GCC使用configure.ac（通过autoconf）来生成configure文件。顶层目录下的configure.ac有三千多行，用来在配置阶段做各种测试检测，并将结果替换到Makefile.in中，生成最终的Makefile。本文简单介绍了configure.ac中所要做的事请，并没有覆盖所有的细节问题。

1、导出原始的configure参数

progname=$0

if PWD already has a value, it is probably wrong.

if test -n “$PWD” ; then PWD=`${PWDCMD-pwd}`; fi

Export original configure arguments for use by sub-configures.

Quote arguments with shell meta charatcers.

TOPLEVEL_CONFIGURE_ARGUMENTS= set – “$progname” “$@” for ac_arg do case “$ac_arg” in *” “*|*” “*|*[[\[\]\~\#\$\^\&\*\{\}\\\|\;\<\>\?\’]]*) ac_arg=`echo “$ac_arg” | sed “s/’/’\\\\\\\\’‘/g”` # if the argument is of the form -foo=baz, quote the baz part only ac_arg=`echo “‘$ac_arg’” | sed “s/^’$[[-a-zA-Z0-9]]*=$/\\1’/”` ;; *) ;; esac # Add the quoted argument to the list. TOPLEVEL_CONFIGURE_ARGUMENTS=”$TOPLEVEL_CONFIGURE_ARGUMENTS $ac_arg” done if test “$silent” = yes; then TOPLEVEL_CONFIGURE_ARGUMENTS=”$TOPLEVEL_CONFIGURE_ARGUMENTS –silent” fi

Remove the initial space we just introduced and, as these will be

expanded by make, quote ‘$’.

TOPLEVEL_CONFIGURE_ARGUMENTS=`echo “x$TOPLEVEL_CONFIGURE_ARGUMENTS” | sed -e ‘s/^x *//’ -e ‘s,\\$,$$,g’` AC_SUBST(TOPLEVEL_CONFIGURE_ARGUMENTS)

这部分代码位于38-64行，用来将configure的参数保存到变量TOPLEVEL_CONFIGURE_ARGUMENTS中，以供子目录的configure使用。

2、一些基本的测试

# Find the build, host, and target systems. ACX_NONCANONICAL_BUILD ACX_NONCANONICAL_HOST ACX_NONCANONICAL_TARGET … … AC_CANONICAL_SYSTEM AC_ARG_PROGRAM

m4_pattern_allow([^AS_FOR_TARGET$])dnl m4_pattern_allow([^AS_FOR_BUILD$])dnl

Get ‘install’ or ‘install-sh’ and its variants.

AC_PROG_INSTALL ACX_PROG_LN AC_PROG_LN_S AC_PROG_SED AC_PROG_AWK …

从66到108行，测试build，host，target系统（这些命令在acx.m4中定义），测试install，ln，sed，awk。

3、确定哪些库和工具将被配置

# these library is used by various programs built for the build

environment

# build_libs=”build-libiberty”

these tools are built for the build environment

build_tools=”build-texinfo build-flex build-bison build-m4 build-fixincludes”

these libraries are used by various programs built for the host environment

# host_libs=”intl libiberty opcodes bfd readline tcl tk itcl libgui zlib libcpp libdecnumber gmp mpfr mpc isl cloog libelf libiconv”

these tools are built for the host environment

Note, the powerpc-eabi build depends on sim occurring before gdb in order to

know that we are building the simulator.

binutils, gas and ld appear in that order because it makes sense to run

“make check” in that particular order.

If –enable-gold is used, “gold” may replace “ld”.

host_tools=”texinfo flex bison binutils gas ld fixincludes gcc cgen sid sim gdb gprof etc expect dejagnu m4 utils guile fastjar gnattools”

libgcj represents the runtime libraries only used by gcj.

libgcj=”target-libffi
target-zlib
target-libjava”

these libraries are built for the target environment, and are built after

the host libraries and the host tools (which may be a cross compiler)

Note that libiberty is not a target library.

target_libraries=”target-libgcc
target-libgloss
target-newlib
target-libgomp
target-libatomic
target-libitm
target-libstdc++-v3
target-libmudflap
target-libssp
target-libquadmath
target-libgfortran
target-boehm-gc
${libgcj}
target-libobjc
target-libada
target-libgo”

these tools are built using the target libraries, and are intended to

run only in the target environment

note: any program that uses libraries that are in the “target_libraries”

list belongs in this list.

# target_tools=”target-rda”

这部分代码位于112-2036行，这部分代码主要是为了确定哪些库和工具将被配置。首先，定义变量build_libs, build_tools, host_libs, host_tools, target_libraries（这里没有使用target_libs，是因为target_libs变量在config-lang.in中已经被定义使用了，参见http://gcc.gnu.org/ml/gcc-patches/2003-06/msg02977.html）, target_tools，用来列出所有可能需要的库和工具。

## All tools belong in one of the four categories, and are assigned above

We assign ${configdirs} this way to remove all embedded newlines. This

is important because configure will choke if they ever get through.

