect_timeout=14 USER connectuser;
Here is an example program that illustrates the use of host variables to specify connection parameters:
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
char *dbname = "testdb"; /* database name */
char *user = "testuser"; /* connection user name */
char *connection = "tcp:postgresql://localhost:5432/testdb";
/* connection string */
char ver[256]; /* buffer to store the version string */
EXEC SQL END DECLARE SECTION;
ECPGdebug(1, stderr);
EXEC SQL CONNECT TO :dbname USER :user;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL SELECT version() INTO :ver;
EXEC SQL DISCONNECT;
printf("version: %s\n", ver);
EXEC SQL CONNECT TO :connection USER :user;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL SELECT version() INTO :ver;
EXEC SQL DISCONNECT;
printf("version: %s\n", ver);
return 0;
}
6.3.23. Compatibility
CONNECT is specified in the SQL standard, but the format of the connection parameters is implementation-specific.
6.3.24. See Also
ecpg-sql-disconnect ecpg-sql-set-connection
DEALLOCATE DESCRIPTOR deallocate an SQL descriptor area
DEALLOCATE DESCRIPTOR name
6.3.25. Description
DEALLOCATE DESCRIPTOR deallocates a named SQL descriptor area.
6.3.26. Parameters
The name of the descriptor which is going to be deallocated. It is case sensitive. This can be an SQL identifier or a host variable.
6.3.27. Examples
EXEC SQL DEALLOCATE DESCRIPTOR mydesc;
6.3.28. Compatibility
DEALLOCATE DESCRIPTOR is specified in the SQL standard.
6.3.29. See Also
ecpg-sql-allocate-descriptor ecpg-sql-get-descriptor ecpg-sql-set-descriptor
DECLARE define a cursor
DECLARE cursor_name [ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ] CURSOR [ { WITH | WITHOUT } HOLD ] FOR prepared_name
DECLARE cursor_name [ BINARY ] [ ASENSITIVE | INSENSITIVE ] [ [ NO ] SCROLL ] CURSOR [ { WITH | WITHOUT } HOLD ] FOR query
6.3.30. Description
DECLARE declares a cursor for iterating over the result set of a prepared statement. This command has slightly different semantics from the direct SQL command DECLARE: Whereas the latter executes a query and prepares the result set for retrieval, this embedded SQL command merely declares a name as a loop variable for iterating over the result set of a query; the actual execution happens when the cursor is opened with the OPEN command.
6.3.31. Parameters
A cursor name, case sensitive. This can be an SQL identifier or a host variable.
The name of a prepared query, either as an SQL identifier or a host variable.
A sql-select or sql-values command which will provide the rows to be returned by the cursor.
For the meaning of the cursor options, see sql-declare.
6.3.32. Examples
Examples declaring a cursor for a query:
EXEC SQL DECLARE C CURSOR FOR SELECT * FROM My_Table;
EXEC SQL DECLARE C CURSOR FOR SELECT Item1 FROM T;
EXEC SQL DECLARE cur1 CURSOR FOR SELECT version();
An example declaring a cursor for a prepared statement:
EXEC SQL PREPARE stmt1 AS SELECT version();
EXEC SQL DECLARE cur1 CURSOR FOR stmt1;
6.3.33. Compatibility
DECLARE is specified in the SQL standard.
6.3.34. See Also
ecpg-sql-open sql-close sql-declare
DECLARE STATEMENT declare SQL statement identifier
EXEC SQL [ AT connection_name ] DECLARE statement_name STATEMENT
6.3.35. Description
DECLARE STATEMENT declares an SQL statement identifier. SQL statement identifier can be associated with the connection. When the identifier is used by dynamic SQL statements, the statements are executed using the associated connection. The namespace of the declaration is the precompile unit, and multiple declarations to the same SQL statement identifier are not allowed. Note that if the precompiler runs in Informix compatibility mode and some SQL statement is declared, «database» can not be used as a cursor name.
6.3.36. Parameters
A database connection name established by the CONNECT command.
AT clause can be omitted, but such statement has no meaning.
The name of an SQL statement identifier, either as an SQL identifier or a host variable.
6.3.37. Notes
This association is valid only if the declaration is physically placed on top of a dynamic statement.
6.3.38. Examples
EXEC SQL CONNECT TO postgres AS con1;
EXEC SQL AT con1 DECLARE sql_stmt STATEMENT;
EXEC SQL DECLARE cursor_name CURSOR FOR sql_stmt;
EXEC SQL PREPARE sql_stmt FROM :dyn_string;
EXEC SQL OPEN cursor_name;
EXEC SQL FETCH cursor_name INTO :column1;
EXEC SQL CLOSE cursor_name;
6.3.39. Compatibility
DECLARE STATEMENT is an extension of the SQL standard, but can be used in famous DBMSs.
6.3.40. See Also
ecpg-sql-connect ecpg-sql-declare ecpg-sql-open
DESCRIBE obtain information about a prepared statement or result set
DESCRIBE [ OUTPUT ] prepared_name USING [ SQL ] DESCRIPTOR descriptor_name
DESCRIBE [ OUTPUT ] prepared_name INTO [ SQL ] DESCRIPTOR descriptor_name
DESCRIBE [ OUTPUT ] prepared_name INTO sqlda_name
6.3.41. Description
DESCRIBE retrieves metadata information about the result columns contained in a prepared statement, without actually fetching a row.
6.3.42. Parameters
The name of a prepared statement. This can be an SQL identifier or a host variable.
A descriptor name. It is case sensitive. It can be an SQL identifier or a host variable.
The name of an SQLDA variable.
6.3.43. Examples
EXEC SQL ALLOCATE DESCRIPTOR mydesc;
EXEC SQL PREPARE stmt1 FROM :sql_stmt;
EXEC SQL DESCRIBE stmt1 INTO SQL DESCRIPTOR mydesc;
EXEC SQL GET DESCRIPTOR mydesc VALUE 1 :charvar = NAME;
EXEC SQL DEALLOCATE DESCRIPTOR mydesc;
6.3.44. Compatibility
DESCRIBE is specified in the SQL standard.
