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|
<?xml version="1.0" encoding="UTF-8"?>
<guide self="general-concepts/autotools/">
<chapter>
<title>The basics of Autotools</title>
<body>
<todo>
This is too long for <uri link="::general-concepts/"/>. It needs to be split up
and moved somewhere, either to a top-level of its own or into
<uri link="::appendices/"/>.
</todo>
<p>
An understanding of GNU autotools (<c>automake</c>, <c>autoconf</c> etc.) can be useful
when working with ebuilds:
</p>
<ul>
<li>
Finding and correcting build issues is often easier if the build system is
not seen simply as a scary black box.
</li>
<li>
The autotools input files can help when determining a package's build-time
dependencies.
</li>
<li>
The risk of accidentally breaking something by patching the wrong file at the
wrong time is greatly reduced if the relationship between the build system
files is understood.
</li>
</ul>
</body>
<section>
<title>Major Autotools components</title>
<body>
<p>
Autotools is a collection of related packages which, when used together, remove
much of the difficulty involved in creating portable software. These tools,
together with a few relatively simple upstream-supplied input files, are used to
create the build system for a package.
</p>
<figure short="How autotools fits together" link="diagram.png"
caption="A basic overview of how the main autotools components fit together."
/>
<p>
In a simple setup:
</p>
<ul>
<li>
The <c>autoconf</c> program produces a <c>configure</c> script from either
<c>configure.in</c> or <c>configure.ac</c> (see note below).
</li>
<li>
The <c>automake</c> program produces a <c>Makefile.in</c> from a <c>Makefile.am</c>.
</li>
<li>
The <c>configure</c> script is run to produce one or more <c>Makefile</c> files from
<c>Makefile.in</c> files.
</li>
<li>
The <c>make</c> program uses the <c>Makefile</c> to compile the program.
</li>
</ul>
<note>
The <c>configure.in</c> name used to be standard. However, the GNU
documentation now recommends <c>configure.ac</c> as it is more obvious which
program should be used when processing it. The files perform the same purpose
and have the same format <d/> the only difference is the name.
</note>
<p>
You may see autotools being used in a variety of phase functions.
The <uri link="::ebuild-writing/functions/src_prepare/"/> function is the most
appropriate place to manipulate the source code prior to configuration
and compilation. In particular, <c>src_prepare</c> is called before
<uri link="::ebuild-writing/functions/src_configure/"/>, which usually expects
the <c>configure</c> script to exist.
</p>
<p>
The <c>autoreconf</c> tool supposedly runs <c>autoconf</c> (and <c>automake</c>,
<c>autoheader</c>, <c>aclocal</c>, <c>autopoint</c> and <c>libtoolize</c>) as necessary.
Sometimes it works. Some packages ship a shell script named <c>autogen.sh</c> which
does the same thing (this is <e>not</e> related to <c>autogen</c>).
The autotools.eclass contains helper functions for the stand-alone tools with their
corresponding names e.g. <c>eautoconf</c> and <c>eautomake</c>.
</p>
<warning>
You must <b>not</b> attempt to modify any of the generated files in
between running <c>configure</c> and <c>make</c>. This can lead to autotools
trying to be clever and recreate the files, in turn leading to your changes
being removed. In some situations this will also result in <c>./configure</c>
arguments being silently dropped, which can give broken dependencies.
The best way to proceed is usually to work with the <c>.ac</c> / <c>.in</c> files
instead.
</warning>
</body>
</section>
<section>
<title>Simple Autotools patching example</title>
<body>
<p>
The following snippet illustrates the correct way to proceed after patching
either <c>Makefile.am</c> or <c>configure.ac</c>:
</p>
<codesample lang="ebuild">
EAPI=8
WANT_AUTOCONF=2.5
WANT_AUTOMAKE=1.9
inherit autotools
IUSE="nls"
BDEPEND="nls? ( sys-devel/gettext )"
src_prepare() {
default
# Remove problematic LDFLAGS declaration
sed -i -e '/^LDFLAGS/d' src/Makefile.am || die
# Rerun autotools
eautoreconf
}
src_configure() {
econf $(use_enable nls)
}
</codesample>
</body>
</section>
<section>
<title>The <c>configure.ac</c> file</title>
<body>
<p>
The <c>configure.ac</c> file is used to create the <c>./configure</c> script. It
consists of a series of macros which are processed and expanded by <c>autoconf</c>.
