What is the difference between a lib and an module with autoconf

Question

I have to check that certain libraries (libm, libdl) are present in order to compile my library.

In my configure.ac template file, there is PKG_CHECK_MODULES macros and PKG_CHECK_LIB macros.

I don't understand which one to use and how?

The PKG_CHECK_MODULES seems the most global one because it checks if a whole library is present and PKG_CHECK_LIB checks only if one function is accessible… But when I do PKG_CHECK_MODULES([LIBM],[libm],[],[exit -1]), it exists and I don't understand why.

I think I'm misunderstanding some concepts. Maybe someone could lead me to good references.

Answer 1

PKG_CHECK_MODULES is for integrating with packages that have pkg-config metadata. This metadata is typically stored in a file called foo.pc (for package foo) in someplace like /usr/share/pkgconfig. This file will say where foo and its associated files (header files, libraries, executables, data, etc.) have actually been installed.

However, most packages don't use the pkg-config system, including the standard C library which is where libm and libdl are. So you'll need to test for them using AC_CHECK_LIB.

Answer 2

You seem to be confused, so I'll go on a bit of a tangent here:

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Once upon a time, there was X11; there were many incompatible installations of X11. To write code that would compile against each variant, people would write crazy autoconf macros to try to figure out automatically what libraries to list before, what libraries to list after, and what extra flags were needed in between. (see AC_PATH_X, AC_PATH_XTRA).

Some people tried more sensible approaches, and wrote shell scripts to install along the libraries; so you would just call them and they would give you all the magic flags needed for that specific libraries. (see sdl-config, wx-config, freetype-config, motif-config, etc)

Then the folks from freedesktop.org decided it was a chore for everyone to maintain those scripts that did essentially the same thing, so they wrote a tool (pkg-config) that would work like all those *-config scripts, and would not require a shell to run (yay for Windows users). All the library authors need to do is to write the metadata in the *.pc files, and install them along the libraries.

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Regarding autoconf, it has low-level ways to poke around the system to find out about the libraries: AC_CHECK_HEADERS, to see if the headers are present and usable, and AC_CHECK_LIB, to see if it's possible to link against them.

The pkg-config tool comes with convenience macros for autoconf, mainly PKG_CHECK_MODULES, which instead of poking around, it simply looks for the metadata the library might have installed.

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Regarding libm, libdl, as ldav1s said, they are part of the system; some systems need explicit link against libm (which provides math functions) and/or libdl (which provides functions for dynamically loading shared objects). Often other tools, like gcc or libtool, take care of linking against them. Unfortunately they don't come with meta-data for pkg-config, so if you have to find them manually, you'll have to poke around with the old AC_CHECK_HEADERS and AC_CHECK_LIB macros to find them.