amhello’s configure.ac Setup Explained ¶Let us begin with the contents of configure.ac.
AC_INIT([amhello], [1.0], [[email protected]]) AM_INIT_AUTOMAKE([-Wall -Werror foreign]) AC_PROG_CC AC_CONFIG_HEADERS([config.h]) AC_CONFIG_FILES([ Makefile src/Makefile ]) AC_OUTPUT
This file is read by both autoconf (to create
configure) and automake (to create the various
Makefile.ins). It contains a series of M4 macros that will be
expanded as shell code to finally form the configure script.
We will not elaborate on the syntax of this file, because the Autoconf
manual has a whole section about it (see Writing configure.ac in The Autoconf Manual).
The macros prefixed with AC_ are Autoconf macros, documented
in the Autoconf manual (see Autoconf Macro
Index in The Autoconf Manual). The macros that start with
AM_ are Automake macros, documented later in this manual
(see Macro Index).
The first two lines of configure.ac initialize Autoconf and
Automake. AC_INIT takes in as parameters the name of the package,
its version number, and a contact address for bug-reports about the
package (this address is output at the end of ./configure
--help, for instance). When adapting this setup to your own package,
by all means please do not blindly copy Automake’s address: use the
mailing list of your package, or your own mail address.
The argument to AM_INIT_AUTOMAKE is a list of options for
automake (see Changing Automake’s Behavior). -Wall and
-Werror ask automake to turn on all warnings and
report them as errors. We are speaking of Automake warnings
here, such as dubious instructions in Makefile.am. This has
absolutely nothing to do with how the compiler will be called, even
though it may support options with similar names. Using -Wall
-Werror is a safe setting when starting to work on a package: you do
not want to miss any issues. Later you may decide to relax things a
bit. The foreign option tells Automake that this package
will not follow the GNU Standards. GNU packages should always
distribute additional files such as ChangeLog, AUTHORS,
etc. We do not want automake to complain about these
missing files in our small example.
The AC_PROG_CC line causes the configure script to
search for a C compiler and define the variable CC with its
name. The src/Makefile.in file generated by Automake uses the
variable CC to build hello, so when configure
creates src/Makefile from src/Makefile.in, it will define
CC with the value it has found. If Automake is asked to create
a Makefile.in that uses CC but configure.ac does
not define it, it will suggest you add a call to AC_PROG_CC.
The AC_CONFIG_HEADERS([config.h]) invocation causes the
configure script to create a config.h file gathering
‘#define’s defined by other macros in configure.ac. In our
case, the AC_INIT macro already defined a few of them. Here
is an excerpt of config.h after configure has run:
... /* Define to the address where bug reports for this package should be sent. */ #define PACKAGE_BUGREPORT "[email protected]" /* Define to the full name and version of this package. */ #define PACKAGE_STRING "amhello 1.0" ...
As you probably noticed, src/main.c includes config.h so
it can use PACKAGE_STRING. In a real-world project,
config.h can grow quite large, with one ‘#define’ per
feature probed on the system.
The AC_CONFIG_FILES macro declares the list of files that
configure should create from their *.in templates.
Automake also scans this list to find the Makefile.am files it must
process. (This is important to remember: when adding a new directory
to your project, you should add its Makefile to this list,
otherwise Automake will never process the new Makefile.am you
wrote in that directory.)
Finally, the AC_OUTPUT line is a closing command that actually
produces the part of the script in charge of creating the files
registered with AC_CONFIG_HEADERS and AC_CONFIG_FILES.
When starting a new project, we suggest you start with such a simple
configure.ac, and gradually add the other tests it requires.
The command autoscan can also suggest a few of the tests
your package may need (see Using
autoscan to Create configure.ac in The
Autoconf Manual).