These macros can check for packages and libraries, handle <c>--enable</c> and
<c>--with</c> switches, and generate various files.
</p>
</body>
<subsection>
<title>Basic format of <c>configure.ac</c></title>
<body>
<p>
The <c>configure.ac</c> file is a basic text file. Indenting and whitespace are
largely irrelevant. Comments are indicated by the string <c>dnl</c> (<c>dnl</c> is
actually a macro which discards the remainder of the input <d/> it stands for
"discard new line").
</p>
<p>
If the previous sentence made you uneasy, you should probably stop reading this
page. It gets far worse.
</p>
<p>
A typical file might start with something like the following:
</p>
<codesample lang="m4">
dnl Process this file with autoconf
AC_INIT(appname, 0.1)
AC_PREREQ(2.5)
AC_CONFIG_SRCDIR(src/main.c)
AC_CONFIG_AUX_DIR(config)
AM_INIT_AUTOMAKE(1.8)
</codesample>
<p>
The <c>AC_PREREQ</c> line, if present, tells you which <c>autoconf</c> version you
need. This is useful, because <c>autoconf</c> is not compatible between versions.
In the above example, we need <c>=autoconf-2.5*</c>. and we should
<c>export WANT_AUTOCONF="2.5"</c> (or use the <c>autoconf-2.59</c> script) when calling
<c>autoconf</c>.
</p>
<p>
The <c>AM_INIT_AUTOMAKE</c> line tells us which <c>automake</c> version we need.
Again, there is little chance that an <c>automake-1.7</c> script will work properly
with <c>automake-1.8</c>. Setting <c>WANT_AUTOMAKE="1.8"</c> in the environment can be
used to make an unversioned <c>automake</c> call run the correct version.
</p>
<note>
The <c>WANT_</c> variables are a Gentoo feature that originally came from
Mandrake. Other distributions may handle things differently.
</note>
<p>
Usually, some standard checks will follow:
</p>
<codesample lang="m4">
dnl Check for toolchain and install components
AC_PROG_CC
AC_PROG_INSTALL
AC_PROG_LN_S
AC_PROG_RANLIB
</codesample>
<p>
For non-standard applications, you may also see manual checks:
</p>
<codesample lang="m4">
dnl Check for sed
AC_CHECK_PROGS(regex_cmd, sed)
if test x$regex_cmd = "x" ; then
AC_MSG_ERROR([sed is required to build the data files.])
fi
</codesample>
<p>
You may also see checks for compiler features:
</p>
<codesample lang="m4">
dnl Check that our compiler can do const and inline
AC_C_CONST
AC_C_INLINE
</codesample>
<p>
Library and header checks:
</p>
<codesample lang="m4">
dnl Check for standard headers:
AC_HEADER_STDC
AC_HEADER_DIRENT
AC_CHECK_HEADERS([stdlib.h stdio.h libintl.h locale.h])
dnl Check for libraries:
AC_CHECK_LIB(ssl, SSL_free)
</codesample>
<p>
And function checks:
</p>
<codesample lang="m4">
dnl Check for functions:
AC_CHECK_FUNCS([setlocale strtol])
</codesample>
<p>
Often these will be all jumbled together without any useful comments. In some
cases, many of the checks done won't even be necessary for the application in
question <d/> most autotools code is copy/pasted rather than written from scratch,
and <c>autoscan</c> (a tool which helps write <c>configure.ac</c>) is sometimes
over-eager to add in checks.