6.3.45. See Also
ecpg-sql-allocate-descriptor ecpg-sql-get-descriptor
DISCONNECT terminate a database connection
DISCONNECT connection_name
DISCONNECT [ CURRENT ]
DISCONNECT DEFAULT
DISCONNECT ALL
6.3.46. Description
DISCONNECT closes a connection (or all connections) to the database.
6.3.47. Parameters
A database connection name established by the CONNECT command.
Close the current connection, which is either the most recently opened connection, or the connection set by the SET CONNECTION command. This is also the default if no argument is given to the DISCONNECT command.
Close the default connection.
Close all open connections.
6.3.48. Examples
int
main(void)
{
EXEC SQL CONNECT TO testdb AS DEFAULT USER testuser;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL CONNECT TO testdb AS con2 USER testuser;
EXEC SQL CONNECT TO testdb AS con3 USER testuser;
EXEC SQL DISCONNECT CURRENT; /* close con3 */
EXEC SQL DISCONNECT DEFAULT; /* close DEFAULT */
EXEC SQL DISCONNECT ALL; /* close con2 and con1 */
return 0;
}
6.3.49. Compatibility
DISCONNECT is specified in the SQL standard.
6.3.50. See Also
ecpg-sql-connect ecpg-sql-set-connection
EXECUTE IMMEDIATE dynamically prepare and execute a statement
EXECUTE IMMEDIATE string
6.3.51. Description
EXECUTE IMMEDIATE immediately prepares and executes a dynamically specified SQL statement, without retrieving result rows.
6.3.52. Parameters
A literal string or a host variable containing the SQL statement to be executed.
6.3.53. Notes
In typical usage, the string is a host variable reference to a string containing a dynamically-constructed SQL statement. The case of a literal string is not very useful; you might as well just write the SQL statement directly, without the extra typing of EXECUTE IMMEDIATE.
If you do use a literal string, keep in mind that any double quotes you might wish to include in the SQL statement must be written as octal escapes (042) not the usual C idiom ". This is because the string is inside an EXEC SQL section, so the ECPG lexer parses it according to SQL rules not C rules. Any embedded backslashes will later be handled according to C rules; but " causes an immediate syntax error because it is seen as ending the literal.
6.3.54. Examples
Here is an example that executes an INSERT statement using EXECUTE IMMEDIATE and a host variable named command:
sprintf(command, "INSERT INTO test (name, amount, letter) VALUES ('db: ''r1''', 1, 'f')");
EXEC SQL EXECUTE IMMEDIATE :command;
6.3.55. Compatibility
EXECUTE IMMEDIATE is specified in the SQL standard.
GET DESCRIPTOR get information from an SQL descriptor area
GET DESCRIPTOR descriptor_name :cvariable = descriptor_header_item [, ... ]
GET DESCRIPTOR descriptor_name VALUE column_number :cvariable = descriptor_item [, ... ]
6.3.56. Description
GET DESCRIPTOR retrieves information about a query result set from an SQL descriptor area and stores it into host variables. A descriptor area is typically populated using FETCH or SELECT before using this command to transfer the information into host language variables.
This command has two forms: The first form retrieves descriptor header items, which apply to the result set in its entirety. One example is the row count. The second form, which requires the column number as additional parameter, retrieves information about a particular column. Examples are the column name and the actual column value.
6.3.57. Parameters
A descriptor name.
A token identifying which header information item to retrieve. Only COUNT, to get the number of columns in the result set, is currently supported.
The number of the column about which information is to be retrieved. The count starts at 1.
A token identifying which item of information about a column to retrieve. See ecpg-named-descriptors for a list of supported items.
A host variable that will receive the data retrieved from the descriptor area.
6.3.58. Examples
An example to retrieve the number of columns in a result set:
EXEC SQL GET DESCRIPTOR d :d_count = COUNT;
An example to retrieve a data length in the first column:
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_returned_octet_length = RETURNED_OCTET_LENGTH;
An example to retrieve the data body of the second column as a string:
EXEC SQL GET DESCRIPTOR d VALUE 2 :d_data = DATA;
Here is an example for a whole procedure of executing SELECT current_database(); and showing the number of columns, the column data length, and the column data:
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
int d_count;
char d_data[1024];
int d_returned_octet_length;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1 USER testuser;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL ALLOCATE DESCRIPTOR d;
/* Declare, open a cursor, and assign a descriptor to the cursor */
EXEC SQL DECLARE cur CURSOR FOR SELECT current_database();
EXEC SQL OPEN cur;
EXEC SQL FETCH NEXT FROM cur INTO SQL DESCRIPTOR d;
/* Get a number of total columns */
EXEC SQL GET DESCRIPTOR d :d_count = COUNT;
printf("d_count = %d\n", d_count);
/* Get length of a returned column */
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_returned_octet_length = RETURNED_OCTET_LENGTH;
printf("d_returned_octet_length = %d\n", d_returned_octet_length);
/* Fetch the returned column as a string */
EXEC SQL GET DESCRIPTOR d VALUE 1 :d_data = DATA;
printf("d_data = %s\n", d_data);
/* Closing */
EXEC SQL CLOSE cur;
EXEC SQL COMMIT;
EXEC SQL DEALLOCATE DESCRIPTOR d;
EXEC SQL DISCONNECT ALL;
return 0;
}
When the example is executed, the result will look like this:
d_count = 1
d_returned_octet_length = 6
d_data = testdb
6.3.59. Compatibility
GET DESCRIPTOR is specified in the SQL standard.
6.3.60. See Also
ecpg-sql-allocate-descriptor ecpg-sql-set-descriptor
OPEN open a dynamic cursor
OPEN cursor_name
OPEN cursor_name USING value [, ... ]
OPEN cursor_name USING SQL DESCRIPTOR descriptor_name
6.3.61. Description
OPEN opens a cursor and optionally binds actual values to the placeholders in the cursor’s declaration. The cursor must previously have been declared with the DECLARE command. The execution of OPEN causes the query to start executing on the server.
6.3.62. Parameters
The name of the cursor to be opened. This can be an SQL identifier or a host variable.
