configure.texi

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@samp{make}.  Doing this is more reliable than trying to rebuild the
files manually, because there are complex order dependencies and it is
easy to forget something.

@node Getting Started Example
@section Example

Let's consider a trivial example.

Suppose we want to write a simple version of @samp{touch}.  Our program,
which we will call @samp{poke}, will take a single file name argument,
and use the @samp{utime} system call to set the modification and access
times of the file to the current time.  We want this program to be
highly portable.

We'll first see what this looks like without using autoconf and
automake, and then see what it looks like with them.

@menu
* Getting Started Example 1::		First Try.
* Getting Started Example 2::		Second Try.
* Getting Started Example 3::		Third Try.
* Generate Files in Example::		Generate Files.
@end menu

@node Getting Started Example 1
@subsection First Try

Here is our first try at @samp{poke.c}.  Note that we've written it
without ANSI/ISO C prototypes, since we want it to be highly portable.

@example
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <utime.h>

int
main (argc, argv)
     int argc;
     char **argv;
@{
  if (argc != 2)
    @{
      fprintf (stderr, "Usage: poke file\n");
      exit (1);
    @}

  if (utime (argv[1], NULL) < 0)
    @{
      perror ("utime");
      exit (1);
    @}

  exit (0);
@}
@end example

We also write a simple @file{Makefile}.

@example
CC = gcc
CFLAGS = -g -O2

all: poke

poke: poke.o
	$(CC) -o poke $(CFLAGS) $(LDFLAGS) poke.o
@end example

So far, so good.

Unfortunately, there are a few problems.

On older Unix systems derived from BSD 4.3, the @samp{utime} system call
does not accept a second argument of @samp{NULL}.  On those systems, we
need to pass a pointer to @samp{struct utimbuf} structure.
Unfortunately, even older systems don't define that structure; on those
systems, we need to pass an array of two @samp{long} values.

The header file @file{stdlib.h} was invented by ANSI C, and older
systems don't have a copy.  We included it above to get a declaration of
@samp{exit}.

We can find some of these portability problems by running
@samp{autoscan}, which will create a @file{configure.scan} file which we
can use as a prototype for our @file{configure.in} file.  I won't show
the output, but it will notice the potential problems with @samp{utime}
and @file{stdlib.h}.

In our @file{Makefile}, we don't provide any way to install the program.
This doesn't matter much for such a simple example, but a real program
will need an @samp{install} target.  For that matter, we will also want
a @samp{clean} target.

@node Getting Started Example 2
@subsection Second Try

Here is our second try at this program.

We modify @file{poke.c} to use preprocessor macros to control what
features are available.  (I've cheated a bit by using the same macro
names which autoconf will use).

@example
#include <stdio.h>

#ifdef STDC_HEADERS
#include <stdlib.h>
#endif

#include <sys/types.h>

#ifdef HAVE_UTIME_H
#include <utime.h>
#endif

#ifndef HAVE_UTIME_NULL

#include <time.h>

#ifndef HAVE_STRUCT_UTIMBUF

struct utimbuf
@{
  long actime;
  long modtime;
@};

#endif

static int
utime_now (file)
     char *file;
@{
  struct utimbuf now;

  now.actime = now.modtime = time (NULL);
  return utime (file, &now);
@}

#define utime(f, p) utime_now (f)

#endif /* HAVE_UTIME_NULL  */

int
main (argc, argv)
     int argc;
     char **argv;
@{
  if (argc != 2)
    @{
      fprintf (stderr, "Usage: poke file\n");
      exit (1);
    @}

  if (utime (argv[1], NULL) < 0)
    @{
      perror ("utime");
      exit (1);
    @}

  exit (0);
@}
@end example

Here is the associated @file{Makefile}.  We've added support for the
preprocessor flags we use.  We've also added @samp{install} and
@samp{clean} targets.

@example
# Set this to your installation directory.
bindir = /usr/local/bin

# Uncomment this if you have the standard ANSI/ISO C header files.
# STDC_HDRS = -DSTDC_HEADERS

# Uncomment this if you have utime.h.
# UTIME_H = -DHAVE_UTIME_H

# Uncomment this if utime (FILE, NULL) works on your system.
# UTIME_NULL = -DHAVE_UTIME_NULL

# Uncomment this if struct utimbuf is defined in utime.h.
# UTIMBUF = -DHAVE_STRUCT_UTIMBUF

CC = gcc
CFLAGS = -g -O2

ALL_CFLAGS = $(STDC_HDRS) $(UTIME_H) $(UTIME_NULL) $(UTIMBUF) $(CFLAGS)

all: poke

poke: poke.o
	$(CC) -o poke $(ALL_CFLAGS) $(LDFLAGS) poke.o

.c.o:
	$(CC) -c $(ALL_CFLAGS) poke.c

install: poke
	cp poke $(bindir)/poke

clean:
	rm poke poke.o
@end example

Some problems with this approach should be clear.

