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5.9.6 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--on some systems the practice is much uglier).

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.

BSD/OS

The compiler flag to create PIC is -fpic. The linker flag to create shared libraries is -shared.

gcc -fpic -c foo.c
ld -shared -o foo.so foo.o

This is applicable as of version 4.0 of BSD/OS.

FreeBSD

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.

HP-UX

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.

IRIX

PIC is the default, no special compiler options are necessary. The linker option to produce shared libraries is -shared.

cc -c foo.c
ld -shared -o foo.so foo.o

Linux

The compiler flag to create PIC is -fpic. On some platforms in some situations -fPIC must be used if -fpic does not work. Refer to the GCC manual for more information. 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

MacOS X

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

NetBSD

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

OpenBSD

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

Solaris

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

Tru64 UNIX

PIC is the default, so the compilation command is the usual one. ld with special options is used to do the linking.

cc -c foo.c
ld -shared -expect_unresolved '*' -o foo.so foo.o

The same procedure is used with GCC instead of the system compiler; no special options are required.

UnixWare

The compiler flag to create PIC is -K PIC with the SCO compiler and -fpic with GCC. To link shared libraries, the compiler option is -G with the SCO compiler and -shared with GCC.

cc -K PIC -c foo.c
cc -G -o foo.so foo.o

or

gcc -fpic -c foo.c
gcc -shared -o foo.so foo.o

Tip: 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 section 5.9.1 Dynamic Loading about where the server expects to find the shared library files.