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The PostgreSQL 9.0 Reference Manual - Volume 2 - Programming Guide
by The PostgreSQL Global Development Group
Paperback (6"x9"), 478 pages
ISBN 9781906966065
RRP £14.95 ($19.95)

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5.9.1 Dynamic Loading

The first time a user-defined function in a particular loadable object file is called in a session, the dynamic loader loads that object file into memory so that the function can be called. The CREATE FUNCTION for a user-defined C function must therefore specify two pieces of information for the function: the name of the loadable object file, and the C name (link symbol) of the specific function to call within that object file. If the C name is not explicitly specified then it is assumed to be the same as the SQL function name.

The following algorithm is used to locate the shared object file based on the name given in the CREATE FUNCTION command:

  1. If the name is an absolute path, the given file is loaded.
  2. If the name starts with the string $libdir, that part is replaced by the PostgreSQL package library directory name, which is determined at build time.
  3. If the name does not contain a directory part, the file is searched for in the path specified by the configuration variable dynamic_library_path.
  4. Otherwise (the file was not found in the path, or it contains a non-absolute directory part), the dynamic loader will try to take the name as given, which will most likely fail. (It is unreliable to depend on the current working directory.)

If this sequence does not work, the platform-specific shared library file name extension (often ‘.so’) is appended to the given name and this sequence is tried again. If that fails as well, the load will fail.

It is recommended to locate shared libraries either relative to $libdir or through the dynamic library path. This simplifies version upgrades if the new installation is at a different location. The actual directory that $libdir stands for can be found out with the command pg_config --pkglibdir.

The user ID the PostgreSQL server runs as must be able to traverse the path to the file you intend to load. Making the file or a higher-level directory not readable and/or not executable by the postgres user is a common mistake.

In any case, the file name that is given in the CREATE FUNCTION command is recorded literally in the system catalogs, so if the file needs to be loaded again the same procedure is applied.

Note: PostgreSQL will not compile a C function automatically. The object file must be compiled before it is referenced in a CREATE FUNCTION command. See section 5.9.6 Compiling and Linking Dynamically-Loaded Functions for additional information.

To ensure that a dynamically loaded object file is not loaded into an incompatible server, PostgreSQL checks that the file contains a “magic block” with the appropriate contents. This allows the server to detect obvious incompatibilities, such as code compiled for a different major version of PostgreSQL. A magic block is required as of PostgreSQL 8.2. To include a magic block, write this in one (and only one) of the module source files, after having included the header ‘fmgr.h’:

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

The #ifdef test can be omitted if the code doesn't need to compile against pre-8.2 PostgreSQL releases.

After it is used for the first time, a dynamically loaded object file is retained in memory. Future calls in the same session to the function(s) in that file will only incur the small overhead of a symbol table lookup. If you need to force a reload of an object file, for example after recompiling it, begin a fresh session.

Optionally, a dynamically loaded file can contain initialization and finalization functions. If the file includes a function named _PG_init, that function will be called immediately after loading the file. The function receives no parameters and should return void. If the file includes a function named _PG_fini, that function will be called immediately before unloading the file. Likewise, the function receives no parameters and should return void. Note that _PG_fini will only be called during an unload of the file, not during process termination. (Presently, unloads are disabled and will never occur, but this may change in the future.)

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