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Chapter 46. Linking Dynamically-Loaded Functions

After you have created and registered a user-defined function, your work is essentially done. Postgres, however, must load the object code (e.g., a .o file, or a shared library) that implements your function. As previously mentioned, Postgres loads your code at runtime, as required. In order to allow your code to be dynamically loaded, you may have to compile and link-edit it in a special way. This section briefly describes how to perform the compilation and link-editing required before you can load your user-defined functions into a running Postgres server. Note that this process has changed as of Version 4.2.

Tip: The old Postgres dynamic loading mechanism required in-depth knowledge in terms of executable format, placement and alignment of executable instructions within memory, etc. on the part of the person writing the dynamic loader. Such loaders tended to be slow and buggy. As of Version 4.2, the Postgres dynamic loading mechanism has been rewritten to use the dynamic loading mechanism provided by the operating system. This approach is generally faster, more reliable and more portable than our previous dynamic loading mechanism. The reason for this is that nearly all modern versions of UNIX use a dynamic loading mechanism to implement shared libraries and must therefore provide a fast and reliable mechanism. On the other hand, the object file must be postprocessed a bit before it can be loaded into Postgres. We hope that the large increase in speed and reliability will make up for the slight decrease in convenience.

You should expect to read (and reread, and re-reread) the manual pages for the C compiler, cc(1), and the link editor, ld(1), if you have specific questions. In addition, the regression test suites in the directory PGROOT/src/regress contain several working examples of this process. If you copy what these tests do, you should not have any problems. The following terminology will be used below:
  • Dynamic loading is what Postgres does to an object file. The object file is copied into the running Postgres server and the functions and variables within the file are made available to the functions within the Postgres process. Postgres does this using the dynamic loading mechanism provided by the operating system.

  • Loading and link editing is what you do to an object file in order to produce another kind of object file (e.g., an executable program or a shared library). You perform this using the link editing program, ld(1).

The following general restrictions and notes also apply to the discussion below:

  • Paths given to the create function command must be absolute paths (i.e., start with "/") that refer to directories visible on the machine on which the Postgres server is running.

    Tip: Relative paths do in fact work, but are relative to the directory where the database resides (which is generally invisible to the frontend application). Obviously, it makes no sense to make the path relative to the directory in which the user started the frontend application, since the server could be running on a completely different machine!

  • The Postgres user must be able to traverse the path given to the create function command and be able to read the object file. This is because the Postgres server runs as the Postgres user, not as the user who starts up the frontend process. (Making the file or a higher-level directory unreadable and/or unexecutable by the "postgres" user is an extremely common mistake.)

  • Symbol names defined within object files must not conflict with each other or with symbols defined in Postgres.

  • The GNU C compiler usually does not provide the special options that are required to use the operating system's dynamic loader interface. In such cases, the C compiler that comes with the operating system must be used.

ULTRIX

It is very easy to build dynamically-loaded object files under ULTRIX. ULTRIX does not have any shared library mechanism and hence does not place any restrictions on the dynamic loader interface. On the other hand, we had to (re)write a non-portable dynamic loader ourselves and could not use true shared libraries. Under ULTRIX, the only restriction is that you must produce each object file with the option -G 0. (Notice that that's the numeral ``0'' and not the letter ``O''). For example,

# simple ULTRIX example
% cc -G 0 -c foo.c
produces an object file called foo.o that can then be dynamically loaded into Postgres. No additional loading or link-editing must be performed.
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