</p>
<p>
The file will finish with some output functions:
</p>
<codesample lang="m4">
AM_CONFIG_HEADER(config.h)
AC_OUTPUT(Makefile src/Makefile nls/Makefile man/Makefile tests/Makefile)
</codesample>
<p>
These are used to make the <c>./configure</c> script generate the relevant files.
</p>
</body>
</subsection>
<subsection>
<title>Enable and disable checks</title>
<body>
<p>
So far we've only seen 'hard' dependencies. Many packages have <e>optional</e>
support for various extras (graphics toolkits, libraries which add
functionality, interpreters, features, ...). This is (if we're lucky) handled
via <c>--enable-foo</c> and <c>--disable-foo</c> switches to <c>./configure</c>. which
are generated from <c>autoconf</c> rules.
</p>
<p>
A simple <c>--enable</c> / <c>--disable</c> function might look something like the
following:
</p>
<codesample lang="m4">
AC_MSG_CHECKING(--enable-cscope argument)
AC_ARG_ENABLE(cscope,
[ --enable-cscope Include cscope interface.],
[enable_cscope=$enableval],
[enable_cscope="no"])
AC_MSG_RESULT($enable_cscope)
if test "$enable_cscope" = "yes"; then
AC_DEFINE(FEAT_CSCOPE)
fi
</codesample>
<p>
Sometimes more complicated checks are included based upon whether or not an
option is enabled. There are also some predefined macros which include
<c>AC_ARG_ENABLE</c>. so grepping <c>configure.ac</c> for <c>AC_ARG_ENABLE</c> may not
give a complete list. A better way is to use <c>./configure --help</c> and check
the output.
</p>
<important>
The third argument is used when a <c>--enable</c> or <c>--disable</c>
switch to <c>./configure</c> is provided, and the fourth is used when such a
switch is <e>not</e> passed. A common misconception is that the third is enable
and the fourth is disable <d/> this is <b>not</b> the case. You may encounter
packages that get this wrong.
</important>
<p>
A simple way to check that a package is using this macro properly is to
install the optional dependency, and then try all of <c>./configure</c>.
<c>./configure --enable-foo</c> and <c>./configure --disable-foo</c>. If the second
and third runs give the same results, something is wrong. If the first run gives
a <e>different</e> result to the second and third, there is a good chance that a
misunderstanding of the <c>AC_ARG_ENABLE</c> arguments is to blame.
</p>
</body>
</subsection>
<subsection>
<title>With and without checks</title>
<body>
<p>
A simple <c>--with</c> / <c>--without</c> check might look like:
</p>
<codesample lang="m4">
AC_MSG_CHECKING([whether to enable foo])
AC_ARG_WITH(foo,
[ --with-foo enable foo support],
with_foo=$withval,
with_foo=yes)
AC_MSG_RESULT($with_foo)
</codesample>
<p>
Again, the third argument is for 'specified' and the fourth for 'not specified',
and there are standard macros which include <c>with</c> options.
</p>
</body>
</subsection>
<subsection>
<title>Automatic checks</title>
<body>
<p>
It's possible to write autoconf rules which bypass the manual enable / disable
convention (or which just ignore what the user asks for). If your package does
this, it must be fixed to avoid dependency problems.
</p>
<p>
The most common form is packages which simply use <c>AC_CHECK_LIB</c> to decide
whether or not to enable a feature. If you find a package which does this, you
<b>must</b> change the behaviour.
</p>
</body>
</subsection>
<subsection>
<title>Quoting rules for <c>configure.ac</c></title>
<body>
<p>
Behind the scenes, <c>autoconf</c> makes heavy use of the <c>m4</c> macro processor to
do the work. The <c>m4</c> quote characters are set by <c>autoconf</c> to be <c>[</c> and
<c>]</c> for opening and closing quotes respectively. Using <c>"</c> or <c>'</c> may
produce unexpected results.
</p>
<p>
To include a literal left square bracket, the easiest thing to do is to use the
special string <c>@<:@</c>. For a right bracket, use <c>@:>@</c>.