A value to be bound to a placeholder in the cursor. This can be an SQL constant, a host variable, or a host variable with indicator.
The name of a descriptor containing values to be bound to the placeholders in the cursor. This can be an SQL identifier or a host variable.
6.3.63. Examples
EXEC SQL OPEN a;
EXEC SQL OPEN d USING 1, 'test';
EXEC SQL OPEN c1 USING SQL DESCRIPTOR mydesc;
EXEC SQL OPEN :curname1;
6.3.64. Compatibility
OPEN is specified in the SQL standard.
6.3.65. See Also
ecpg-sql-declare sql-close
PREPARE prepare a statement for execution
PREPARE prepared_name FROM string
6.3.66. Description
PREPARE prepares a statement dynamically specified as a string for execution. This is different from the direct SQL statement sql-prepare, which can also be used in embedded programs. The sql-execute command is used to execute either kind of prepared statement.
6.3.67. Parameters
An identifier for the prepared query.
A literal string or a host variable containing a preparable SQL statement, one of SELECT, INSERT, UPDATE, or DELETE. Use question marks (?) for parameter values to be supplied at execution.
6.3.68. Notes
In typical usage, the string is a host variable reference to a string containing a dynamically-constructed SQL statement. The case of a literal string is not very useful; you might as well just write a direct SQL PREPARE statement.
If you do use a literal string, keep in mind that any double quotes you might wish to include in the SQL statement must be written as octal escapes (042) not the usual C idiom ". This is because the string is inside an EXEC SQL section, so the ECPG lexer parses it according to SQL rules not C rules. Any embedded backslashes will later be handled according to C rules; but " causes an immediate syntax error because it is seen as ending the literal.
6.3.69. Examples
char *stmt = "SELECT * FROM test1 WHERE a = ? AND b = ?";
EXEC SQL ALLOCATE DESCRIPTOR outdesc;
EXEC SQL PREPARE foo FROM :stmt;
EXEC SQL EXECUTE foo USING SQL DESCRIPTOR indesc INTO SQL DESCRIPTOR outdesc;
6.3.70. Compatibility
PREPARE is specified in the SQL standard.
6.3.71. See Also
sql-execute
SET AUTOCOMMIT set the autocommit behavior of the current session
SET AUTOCOMMIT { = | TO } { ON | OFF }
6.3.72. Description
SET AUTOCOMMIT sets the autocommit behavior of the current database session. By default, embedded SQL programs are not in autocommit mode, so COMMIT needs to be issued explicitly when desired. This command can change the session to autocommit mode, where each individual statement is committed implicitly.
6.3.73. Compatibility
SET AUTOCOMMIT is an extension of PostgreSQL ECPG.
SET CONNECTION select a database connection
SET CONNECTION [ TO | = ] connection_name
6.3.74. Description
SET CONNECTION sets the current database connection, which is the one that all commands use unless overridden.
6.3.75. Parameters
A database connection name established by the CONNECT command.
Set the connection to the default connection.
6.3.76. Examples
EXEC SQL SET CONNECTION TO con2;
EXEC SQL SET CONNECTION = con1;
6.3.77. Compatibility
SET CONNECTION is specified in the SQL standard.
6.3.78. See Also
ecpg-sql-connect ecpg-sql-disconnect
SET DESCRIPTOR set information in an SQL descriptor area
SET DESCRIPTOR descriptor_name descriptor_header_item = value [, ... ]
SET DESCRIPTOR descriptor_name VALUE number descriptor_item = value [, ...]
6.3.79. Description
SET DESCRIPTOR populates an SQL descriptor area with values. The descriptor area is then typically used to bind parameters in a prepared query execution.
This command has two forms: The first form applies to the descriptor header, which is independent of a particular datum. The second form assigns values to particular datums, identified by number.
6.3.80. Parameters
A descriptor name.
A token identifying which header information item to set. Only COUNT, to set the number of descriptor items, is currently supported.
The number of the descriptor item to set. The count starts at 1.
A token identifying which item of information to set in the descriptor. See ecpg-named-descriptors for a list of supported items.
A value to store into the descriptor item. This can be an SQL constant or a host variable.
6.3.81. Examples
EXEC SQL SET DESCRIPTOR indesc COUNT = 1;
EXEC SQL SET DESCRIPTOR indesc VALUE 1 DATA = 2;
EXEC SQL SET DESCRIPTOR indesc VALUE 1 DATA = :val1;
EXEC SQL SET DESCRIPTOR indesc VALUE 2 INDICATOR = :val1, DATA = 'some string';
EXEC SQL SET DESCRIPTOR indesc VALUE 2 INDICATOR = :val2null, DATA = :val2;
6.3.82. Compatibility
SET DESCRIPTOR is specified in the SQL standard.
6.3.83. See Also
ecpg-sql-allocate-descriptor ecpg-sql-get-descriptor
TYPE define a new data type
TYPE type_name IS ctype
6.3.84. Description
The TYPE command defines a new C type. It is equivalent to putting a typedef into a declare section.
This command is only recognized when ecpg is run with the -c option.
6.3.85. Parameters
The name for the new type. It must be a valid C type name.
A C type specification.
6.3.86. Examples
EXEC SQL TYPE customer IS
struct
{
varchar name[50];
int phone;
};
EXEC SQL TYPE cust_ind IS
struct ind
{
short name_ind;
short phone_ind;
};
EXEC SQL TYPE c IS char reference;
EXEC SQL TYPE ind IS union { int integer; short smallint; };
EXEC SQL TYPE intarray IS int[AMOUNT];
EXEC SQL TYPE str IS varchar[BUFFERSIZ];
EXEC SQL TYPE string IS char[11];
Here is an example program that uses EXEC SQL TYPE:
EXEC SQL WHENEVER SQLERROR SQLPRINT;
EXEC SQL TYPE tt IS
struct
{
varchar v[256];
int i;
};
EXEC SQL TYPE tt_ind IS
struct ind {
short v_ind;
short i_ind;
};
int
main(void)
{
EXEC SQL BEGIN DECLARE SECTION;
tt t;
tt_ind t_ind;
EXEC SQL END DECLARE SECTION;
EXEC SQL CONNECT TO testdb AS con1;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL SELECT current_database(), 256 INTO :t:t_ind LIMIT 1;
printf("t.v = %s\n", t.v.arr);
printf("t.i = %d\n", t.i);
printf("t_ind.v_ind = %d\n", t_ind.v_ind);
printf("t_ind.i_ind = %d\n", t_ind.i_ind);
EXEC SQL DISCONNECT con1;
return 0;
}
The output from this program looks like this:
t.v = testdb
t.i = 256
t_ind.v_ind = 0
t_ind.i_ind = 0
6.3.87. Compatibility
The TYPE command is a PostgreSQL extension.
