Feature tests offered by Autoconf do not cover all needs. People often have to supplement existing tests with their own macros, or with third-party macros.
There are two ways to organize custom macros in a package.
The first possibility (the historical practice) is to list all your macros in acinclude.m4. This file will be included in aclocal.m4 when you run aclocal, and its macro(s) will henceforth be visible to autoconf. However if it contains numerous macros, it will rapidly become difficult to maintain, and it will be almost impossible to share macros between packages.
The second possibility, which we do recommend, is to write each macro
in its own file and gather all these files in a directory. This
directory is usually called m4/. Then it's enough to update
configure.ac by adding a proper call to
aclocal will then take care of automatically adding m4/ to its search path for m4 files.
When ‘aclocal’ is run, it will build an aclocal.m4
m4_includes any file from m4/ that defines a
required macro. Macros not found locally will still be searched in
system-wide directories, as explained in Macro Search Path.
Custom macros should be distributed for the same reason that
configure.ac is: so that other people have all the sources of
your package if they want to work on it. Actually, this distribution
happens automatically because all
m4_included files are
However there is no consensus on the distribution of third-party
macros that your package may use. Many libraries install their own
macro in the system-wide aclocal directory (see Extending aclocal). For instance, Guile ships with a file called
guile.m4 that contains the macro
GUILE_FLAGS that can
be used to define setup compiler and linker flags appropriate for
using Guile. Using
GUILE_FLAGS in configure.ac will
cause aclocal to copy guile.m4 into
aclocal.m4, but as guile.m4 is not part of the project,
it will not be distributed. Technically, that means a user who
needs to rebuild aclocal.m4 will have to install Guile first.
This is probably OK, if Guile already is a requirement to build the
package. However, if Guile is only an optional feature, or if your
package might run on architectures where Guile cannot be installed,
this requirement will hinder development. An easy solution is to copy
such third-party macros in your local m4/ directory so they get
Since Automake 1.10, aclocal offers the option
to copy these system-wide third-party macros in your local macro directory,
helping to solve the above problem.
With this setup, system-wide macros will be copied to m4/ the first time you run aclocal. Then the locally installed macros will have precedence over the system-wide installed macros each time aclocal is run again.
One reason why you should keep --install in the flags even after the first run is that when you later edit configure.ac and depend on a new macro, this macro will be installed in your m4/ automatically. Another one is that serial numbers (see Serials) can be used to update the macros in your source tree automatically when new system-wide versions are installed. A serial number should be a single line of the form
where nnn contains only digits and dots. It should appear in the M4 file before any macro definition. It is a good practice to maintain a serial number for each macro you distribute, even if you do not use the --install option of aclocal: this allows other people to use it.