${configdirs} is directories we build using the host tools.

${target_configdirs} is directories we build using the target tools.

configdirs=`echo ${host_libs} ${host_tools}` target_configdirs=`echo ${target_libraries} ${target_tools}` build_configdirs=`echo ${build_libs} ${build_tools}`

然后，定义变量configdirs, target_configdirs, build_configdirs，用来列出这些库和工具所对应的将要进行配置的源码目录。

# Skipdirs are removed silently. skipdirs=

Noconfigdirs are removed loudly.

noconfigdirs=””

然后，定义变量skipdirs和noconfigdirs，用来列出哪些目录将被跳过，或者不进行配置。其中skipdirs中的目录将被隐式的跳过，而noconfigdirs中的目录将会在终端显式的打印出消息，告知这些目录将不被配置。

use_gnu_ld=

Make sure we don’t let GNU ld be added if we didn’t want it.

if test x$with_gnu_ld = xno ; then use_gnu_ld=no noconfigdirs=”$noconfigdirs ld gold” fi

use_gnu_as=

Make sure we don’t let GNU as be added if we didn’t want it.

if test x$with_gnu_as = xno ; then use_gnu_as=no noconfigdirs=”$noconfigdirs gas” fi … …

接下来，就是根据configure的各种选项，来设置skipdirs和noconfigdirs的值。

# Remove the entries in $skipdirs and $noconfigdirs from $configdirs,

$build_configdirs and $target_configdirs.

If we have the source for $noconfigdirs entries, add them to $notsupp.

notsupp=”” for dir in . $skipdirs $noconfigdirs ; do dirname=`echo $dir | sed -e s/target-//g -e s/build-//g` if test $dir != . && echo “ ${configdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then configdirs=`echo “ ${configdirs} “ | sed -e “s/ ${dir} / /”` if test -r $srcdir/$dirname/configure ; then if echo “ ${skipdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then true else notsupp=”$notsupp $dir” fi fi fi if test $dir != . && echo “ ${build_configdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then build_configdirs=`echo “ ${build_configdirs} “ | sed -e “s/ ${dir} / /”` if test -r $srcdir/$dirname/configure ; then if echo “ ${skipdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then true else notsupp=”$notsupp $dir” fi fi fi if test $dir != . && echo “ ${target_configdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then target_configdirs=`echo “ ${target_configdirs} “ | sed -e “s/ ${dir} / /”` if test -r $srcdir/$dirname/configure ; then if echo “ ${skipdirs} “ | grep “ ${dir} “ >/dev/null 2>&1; then true else notsupp=”$notsupp $dir” fi fi fi done … …

最后，从configdirs, target_configdirs, build_configdirs中，移除在skipdirs和noconfigdirs中存在的目录项。并且，移除那些无法进行配置（没有configure文件）的目录。

4、测试flags，复制目录

AC_ARG_WITH([build-sysroot], [AS_HELP_STRING([–with-build-sysroot=SYSROOT], [use sysroot as the system root during the build])], [if test x”$withval” != x ; then SYSROOT_CFLAGS_FOR_TARGET=”–sysroot=$withval” fi], [SYSROOT_CFLAGS_FOR_TARGET=]) AC_SUBST(SYSROOT_CFLAGS_FOR_TARGET) … …

从2046到2206行，这部分代码设定了CFLAGS_FOR_TARGET，CXXFLAGS_FOR_TARGET，LDFLAGS_FOR_TARGET；并且对选项–with-headers=XXX，–with-libs=XXX做了处理，分别将目录下的内容复制到$(tooldir)/sys-include和$(tooldir)/lib下。

5、设定Makefile片段（frag）

# Work in distributions that contain no compiler tools, like Autoconf. host_makefile_frag=/dev/null if test -d ${srcdir}/config ; then case “${host}” in i[[3456789]]86-*-msdosdjgpp*) host_makefile_frag=”config/mh-djgpp” ;; *-cygwin*) ACX_CHECK_CYGWIN_CAT_WORKS host_makefile_frag=”config/mh-cygwin” ;; … …

Makefile fragments.

for frag in host_makefile_frag target_makefile_frag alphaieee_frag ospace_frag; do eval fragval=\$$frag if test $fragval != /dev/null; then eval $frag=${srcdir}/$fragval fi done AC_SUBST_FILE(host_makefile_frag) AC_SUBST_FILE(target_makefile_frag) AC_SUBST_FILE(alphaieee_frag) AC_SUBST_FILE(ospace_frag)

这部分代码位于1074-1107，2226-2280，2871-2882行，用来设定host_makefile_frag，target_makefile_frag，alphaieee_frag，ospace_frag这些makefile片段的值。

6、gdb相关

# Create a .gdbinit file which runs the one in srcdir

and tells GDB to look there for source files.

if test -r ${srcdir}/.gdbinit ; then case ${srcdir} in .) ;; *) cat > ./.gdbinit < 代码2287-2301行，用来创建.gdbinit文件，方便调试gcc。

# Determine whether gdb needs tk/tcl or not.

Use ‘maybe’ since enable_gdbtk might be true even if tk isn’t available

and in that case we want gdb to be built without tk. Ugh!