VAR define a variable
VAR varname IS ctype
6.3.88. Description
The VAR command assigns a new C data type to a host variable. The host variable must be previously declared in a declare section.
6.3.89. Parameters
A C variable name.
A C type specification.
6.3.90. Examples
Exec sql begin declare section;
short a;
exec sql end declare section;
EXEC SQL VAR a IS int;
6.3.91. Compatibility
The VAR command is a PostgreSQL extension.
WHENEVER specify the action to be taken when an SQL statement causes a specific class condition to be raised
WHENEVER { NOT FOUND | SQLERROR | SQLWARNING } action
6.3.92. Description
Define a behavior which is called on the special cases (Rows not found, SQL warnings or errors) in the result of SQL execution.
6.3.93. Parameters
See ecpg-whenever for a description of the parameters.
6.3.94. Examples
EXEC SQL WHENEVER NOT FOUND CONTINUE;
EXEC SQL WHENEVER NOT FOUND DO BREAK;
EXEC SQL WHENEVER NOT FOUND DO CONTINUE;
EXEC SQL WHENEVER SQLWARNING SQLPRINT;
EXEC SQL WHENEVER SQLWARNING DO warn();
EXEC SQL WHENEVER SQLERROR sqlprint;
EXEC SQL WHENEVER SQLERROR CALL print2();
EXEC SQL WHENEVER SQLERROR DO handle_error("select");
EXEC SQL WHENEVER SQLERROR DO sqlnotice(NULL, NONO);
EXEC SQL WHENEVER SQLERROR DO sqlprint();
EXEC SQL WHENEVER SQLERROR GOTO error_label;
EXEC SQL WHENEVER SQLERROR STOP;
A typical application is the use of WHENEVER NOT FOUND BREAK to handle looping through result sets:
int
main(void)
{
EXEC SQL CONNECT TO testdb AS con1;
EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
EXEC SQL ALLOCATE DESCRIPTOR d;
EXEC SQL DECLARE cur CURSOR FOR SELECT current_database(), 'hoge', 256;
EXEC SQL OPEN cur;
/* when end of result set reached, break out of while loop */
EXEC SQL WHENEVER NOT FOUND DO BREAK;
while (1)
{
EXEC SQL FETCH NEXT FROM cur INTO SQL DESCRIPTOR d;
...
}
EXEC SQL CLOSE cur;
EXEC SQL COMMIT;
EXEC SQL DEALLOCATE DESCRIPTOR d;
EXEC SQL DISCONNECT ALL;
return 0;
}
6.3.95. Compatibility
WHENEVER is specified in the SQL standard, but most of the actions are PostgreSQL extensions.
6.3.96. Informix Compatibility Mode
ecpg can be run in a so-called Informix compatibility mode. If this mode is active, it tries to behave as if it were the Informix precompiler for Informix E/SQL. Generally spoken this will allow you to use the dollar sign instead of the EXEC SQL primitive to introduce embedded SQL commands:
$int j = 3;
$CONNECT TO :dbname;
$CREATE TABLE test(i INT PRIMARY KEY, j INT);
$INSERT INTO test(i, j) VALUES (7, :j);
$COMMIT;
Примечание
There must not be any white space between the $ and a following preprocessor directive, that is, include, define, ifdef, etc. Otherwise, the preprocessor will parse the token as a host variable.
There are two compatibility modes: INFORMIX, INFORMIX_SE
When linking programs that use this compatibility mode, remember to link against libcompat that is shipped with ECPG.
Besides the previously explained syntactic sugar, the Informix compatibility mode ports some functions for input, output and transformation of data as well as embedded SQL statements known from E/SQL to ECPG.
Informix compatibility mode is closely connected to the pgtypeslib library of ECPG. pgtypeslib maps SQL data types to data types within the C host program and most of the additional functions of the Informix compatibility mode allow you to operate on those C host program types. Note however that the extent of the compatibility is limited. It does not try to copy Informix behavior; it allows you to do more or less the same operations and gives you functions that have the same name and the same basic behavior but it is no drop-in replacement if you are using Informix at the moment. Moreover, some of the data types are different. For example, PostgreSQL’s datetime and interval types do not know about ranges like for example YEAR TO MINUTE so you won’t find support in ECPG for that either.
6.3.96.1. Additional Types
The Informix-special «string» pseudo-type for storing right-trimmed character string data is now supported in Informix-mode without using typedef. In fact, in Informix-mode, ECPG refuses to process source files that contain typedef sometype string;
EXEC SQL BEGIN DECLARE SECTION;
string userid; /* this variable will contain trimmed data */
EXEC SQL END DECLARE SECTION;
EXEC SQL FETCH MYCUR INTO :userid;
6.3.96.2. Additional/Missing Embedded SQL Statements
This statement closes the current connection. In fact, this is a synonym for ECPG’s DISCONNECT CURRENT:
$CLOSE DATABASE; /* close the current connection */ EXEC SQL CLOSE DATABASE;
Due to differences in how ECPG works compared to Informix’s ESQL/C (namely, which steps are purely grammar transformations and which steps rely on the underlying run-time library) there is no FREE cursor_name statement in ECPG. This is because in ECPG, DECLARE CURSOR doesn’t translate to a function call into the run-time library that uses to the cursor name. This means that there’s no run-time bookkeeping of SQL cursors in the ECPG run-time library, only in the PostgreSQL server.
FREE statement_name is a synonym for DEALLOCATE PREPARE statement_name.