</p>
<p>
For example:
</p>
<codesample lang="m4">
AC_MSG_RESULT(the first)
AC_MSG_RESULT([the second])
AC_MSG_RESULT("the third")
AC_MSG_RESULT(@<:@the fourth@:>@)
AC_MSG_RESULT([@<:@the fifth@:>@])
</codesample>
<p>
gives:
</p>
<pre>
the first
the second
"the third"
[the fourth]
[the fifth]
</pre>
<p>
When in doubt, it is generally safest to quote macro arguments using <c>[ ]</c>
rather than leaving things unquoted.
</p>
</body>
</subsection>
</section>
<section>
<title>The <c>Makefile.am</c> files</title>
<body>
<p>
The <c>Makefile.am</c> file is processed by <c>automake</c> to create <c>Makefile.in</c>,
which is in turn processed by <c>configure</c> to create <c>Makefile</c>, which is in
turn used by <c>make</c> to build the software.
</p>
<p>
The basic format is like that of a <c>Makefile</c>. However, you will see various
'special' variables being set, rather than every rule being written manually.
</p>
<p>
A very simple example:
</p>
<codesample lang="make">
bin_PROGRAMS = myapp
myapp_SOURCES = myapp_commandline.c myapp.c
</codesample>
<p>
All the standard GNU rules will be generated, so <c>make</c>. <c>make clean</c>.
<c>make distclean</c>. <c>make dist</c> and so on will all work here.
</p>
<p>
You may also see some standard <c>Makefile</c> constructs showing up whenever there
isn't a standard <c>automake</c> way of handling a certain task. For example:
</p>
<codesample lang="make">
CLEANFILES = glep31check.1
man_MANS = glep31check.1
EXTRA_DIST = glep31check.in
%.1 : %.in
@regex_cmd@ -e "s,\@VERSION\@,$(VERSION),g" $? > $@
</codesample>
<p>
Here, the <c>@regex_cmd@</c> variable will be substituted with whatever
<c>configure</c> detects (<c>sed</c> in this case) when creating the <c>Makefile</c>.
This is handled via the macro <c>AC_SUBST(VARNAME)</c> in <c>configure.ac</c>.
</p>
</body>
<subsection>
<title>Makefile variables</title>
<body>
<p>
Sometimes, badly behaved <c>Makefile.am</c> files will override user variables such
as <c>CFLAGS</c>. This must not be allowed <d/> see
<uri link="::general-concepts/user-environment/#Not filtering variables"/>.
There are separate special variables which should be used in these situations
<d/> for setting <c>CFLAGS</c>. for example, a <c>Makefile.am</c> should use
<c>AM_CFLAGS</c> so that user preferences are not ignored.
</p>
<p>
So, if a <c>Makefile.am</c> contains, say:
</p>
<codesample lang="ebuild">
CFLAGS="-Wall"
</codesample>
<p>
You should <c>sed</c> or <c>patch</c> it to use:
</p>
<codesample lang="ebuild">
AM_CFLAGS="-Wall"
</codesample>
<p>
Remember to manually run <c>autoconf</c> then <c>automake</c> if you do this.
</p>
</body>
</subsection>
</section>
<section>
<title>The <c>config.h.in</c> file</title>
<body>
<p>
The <c>config.h.in</c> file is generated by <c>autoheader</c>. You shouldn't have to
worry about this usually, but occasionally you may need to run <c>autoheader</c>
manually as part of the build process if upstream do not ship a pre-generated
version.
</p>
</body>
</section>
<section>
<title><c>aclocal</c> and <c>m4</c> files</title>
<body>
<p>
In the <c>configure.ac</c> or <c>configure.in</c> files you can call not only the
default macros defined by <c>autoconf</c> and <c>automake</c>. but also other
functions which can be defined by specific packages like libraries and programs
to check for their features in the correct manner.
