configure配置过程学习

本文章是自己通过阅读configure的总结笔记，可能部分理解有错误，希望大家不吝指出，大家共同学习

目标

通过configure用户选项进行配置，加上测试系统支持的特性，生成配置结果，根据配置结果替换编译系统中的变量，从而实现根据配置改变编译的结果
一般是将 xxx.in 根据配置替换其中的变量 变为 xxx, 后缀in表示input的意思

实现细节

在configure中定义了ac_subst_vars，这里面就是包含可以替换的变量

ac_user_opts 用户的选项

ac_precious_vars 配置变量，可以通过环境变量或者命令行进行赋值

configure是一个shell脚本，通过用户选项ac_user_opts 和 配置变量ac_precious_vars 的值，然后对系统环境进行测试，生成config.status 文件

: "${CONFIG_STATUS=./config.status}" 
cat >$CONFIG_STATUS <<_ASEOF || as_write_fail=1
#! $SHELL
# Generated by $as_me.
# Run this file to recreate the current configuration.
# Compiler output produced by configure, useful for debugging
# configure, is in config.log if it exists.

debug=false
ac_cs_recheck=false
ac_cs_silent=false

SHELL=\${CONFIG_SHELL-$SHELL}
export SHELL
_ASEOF
...
cat >>$CONFIG_STATUS <<\_ACEOF || ac_write_fail=1
exec 5>>config.log
{
  echo
  sed 'h;s/./-/g;s/^.../## /;s/...$/ ##/;p;x;p;x' <<_ASBOX
## Running $as_me. ##
_ASBOX
  $as_echo "$ac_log"
} >&5

_ACEOF

# configure is writing to config.log, and then calls config.status.
# config.status does its own redirection, appending to config.log.
# Unfortunately, on DOS this fails, as config.log is still kept open
# by configure, so config.status won't be able to write to it; its
# output is simply discarded.  So we exec the FD to /dev/null,
# effectively closing config.log, so it can be properly (re)opened and
# appended to by config.status.  When coming back to configure, we
# need to make the FD available again.
if test "$no_create" != yes; then
  ac_cs_success=:
  ac_config_status_args=
  test "$silent" = yes &&
    ac_config_status_args="$ac_config_status_args --quiet"
  exec 5>/dev/null
  $SHELL $CONFIG_STATUS $ac_config_status_args || ac_cs_success=false
  exec 5>>config.log
  # Use ||, not &&, to avoid exiting from the if with $? = 1, which
  # would make configure fail if this is the last instruction.
  $ac_cs_success || as_fn_exit 1
fi

config.status是一个shell脚本，configure生成config.status后，然后执行config.status
$SHELL $CONFIG_STATUS $ac_config_status_args
configure生成的log放在config.log中，执行config.status生成的log也append在config.log后面

confdefs.h只是配置的中间结果，最终会被处理写入config.status文件中

  { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether to use libcap-ng" >&5
$as_echo_n "checking whether to use libcap-ng... " >&6; }
  if test x$CAPNG_LDADD != x ; then

$as_echo "#define HAVE_LIBCAP_NG 1" >>confdefs.h

      { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
$as_echo "yes" >&6; }
  else
      { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
$as_echo "no" >&6; }
...

# Transform confdefs.h into an awk script `defines.awk', embedded as
# here-document in config.status, that substitutes the proper values into
# config.h.in to produce config.h.

config.status需要替换哪些文件中的变量呢？
config_files=" Makefile lib/Makefile lib/audit.pc lib/test/Makefile"
config_headers=" config.h"
config_commands=" depfiles libtool"
分别记录了哪些文件需要config，也即是需要替换

# Handling of arguments.
for ac_config_target in $ac_config_targets                                                                                                                               
do
  case $ac_config_target in
    "config.h") CONFIG_HEADERS="$CONFIG_HEADERS config.h" ;;
    "depfiles") CONFIG_COMMANDS="$CONFIG_COMMANDS depfiles" ;;
    "libtool") CONFIG_COMMANDS="$CONFIG_COMMANDS libtool" ;;
    "Makefile") CONFIG_FILES="$CONFIG_FILES Makefile" ;;
    "lib/Makefile") CONFIG_FILES="$CONFIG_FILES lib/Makefile" ;

从配置的过程在终端中的打印信息可以验证上面的分析过程：

其他的细节
1.设置好环境，然后重新执行自己，也就是重新执行configure

if test "x$as_myself" = x; then                                                                                                                                          
  as_myself=$0
fi
exec $CONFIG_SHELL $as_opts "$as_myself" ${1+"$@"} 

2. 日志log

exec 6>&1
test "$silent" = yes && exec 6>/dev/null 

cat >config.log <<_ACEOF
This file contains any messages produced by compilers while
running configure, to aid debugging if configure makes a mistake.

It was created by audit $as_me 2.8.4, which was
generated by GNU Autoconf 2.69.  Invocation command line was

  $ $0 $@

_ACEOF
exec 5>>config.log

后面配置过程中的打印

{ $as_echo "$as_me:${as_lineno-$LINENO}: checking build system type" >&5
$as_echo_n "checking build system type... " >&6; }

&6主要打印在终端，&5打印到config.log中，如果设置了silent=yes，那么终端将不打印任何消息。

3. config.guess config.sub

ac_build_alias=`$SHELL "$ac_aux_dir/config.guess"`
ac_cv_build=`$SHELL "$ac_aux_dir/config.sub" $ac_build_alias`

ac_cv_host=`$SHELL "$ac_aux_dir/config.sub" $host_alias`

ac_cv_target=`$SHELL "$ac_aux_dir/config.sub" $target_alias`

config.guess是一个shell脚本，主要是在没有指定build时去猜测
config.sub 也是一个shell脚本， 主要用于构建echo $basic_machine$os
例如：
ac_cv_build=x86_64-pc-linux-gnu
ac_cv_host=arm-unknown-linux-gnueabihf
ac_cv_target=arm-unknown-linux-gnueabihf

至于是build，host， target的区别：
build： 当前构建系统的类型
host ： 构建的程序哪个系统上运行
target ： 程序运行生成的文件在哪个系统上运行，例如编译器gcc