6.3.96.3. Informix-compatible SQLDA Descriptor Areas
Informix-compatible mode supports a different structure than the one described in ecpg-sqlda-descriptors. See below:
struct sqlvar_compat
{
short sqltype;
int sqllen;
char *sqldata;
short *sqlind;
char *sqlname;
char *sqlformat;
short sqlitype;
short sqlilen;
char *sqlidata;
int sqlxid;
char *sqltypename;
short sqltypelen;
short sqlownerlen;
short sqlsourcetype;
char *sqlownername;
int sqlsourceid;
char *sqlilongdata;
int sqlflags;
void *sqlreserved;
};
struct sqlda_compat
{
short sqld;
struct sqlvar_compat *sqlvar;
char desc_name[19];
short desc_occ;
struct sqlda_compat *desc_next;
void *reserved;
};
typedef struct sqlvar_compat sqlvar_t;
typedef struct sqlda_compat sqlda_t;
The global properties are:
The number of fields in the SQLDA descriptor.
Pointer to the per-field properties.
Unused, filled with zero-bytes.
Size of the allocated structure.
Pointer to the next SQLDA structure if the result set contains more than one record.
Unused pointer, contains NULL. Kept for Informix-compatibility.
The per-field properties are below, they are stored in the sqlvar array:
Type of the field. Constants are in sqltypes.h
Length of the field data.
Pointer to the field data. The pointer is of char * type, the data pointed by it is in a binary format. Example:
int intval; switch (sqldata->sqlvar[i].sqltype) { case SQLINTEGER: intval = *(int *)sqldata->sqlvar[i].sqldata; break; ... }
Pointer to the NULL indicator. If returned by DESCRIBE or FETCH then it’s always a valid pointer. If used as input for EXECUTE … USING sqlda; then NULL-pointer value means that the value for this field is non-NULL. Otherwise a valid pointer and sqlitype has to be properly set. Example:
if (*(int2 *)sqldata->sqlvar[i].sqlind != 0) printf("value is NULL\n");
Name of the field. 0-terminated string.
Reserved in Informix, value of libpq-PQfformat for the field.
Type of the NULL indicator data. It’s always SQLSMINT when returning data from the server. When the SQLDA is used for a parameterized query, the data is treated according to the set type.
Length of the NULL indicator data.
Extended type of the field, result of libpq-PQftype.
Unused.
It equals to sqldata if sqllen is larger than 32kB.
Example:
EXEC SQL INCLUDE sqlda.h;
sqlda_t *sqlda; /* This doesn't need to be under embedded DECLARE SECTION */
EXEC SQL BEGIN DECLARE SECTION;
char *prep_stmt = "select * from table1";
int i;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL PREPARE mystmt FROM :prep_stmt;
EXEC SQL DESCRIBE mystmt INTO sqlda;
printf("# of fields: %d\n", sqlda->sqld);
for (i = 0; i < sqlda->sqld; i++)
printf("field %d: \"%s\"\n", sqlda->sqlvar[i]->sqlname);
EXEC SQL DECLARE mycursor CURSOR FOR mystmt;
EXEC SQL OPEN mycursor;
EXEC SQL WHENEVER NOT FOUND GOTO out;
while (1)
{
EXEC SQL FETCH mycursor USING sqlda;
}
EXEC SQL CLOSE mycursor;
free(sqlda); /* The main structure is all to be free(),
* sqlda and sqlda->sqlvar is in one allocated area */
For more information, see the **sqlda.h** header and the
src/interfaces/ecpg/test/compat_informix/sqlda.pgc regression test.
6.3.96.4. Additional Functions
Add two decimal type values.
int decadd(decimal *arg1, decimal *arg2, decimal *sum); The function receives a pointer to the first operand of type decimal
(arg1), a pointer to the second operand of type decimal (arg2) and a pointer to a value of type decimal that will contain the sum (sum). On success, the function returns 0. ECPG_INFORMIX_NUM_OVERFLOW is returned in case of overflow and ECPG_INFORMIX_NUM_UNDERFLOW in case of underflow. -1 is returned for other failures and errno is set to the respective errno number of the pgtypeslib.
Compare two variables of type decimal.
int deccmp(decimal *arg1, decimal *arg2); The function receives a pointer to the first decimal value
(arg1), a pointer to the second decimal value (arg2) and returns an integer value that indicates which is the bigger value.
1, if the value that arg1 points to is bigger than the value that var2 points to
-1, if the value that arg1 points to is smaller than the value that arg2 points to
0, if the value that arg1 points to and the value that arg2 points to are equal
Copy a decimal value.
void deccopy(decimal *src, decimal *target); The function receives a pointer to the decimal value that should be
copied as the first argument (src) and a pointer to the target structure of type decimal (target) as the second argument.
Convert a value from its ASCII representation into a decimal type.
int deccvasc(char *cp, int len, decimal *np); The function receives a pointer to string that contains the string
representation of the number to be converted (cp) as well as its length len. np is a pointer to the decimal value that saves the result of the operation.
Valid formats are for example: -2, .794, +3.44, 592.49E07 or -32.84e-4.
The function returns 0 on success. If overflow or underflow occurred, ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW is returned. If the ASCII representation could not be parsed, ECPG_INFORMIX_BAD_NUMERIC is returned or ECPG_INFORMIX_BAD_EXPONENT if this problem occurred while parsing the exponent.
Convert a value of type double to a value of type decimal.
int deccvdbl(double dbl, decimal *np); The function receives the variable of type double that should be
converted as its first argument (dbl). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation.
The function returns 0 on success and a negative value if the conversion failed.
Convert a value of type int to a value of type decimal.
int deccvint(int in, decimal *np); The function receives the variable of type int that should be
converted as its first argument (in). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation.
The function returns 0 on success and a negative value if the conversion failed.
Convert a value of type long to a value of type decimal.
int deccvlong(long lng, decimal *np); The function receives the variable of type long that should be
converted as its first argument (lng). As the second argument (np), the function receives a pointer to the decimal variable that should hold the result of the operation.
The function returns 0 on success and a negative value if the conversion failed.
