(automake-1.16.info)amhello's configure.ac Setup Explained

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2.4.2 ‘amhello’’s ‘configure.ac’ Setup Explained
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Let us begin with the contents of ‘configure.ac’.

     AC_INIT([amhello], [1.0], [bug-automake@gnu.org])
     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.in’s).  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 (Note: Writing
‘configure.ac’.).

   The macros prefixed with ‘AC_’ are Autoconf macros, documented in the
Autoconf manual (Note: Autoconf Macro Index.
).  The macros that start with ‘AM_’ are Automake macros,
documented later in this manual (Note: 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’ (Note: Options).  ‘-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 "bug-automake@gnu.org"

     /* 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
really big, 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 (*note Using ‘autoscan’ to Create ‘configure.ac’:
(autoconf)autoscan Invocation.).