</p>
<p>
Those functions are (usually) defined in <c>m4</c> files placed in
the <c>/usr/share/aclocal</c> directory by packages. Problems can arise when you
need to regenerate the <c>autotools</c> files, because the functions used by the
<c>configure.ac</c> file can be defined in an <c>m4</c> macro file which isn't
installed on the user's system. This is the case for example for some optional
features which require libraries and are disabled by <c>USE</c> flags. If the
<c>m4</c> files aren't installed in the user's system, the <c>autoconf</c> step will
fail.
</p>
<p>
To resolve this, most packages just ship the <c>m4</c> macro files needed for their
<c>configure.ac</c> in an <c>m4</c> subdirectory in the source package. Unfortunately,
not all <c>m4</c> directories are complete, even if they are present.
</p>
<p>
In those cases you need to find out the <c>m4</c> file, usually installed by the
dependency in <c>/usr/share/aclocal</c>. and make sure that the <c>src_unpack</c>
stage both makes available those files to the autotools and calls <c>aclocal</c> to
recreate the <c>aclocal.m4</c> file which is used by <c>autoconf</c> when creating the
configure script.
</p>
<p>
Usually it's more than one <c>m4</c> file which is missing, so you probably want to
package them in a tarball, and add it to <c>SRC_URI</c>. After making sure that the
tarball is extracted somewhere in <c>${WORKDIR}</c> (say, in a <c>gentoo-m4</c>
directory), you usually have two general ways to handle those macro files, one
for packages which ships with an incomplete <c>m4</c> directory and one for
packages which ships without the <c>m4</c> directory.
</p>
<p>
In the first case you usually want to do something like:
</p>
<codesample lang="ebuild">
WANT_AUTOCONF="2.5"
inherit autotools
src_prepare() {
default
einfo "Regenerating autotools files..."
cp "${WORKDIR}/gentoo-m4" "${S}/m4" || die "m4 copy failed"
eaclocal -I "${S}/m4"
eautoconf
}
</codesample>
<p>
and so on. In the second case you can simplify it in this way:
</p>
<codesample lang="ebuild">
WANT_AUTOCONF="2.5"
inherit autotools
src_prepare() {
default
einfo "Regenerating autotools files..."
eaclocal -I "${WORKDIR}/gentoo-m4"
eautoconf
}
</codesample>
<p>
without need to copy the files.
</p>
<p>
It's always better inform upstream when the needed <c>m4</c> files are missing in
the distribution, asking them to add all the needed files in the source tarball
to avoid version conflicts if the macro changes.
</p>
</body>
</section>
<section>
<title>Libtool</title>
<body>
<todo>
Stuff about libtool, libtoolize, elibtoolize, libtool.eclass. Which I
know almost nothing about... Yay!
</todo>
</body>
</section>
<section>
<title>Further autotools reading</title>
<body>
<p>
For more details on autotools:
</p>
<ul>
<li>
The book "GNU Autoconf, Automake and Libtool" by Gary V. Vaughan, Ben
Elliston, Tom Tromey and Ian Lance Taylor (often called "The Autobook")
provides a good but somewhat outdated description of autotools. It is
<uri link="https://sourceware.org/autobook/">freely available online</uri>.
</li>
<li>
The GNU documentation for the various autotools components is of varying
quality and completeness:
<ul>
<li>
<uri link="https://www.gnu.org/software/automake/manual/automake.html">
GNU automake Manual
</uri>
</li>
<li>
<uri link="https://www.gnu.org/software/autoconf/manual/">GNU autoconf Manual</uri>
</li>
<li>
<uri link="https://www.gnu.org/software/libtool/manual/">GNU libtool Manual</uri>
</li>
<li>
<uri link="https://www.gnu.org/software/m4/manual/m4.html">GNU m4 Manual</uri>
</li>
<li>
<uri link="https://autotools.io">Autotools Mythbuster</uri>
</li>
</ul>
</li>
<li>
There are some good overview lectures available online.
<uri link="https://www.shlomifish.org/lecture/Autotools/">These slides</uri>
are one example.
</li>
</ul>
</body>
</section>
</chapter>
</guide>
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