Divide two variables of type decimal.
int decdiv(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first
(n1) and the second (n2) operands and calculates n1/n2. result is a pointer to the variable that should hold the result of the operation.
On success, 0 is returned and a negative value if the division fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively. If an attempt to divide by zero is observed, the function returns ECPG_INFORMIX_DIVIDE_ZERO.
Multiply two decimal values.
int decmul(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first
(n1) and the second (n2) operands and calculates n1*****n2. result is a pointer to the variable that should hold the result of the operation.
On success, 0 is returned and a negative value if the multiplication fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively.
Subtract one decimal value from another.
int decsub(decimal *n1, decimal *n2, decimal *result); The function receives pointers to the variables that are the first
(n1) and the second (n2) operands and calculates n1-n2. result is a pointer to the variable that should hold the result of the operation.
On success, 0 is returned and a negative value if the subtraction fails. If overflow or underflow occurred, the function returns ECPG_INFORMIX_NUM_OVERFLOW or ECPG_INFORMIX_NUM_UNDERFLOW respectively.
Convert a variable of type decimal to its ASCII representation in a C char* string.
int dectoasc(decimal *np, char *cp, int len, int right) The function receives a pointer to a variable of type decimal
(np) that it converts to its textual representation. cp is the buffer that should hold the result of the operation. The parameter right specifies, how many digits right of the decimal point should be included in the output. The result will be rounded to this number of decimal digits. Setting right to -1 indicates that all available decimal digits should be included in the output. If the length of the output buffer, which is indicated by len is not sufficient to hold the textual representation including the trailing zero byte, only a single * character is stored in the result and -1 is returned.
The function returns either -1 if the buffer cp was too small or ECPG_INFORMIX_OUT_OF_MEMORY if memory was exhausted.
Convert a variable of type decimal to a double.
int dectodbl(decimal *np, double *dblp); The function receives a pointer to the decimal value to convert
(np) and a pointer to the double variable that should hold the result of the operation (dblp).
On success, 0 is returned and a negative value if the conversion failed.
Convert a variable to type decimal to an integer.
int dectoint(decimal *np, int *ip); The function receives a pointer to the decimal value to convert
(np) and a pointer to the integer variable that should hold the result of the operation (ip).
On success, 0 is returned and a negative value if the conversion failed. If an overflow occurred, ECPG_INFORMIX_NUM_OVERFLOW is returned.
Note that the ECPG implementation differs from the Informix implementation. Informix limits an integer to the range from -32767 to 32767, while the limits in the ECPG implementation depend on the architecture (INT_MIN .. INT_MAX).
Convert a variable to type decimal to a long integer.
int dectolong(decimal *np, long *lngp); The function receives a pointer to the decimal value to convert
(np) and a pointer to the long variable that should hold the result of the operation (lngp).
On success, 0 is returned and a negative value if the conversion failed. If an overflow occurred, ECPG_INFORMIX_NUM_OVERFLOW is returned.
Note that the ECPG implementation differs from the Informix implementation. Informix limits a long integer to the range from -2,147,483,647 to 2,147,483,647, while the limits in the ECPG implementation depend on the architecture (-LONG_MAX .. LONG_MAX).
Converts a date to a C char* string.
int rdatestr(date d, char *str); The function receives two arguments, the first one is the date to
convert (d) and the second one is a pointer to the target string. The output format is always yyyy-mm-dd, so you need to allocate at least 11 bytes (including the zero-byte terminator) for the string.
The function returns 0 on success and a negative value in case of error.
Note that ECPG’s implementation differs from the Informix implementation. In Informix the format can be influenced by setting environment variables. In ECPG however, you cannot change the output format.
Parse the textual representation of a date.
int rstrdate(char *str, date *d); The function receives the textual representation of the date to convert
(str) and a pointer to a variable of type date (d). This function does not allow you to specify a format mask. It uses the default format mask of Informix which is mm/dd/yyyy. Internally, this function is implemented by means of rdefmtdate. Therefore, rstrdate is not faster and if you have the choice you should opt for rdefmtdate which allows you to specify the format mask explicitly.
The function returns the same values as rdefmtdate.
Get the current date.
void rtoday(date *d); The function receives a pointer to a date variable (**d**)
that it sets to the current date.
Internally this function uses the pgtypesdatetoday function.
Extract the values for the day, the month and the year from a variable of type date.
int rjulmdy(date d, short mdy[3]); The function receives the date **d** and a pointer to an array
of 3 short integer values mdy. The variable name indicates the sequential order: mdy[0] will be set to contain the number of the month, mdy[1] will be set to the value of the day and mdy[2] will contain the year.
The function always returns 0 at the moment.
Internally the function uses the pgtypesdatejulmdy function.
Use a format mask to convert a character string to a value of type date.
int rdefmtdate(date *d, char *fmt, char *str); The function receives a pointer to the date value that should hold the
result of the operation (d), the format mask to use for parsing the date (fmt) and the C char* string containing the textual representation of the date (str). The textual representation is expected to match the format mask. However you do not need to have a 1:1 mapping of the string to the format mask. The function only analyzes the sequential order and looks for the literals yy or yyyy that indicate the position of the year, mm to indicate the position of the month and dd to indicate the position of the day.
The function returns the following values:
0 - The function terminated successfully.
ECPG_INFORMIX_ENOSHORTDATE - The date does not contain delimiters between day, month and year. In this case the input string must be exactly 6 or 8 bytes long but isn’t.
ECPG_INFORMIX_ENOTDMY - The format string did not correctly indicate the sequential order of year, month and day.
ECPG_INFORMIX_BAD_DAY - The input string does not contain a valid day.
ECPG_INFORMIX_BAD_MONTH - The input string does not contain a valid month.
ECPG_INFORMIX_BAD_YEAR - The input string does not contain a valid year.
Internally this function is implemented to use the pgtypesdatedefmtasc function. See the reference there for a table of example input.
Convert a variable of type date to its textual representation using a format mask.
int rfmtdate(date d, char *fmt, char *str); The function receives the date to convert (**d**), the format
mask (fmt) and the string that will hold the textual representation of the date (str).
