20.5 How Do I #define Installation Directories?

My program needs library files, installed in datadir and
similar.  If I use

AC_DEFINE_UNQUOTED([DATADIR], [$datadir],
  [Define to the read-only architecture-independent
   data directory.])

I get

#define DATADIR "${prefix}/share"

As already explained, this behavior is on purpose, mandated by the GNU Coding Standards, see Installation Directory Variables. There are several means to achieve a similar goal:

• Do not use AC_DEFINE but use your makefile to pass the actual value of datadir via compilation flags. See Installation Directory Variables, for the details.
• This solution can be simplified when compiling a program: you may either extend the CPPFLAGS:

  CPPFLAGS = -DDATADIR='"$(datadir)"' @CPPFLAGS@
  

  If you are using Automake, you should use AM_CPPFLAGS instead:

  AM_CPPFLAGS = -DDATADIR='"$(datadir)"'
  

  Alternatively, create a dedicated header file:

  DISTCLEANFILES = myprog-paths.h
  myprog-paths.h: Makefile
          echo '#define DATADIR "$(datadir)"' >$@
  

  The Gnulib module ‘configmake’ provides such a header with all the standard directory variables defined, see configmake in GNU Gnulib.

• Use AC_DEFINE but have configure compute the literal value of datadir and others. Many people have wrapped macros to automate this task; for an example, see the macro AC_DEFINE_DIR from the Autoconf Macro Archive.

  This solution does not conform to the GNU Coding Standards.

• Note that all the previous solutions hard wire the absolute name of these directories in the executables, which is not a good property. You may try to compute the names relative to prefix, and try to find prefix at runtime, this way your package is relocatable.