In fact I believe gdb is the only package directly dependent on tk,

so we should be able to put the ‘maybe’s in unconditionally and

leave out the maybe dependencies when enable_gdbtk is false. I’m not

100% sure that that’s safe though.

gdb_tk=”maybe-all-tcl maybe-all-tk maybe-all-itcl maybe-all-libgui” case “$enable_gdbtk” in no) GDB_TK=”” ;; yes) GDB_TK=”${gdb_tk}” ;; *) # Only add the dependency on gdbtk when GDBtk is part of the gdb # distro. Eventually someone will fix this and move Insight, nee # gdbtk to a separate directory. if test -d ${srcdir}/gdb/gdbtk ; then GDB_TK=”${gdb_tk}” else GDB_TK=”” fi ;; esac CONFIGURE_GDB_TK=`echo ${GDB_TK} | sed s/-all-/-configure-/g` INSTALL_GDB_TK=`echo ${GDB_TK} | sed s/-all-/-install-/g`

代码2352-2378行，用来确定gdb是否需要tk/tcl。这里作者提到一个他认为可以进行改进，但又不太确定的地方。

7、去掉Makefile中不需要的目标（target）

extrasub_build= for module in ${build_configdirs} ; do if test -z “${no_recursion}”
&& test -f ${build_subdir}/${module}/Makefile; then echo 1>&2 “*** removing ${build_subdir}/${module}/Makefile to force reconfigure” rm -f ${build_subdir}/${module}/Makefile fi extrasub_build=”$extrasub_build /^@if build-$module\$/d /^@endif build-$module\$/d /^@if build-$module-$bootstrap_suffix\$/d /^@endif build-$module-$bootstrap_suffix\$/d” done … … AC_CONFIG_FILES([Makefile], [sed “$extrasub_build” Makefile | sed “$extrasub_host” | sed “$extrasub_target” > mf$mv -f mf$ Makefile], [extrasub_build=”$extrasub_build” extrasub_host=”$extrasub_host” extrasub_target=”$extrasub_target”]) AC_OUTPUT

这部分代码位于2382-2392，2445-2497，3199-3207行，用来删除掉Makefile（或者Makefile.in）中不需要的目标。 Makefile.in文件中有许多@if和@endif包裹的模块。configure.ac中定义了变量extrasub_build，extrasub_host， extrasub_target，它们是根据build_configdirs，configdirs，target_configdirs的内容而定义的sed语句。

在最后的AC_CONFIG_FILES命令中，将会调用这些sed语句，修改Makefile，对@if语句块进行选择性的删除。

8、设定BUILD_CONFIG变量，以调整顶层makefile

# Adjust the toplevel makefile according to whether bootstrap was selected. case $enable_bootstrap in yes) bootstrap_suffix=bootstrap BUILD_CONFIG=bootstrap-debug ;; no) bootstrap_suffix=no-bootstrap BUILD_CONFIG= ;; esac … … AC_MSG_RESULT($BUILD_CONFIG)

这部分代码位于2401-2443行，根据是否选择了bootstrap，来设定BUILD_CONFIG变量的值。 Makefile中会根据该变量的值将config子目录下相应的mk文件包含进来。

9、串行化configure

# Create the serialization dependencies. This uses a temporary file.

AC_ARG_ENABLE([serial-configure], [AS_HELP_STRING([[–enable-serial-[{host,target,build}-]configure]], [force sequential configuration of sub-packages for the host, target or build machine, or all sub-packages])]) … … serialization_dependencies=serdep.tmp AC_SUBST_FILE(serialization_dependencies)

这部分代码位于2499-2547行，用来实现对串行化configure的支持。

10、对自举（bootstrap）的支持

# ———————

GCC bootstrap support

———————

Stage specific cflags for build.

stage1_cflags=”-g” case $build in vax-*-*) case ${GCC} in yes) stage1_cflags=”-g -Wa,-J” ;; *) stage1_cflags=”-g -J” ;; esac ;; esac … …

这部分代码位于3118-3197行，用来设定不同阶段的命令行选项（比如stage1_cflags），指定哪些文件在自举过程中不需要进行比较操作。