On success, 0 is returned and a negative value if an error occurred.
Internally this function uses the pgtypesdatefmtasc function, see the reference there for examples.
Create a date value from an array of 3 short integers that specify the day, the month and the year of the date.
int rmdyjul(short mdy[3], date *d); The function receives the array of the 3 short integers
(mdy) and a pointer to a variable of type date that should hold the result of the operation.
Currently the function returns always 0.
Internally the function is implemented to use the function pgtypesdatemdyjul.
Return a number representing the day of the week for a date value.
int rdayofweek(date d); The function receives the date variable **d** as its only
argument and returns an integer that indicates the day of the week for this date.
0 - Sunday
1 - Monday
2 - Tuesday
3 - Wednesday
4 - Thursday
5 - Friday
6 - Saturday
Internally the function is implemented to use the function pgtypesdatedayofweek.
Retrieve the current timestamp.
void dtcurrent(timestamp *ts); The function retrieves the current timestamp and saves it into the
timestamp variable that ts points to.
Parses a timestamp from its textual representation into a timestamp variable.
int dtcvasc(char *str, timestamp *ts); The function receives the string to parse (**str**) and a
pointer to the timestamp variable that should hold the result of the operation (ts).
The function returns 0 on success and a negative value in case of error.
Internally this function uses the pgtypestimestampfromasc function. See the reference there for a table with example inputs.
Parses a timestamp from its textual representation using a format mask into a timestamp variable.
dtcvfmtasc(char *inbuf, char *fmtstr, timestamp *dtvalue) The function receives the string to parse (**inbuf**), the
format mask to use (fmtstr) and a pointer to the timestamp variable that should hold the result of the operation (dtvalue).
This function is implemented by means of the pgtypestimestampdefmtasc function. See the documentation there for a list of format specifiers that can be used.
The function returns 0 on success and a negative value in case of error.
Subtract one timestamp from another and return a variable of type interval.
int dtsub(timestamp *ts1, timestamp *ts2, interval *iv); The function will subtract the timestamp variable that **ts2**
points to from the timestamp variable that ts1 points to and will store the result in the interval variable that iv points to.
Upon success, the function returns 0 and a negative value if an error occurred.
Convert a timestamp variable to a C char* string.
int dttoasc(timestamp *ts, char *output); The function receives a pointer to the timestamp variable to convert
(ts) and the string that should hold the result of the operation (output). It converts ts to its textual representation according to the SQL standard, which is be YYYY-MM-DD HH:MM:SS.
Upon success, the function returns 0 and a negative value if an error occurred.
Convert a timestamp variable to a C char* using a format mask.
int dttofmtasc(timestamp *ts, char *output, int str_len, char *fmtstr); The function receives a pointer to the timestamp to convert as its
first argument (ts), a pointer to the output buffer (output), the maximal length that has been allocated for the output buffer (str_len) and the format mask to use for the conversion (fmtstr).
Upon success, the function returns 0 and a negative value if an error occurred.
Internally, this function uses the pgtypestimestampfmtasc function. See the reference there for information on what format mask specifiers can be used.
Convert an interval variable to a C char* string.
int intoasc(interval *i, char *str); The function receives a pointer to the interval variable to convert
(i) and the string that should hold the result of the operation (str). It converts i to its textual representation according to the SQL standard, which is be YYYY-MM-DD HH:MM:SS.
Upon success, the function returns 0 and a negative value if an error occurred.
Convert a long integer value to its textual representation using a format mask.
int rfmtlong(long lng_val, char *fmt, char *outbuf); The function receives the long value **lng_val**, the format
mask fmt and a pointer to the output buffer outbuf. It converts the long value according to the format mask to its textual representation.
The format mask can be composed of the following format specifying characters:
* (asterisk) - if this position would be blank otherwise, fill it with an asterisk.
& (ampersand) - if this position would be blank otherwise, fill it with a zero.
# - turn leading zeroes into blanks.
< - left-justify the number in the string.
, (comma) - group numbers of four or more digits into groups of three digits separated by a comma.
. (period) - this character separates the whole-number part of the number from the fractional part.
- (minus) - the minus sign appears if the number is a negative value.
+ (plus) - the plus sign appears if the number is a positive value.
( - this replaces the minus sign in front of the negative number. The minus sign will not appear.
) - this character replaces the minus and is printed behind the negative value.
$ - the currency symbol.
Convert a string to upper case.
void rupshift(char *str); The function receives a pointer to the string and transforms every
lower case character to upper case.
Return the number of characters in a string without counting trailing blanks.
int byleng(char *str, int len); The function expects a fixed-length string as its first argument
(str) and its length as its second argument (len). It returns the number of significant characters, that is the length of the string without trailing blanks.
Copy a fixed-length string into a null-terminated string.
void ldchar(char *src, int len, char *dest); The function receives the fixed-length string to copy
(src), its length (len) and a pointer to the destination memory (dest). Note that you need to reserve at least len+1 bytes for the string that dest points to. The function copies at most len bytes to the new location (less if the source string has trailing blanks) and adds the null-terminator.
int rgetmsg(int msgnum, char *s, int maxsize); This function exists but is not implemented at the moment!int rtypalign(int offset, int type); This function exists but is not implemented at the moment!int rtypmsize(int type, int len); This function exists but is not implemented at the moment!int rtypwidth(int sqltype, int sqllen); This function exists but is not implemented at the moment!Set a variable to NULL.
int rsetnull(int t, char *ptr); The function receives an integer that indicates the type of the
variable and a pointer to the variable itself that is cast to a C char* pointer.
The following types exist:
CCHARTYPE - For a variable of type char or char*
CSHORTTYPE - For a variable of type short int
CINTTYPE - For a variable of type int
CBOOLTYPE - For a variable of type boolean
CFLOATTYPE - For a variable of type float
CLONGTYPE - For a variable of type long
CDOUBLETYPE - For a variable of type double
CDECIMALTYPE - For a variable of type decimal
CDATETYPE - For a variable of type date
CDTIMETYPE - For a variable of type timestamp
Here is an example of a call to this function:
$char c[] = "abc "; $short s = 17; $int i = -74874; rsetnull(CCHARTYPE, (char *) c); rsetnull(CSHORTTYPE, (char *) &s); rsetnull(CINTTYPE, (char *) &i);
Test if a variable is NULL.
int risnull(int t, char *ptr); The function receives the type of the variable to test (**t**)
as well a pointer to this variable (ptr). Note that the latter needs to be cast to a char*. See the function rsetnull for a list of possible variable types.
