7.1.2.5.8.4.1. Compiling and Linking Dynamically-Loaded Functions

Before you are able to use your PostgreSQL extension functions written in C, they must be compiled and linked in a special way to produce a file that can be dynamically loaded by the server. To be precise, a shared library needs to be created.

For information beyond what is contained in this section you should read the documentation of your operating system, in particular the manual pages for the C compiler, cc, and the link editor, ld. In addition, the PostgreSQL source code contains several working examples in the contrib directory. If you rely on these examples you will make your modules dependent on the availability of the PostgreSQL source code, however.

Creating shared libraries is generally analogous to linking executables: first the source files are compiled into object files, then the object files are linked together. The object files need to be created as position-independent code (PIC),which conceptually means that they can be placed at an arbitrary location in memory when they are loaded by the executable. (Object files intended for executables are usually not compiled that way.) The command to link a shared library contains special flags to distinguish it from linking an executable (at least in theory

In the following examples we assume that your source code is in a file foo.c and we will create a shared library foo.so. The intermediate object file will be called foo.o unless otherwise noted. A shared library can contain more than one object file, but we only use one here.

The compiler flag to create PIC is -fPIC. To create shared libraries the compiler flag is -shared.

gcc -fPIC -c foo.c
gcc -shared -o foo.so foo.o
    This is applicable as of version 3.0 of

FreeBSD.

The compiler flag of the system compiler to create PIC is +z. When using GCC it’s -fPIC. The linker flag for shared libraries is -b. So:
```
cc +z -c foo.c
    or:
```
```
gcc -fPIC -c foo.c
    and then:
```
```
ld -b -o foo.sl foo.o
    HP-UX uses the extension
```
.sl for shared libraries, unlike most other systems.
The compiler flag to create PIC is -fPIC. The compiler flag to create a shared library is -shared. A complete example looks like this:
```
cc -fPIC -c foo.c
cc -shared -o foo.so foo.o
```

Here is an example. It assumes the developer tools are installed.

cc -c foo.c
cc -bundle -flat_namespace -undefined suppress -o foo.so foo.o

The compiler flag to create PIC is -fPIC. For ELF systems, the compiler with the flag -shared is used to link shared libraries. On the older non-ELF systems, ld -Bshareable is used.
```
gcc -fPIC -c foo.c
gcc -shared -o foo.so foo.o
```
The compiler flag to create PIC is -fPIC. ld -Bshareable is used to link shared libraries.
```
gcc -fPIC -c foo.c
ld -Bshareable -o foo.so foo.o
```
The compiler flag to create PIC is -KPIC with the Sun compiler and -fPIC with GCC. To link shared libraries, the compiler option is -G with either compiler or alternatively -shared with GCC.
```
cc -KPIC -c foo.c
cc -G -o foo.so foo.o
    or
```
```
gcc -fPIC -c foo.c
gcc -G -o foo.so foo.o
```

Совет

If this is too complicated for you, you should consider using **GNU Libtool**, which hides the platform differences behind a uniform interface.

The resulting shared library file can then be loaded into PostgreSQL. When specifying the file name to the CREATE FUNCTION command, one must give it the name of the shared library file, not the intermediate object file. Note that the system’s standard shared-library extension (usually .so or .sl) can be omitted from the CREATE FUNCTION command, and normally should be omitted for best portability.

Refer back to xfunc-c-dynload about where the server expects to find the shared library files.