Users who want to compile poke will have to know how @samp{utime} works
on their systems, so that they can uncomment the @file{Makefile}
correctly.

The installation is done using @samp{cp}, but many systems have an
@samp{install} program which may be used, and which supports optional
features such as stripping debugging information out of the installed
binary.

The use of @file{Makefile} variables like @samp{CC}, @samp{CFLAGS} and
@samp{LDFLAGS} follows the requirements of the GNU standards.  This is
convenient for all packages, since it reduces surprises for users.
However, it is easy to get the details wrong, and wind up with a
slightly nonstandard distribution.

@node Getting Started Example 3
@subsection Third Try

For our third try at this program, we will write a @file{configure.in}
script to discover the configuration features on the host system, rather
than requiring the user to edit the @file{Makefile}.  We will also write
a @file{Makefile.am} rather than a @file{Makefile}.

The only change to @file{poke.c} is to add a line at the start of the
file:
@smallexample
#include "config.h"
@end smallexample

The new @file{configure.in} file is as follows.

@example
AC_INIT(poke.c)
AM_INIT_AUTOMAKE(poke, 1.0)
AM_CONFIG_HEADER(config.h:config.in)
AC_PROG_CC
AC_HEADER_STDC
AC_CHECK_HEADERS(utime.h)
AC_EGREP_HEADER(utimbuf, utime.h, AC_DEFINE(HAVE_STRUCT_UTIMBUF))
AC_FUNC_UTIME_NULL
AC_OUTPUT(Makefile)
@end example

The first four macros in this file, and the last one, were described
above; see @ref{Write configure.in}.  If we omit these macros, then when
we run @samp{automake} we will get a reminder that we need them.

The other macros are standard autoconf macros.

@table @samp
@item AC_HEADER_STDC
Check for standard C headers.
@item AC_CHECK_HEADERS
Check whether a particular header file exists.
@item AC_EGREP_HEADER
Check for a particular string in a particular header file, in this case
checking for @samp{utimbuf} in @file{utime.h}.
@item AC_FUNC_UTIME_NULL
Check whether @samp{utime} accepts a NULL second argument to set the
file change time to the current time.
@end table

See the autoconf manual for a more complete description.

The new @file{Makefile.am} file is as follows.  Note how simple this is
compared to our earlier @file{Makefile}.

@example
bin_PROGRAMS = poke

poke_SOURCES = poke.c
@end example

This means that we should build a single program name @samp{poke}.  It
should be installed in the binary directory, which we called
@samp{bindir} earlier.  The program @samp{poke} is built from the source
file @file{poke.c}.

We must also write a @file{acconfig.h} file.  Besides @samp{PACKAGE} and
@samp{VERSION}, which must be mentioned for all packages which use
automake, we must include @samp{HAVE_STRUCT_UTIMBUF}, since we mentioned
it in an @samp{AC_DEFINE}.

@example
/* Name of package.  */
#undef PACKAGE

/* Version of package.  */
#undef VERSION

/* Whether utime.h defines struct utimbuf.  */
#undef HAVE_STRUCT_UTIMBUF
@end example

@node Generate Files in Example
@subsection Generate Files

We must now generate the other files, using the following commands.

@smallexample
aclocal
autoconf
autoheader
automake
@end smallexample

When we run @samp{autoheader}, it will remind us of any macros we forgot
to add to @file{acconfig.h}.

When we run @samp{automake}, it will want to add some files to our
distribution.  It will add them automatically if we use the
@samp{--add-missing} option.

By default, @samp{automake} will run in GNU mode, which means that it
will want us to create certain additional files; as of this writing, it
will want @file{NEWS}, @file{README}, @file{AUTHORS}, and
@file{ChangeLog}, all of which are files which should appear in a
standard GNU distribution.  We can either add those files, or run
@samp{automake} with the @samp{--foreign} option.

Running these tools will generate the following files, all of which are
described in the next chapter.

@itemize @bullet
@item
@file{aclocal.m4}
@item
@file{configure}
@item
@file{config.in}
@item
@file{Makefile.in}
@item
@file{stamp-h.in}
@end itemize

@node Files
@chapter Files

As was seen in the previous chapter, the GNU configure and build system
uses a number of different files.  The developer must write a few files.