Here is an example of how to use this function:
$char c[] = "abc "; $short s = 17; $int i = -74874; risnull(CCHARTYPE, (char *) c); risnull(CSHORTTYPE, (char *) &s); risnull(CINTTYPE, (char *) &i);
6.3.96.5. Additional Constants
Note that all constants here describe errors and all of them are defined to represent negative values. In the descriptions of the different constants you can also find the value that the constants represent in the current implementation. However you should not rely on this number. You can however rely on the fact all of them are defined to represent negative values.
Functions return this value if an overflow occurred in a calculation. Internally it is defined as -1200 (the Informix definition).
Functions return this value if an underflow occurred in a calculation. Internally it is defined as -1201 (the Informix definition).
Functions return this value if an attempt to divide by zero is observed. Internally it is defined as -1202 (the Informix definition).
Functions return this value if a bad value for a year was found while parsing a date. Internally it is defined as -1204 (the Informix definition).
Functions return this value if a bad value for a month was found while parsing a date. Internally it is defined as -1205 (the Informix definition).
Functions return this value if a bad value for a day was found while parsing a date. Internally it is defined as -1206 (the Informix definition).
Functions return this value if a parsing routine needs a short date representation but did not get the date string in the right length. Internally it is defined as -1209 (the Informix definition).
Functions return this value if an error occurred during date formatting. Internally it is defined as -1210 (the Informix definition).
Functions return this value if memory was exhausted during their operation. Internally it is defined as -1211 (the Informix definition).
Functions return this value if a parsing routine was supposed to get a format mask (like mmddyy) but not all fields were listed correctly. Internally it is defined as -1212 (the Informix definition).
Functions return this value either if a parsing routine cannot parse the textual representation for a numeric value because it contains errors or if a routine cannot complete a calculation involving numeric variables because at least one of the numeric variables is invalid. Internally it is defined as -1213 (the Informix definition).
Functions return this value if a parsing routine cannot parse an exponent. Internally it is defined as -1216 (the Informix definition).
Functions return this value if a parsing routine cannot parse a date. Internally it is defined as -1218 (the Informix definition).
Functions return this value if a parsing routine is passed extra characters it cannot parse. Internally it is defined as -1264 (the Informix definition).
6.3.97. Oracle Compatibility Mode
ecpg can be run in a so-called Oracle compatibility mode. If this mode is active, it tries to behave as if it were Oracle Pro*C.
Specifically, this mode changes ecpg in three ways:
Pad character arrays receiving character string types with trailing spaces to the specified length
Zero byte terminate these character arrays, and set the indicator variable if truncation occurs
Set the null indicator to -1 when character arrays receive empty character string types
6.3.98. Internals
This section explains how ECPG works internally. This information can occasionally be useful to help users understand how to use ECPG.
The first four lines written by ecpg to the output are fixed lines. Two are comments and two are include lines necessary to interface to the library. Then the preprocessor reads through the file and writes output. Normally it just echoes everything to the output.
When it sees an EXEC SQL statement, it intervenes and changes it. The command starts with EXEC SQL and ends with ;. Everything in between is treated as an SQL statement and parsed for variable substitution.
Variable substitution occurs when a symbol starts with a colon (:). The variable with that name is looked up among the variables that were previously declared within a EXEC SQL DECLARE section.
The most important function in the library is ECPGdo, which takes care of executing most commands. It takes a variable number of arguments. This can easily add up to 50 or so arguments, and we hope this will not be a problem on any platform.
The arguments are:
This is the line number of the original line; used in error messages only.
This is the SQL command that is to be issued. It is modified by the input variables, i.e., the variables that where not known at compile time but are to be entered in the command. Where the variables should go the string contains ?.
Every input variable causes ten arguments to be created. (See below.)
An enum telling that there are no more input variables.
Every output variable causes ten arguments to be created. (See below.) These variables are filled by the function.
An enum telling that there are no more variables.
For every variable that is part of the SQL command, the function gets ten arguments:
The type as a special symbol.
A pointer to the value or a pointer to the pointer.
The size of the variable if it is a char or varchar.
The number of elements in the array (for array fetches).
The offset to the next element in the array (for array fetches).
The type of the indicator variable as a special symbol.
A pointer to the indicator variable.
0
The number of elements in the indicator array (for array fetches).
The offset to the next element in the indicator array (for array fetches).
Note that not all SQL commands are treated in this way. For instance, an open cursor statement like:
EXEC SQL OPEN cursor;
is not copied to the output. Instead, the cursor's
DECLARE command is used at the position of the OPEN command because it indeed opens the cursor.
Here is a complete example describing the output of the preprocessor of a file foo.pgc (details might change with each particular version of the preprocessor):
EXEC SQL BEGIN DECLARE SECTION;
int index;
int result;
EXEC SQL END DECLARE SECTION;
...
EXEC SQL SELECT res INTO :result FROM mytable WHERE index = :index;
is translated into:
/* Processed by ecpg (2.6.0) */
/* These two include files are added by the preprocessor */
#include <ecpgtype.h>;
#include <ecpglib.h>;
/* exec sql begin declare section */
#line 1 "foo.pgc"
int index;
int result;
/* exec sql end declare section */
...
ECPGdo(__LINE__, NULL, "SELECT res FROM mytable WHERE index = ? ",
ECPGt_int,&(index),1L,1L,sizeof(int),
ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EOIT,
ECPGt_int,&(result),1L,1L,sizeof(int),
ECPGt_NO_INDICATOR, NULL , 0L, 0L, 0L, ECPGt_EORT);
#line 147 "foo.pgc"
(The indentation here is added for readability and not
something the preprocessor does.)