How to get a configure script to look for a library

Question

I'm trying to write a configure.ac file such that the resulting configure script searches for a library directory containing a given static library e.g. libsomething.a. How can I do this? At the moment I have it check just one location with:

AC_CHECK_FILE([/usr/local/lib/libsomething.a],[AC_SUBST(libsomething,"-L/usr/local/lib -lsomething")],[AC_SUBST(libcfitsio,'')])

But I want it to try and find it automatically. And if the library isn't in one of the default locations, I'd like configure to say that the library wasn't found and that a custom location can be specified with --use-something=path as is usually done. So I also need to then check if --use-something=path is provided. I'm pretty new at creating configure files, and the M4 documentation isn't very easy to follow, so would appreciate any help.

Thanks!

Answer 1

The best way to figure this out is to look at other autoconf macros that do something similar. Autoconf macros are an amalgam of Bourne shell script and M4 code, so they can literally solve any computable problem.

Here's a link to a macro I wrote for MySQL++ that does this: mysql++.m4.

Answer 2

It's not the job of configure to search where libraries are installed. it should only make sure they are available to the linker. If the user installed them in a different location, he knows how to call ./configure CPPFLAGS=-I/the/location/include LDFLAGS=-L/the/location/lib so that the tools will find the library (this is explained in the --help output of configure and in the standard INSTALL file).

Also --with-package and --enable-package macros are not supposed to be used to specify paths, contrary to what many third-party macros will do. The GNU Coding Standards explicitly prohibit this usage:

Do not use a --with option to specify the file name to use to find certain files. That is outside the scope of what --with options are for.

CPPFLAGS and LDFLAGS are already here to address the problem, so why redevelop and maintain another mechanism?