Autotools：如何设置全局编译标志 [英] Autotools : how to set global compilation flag

问题描述

我有一个有多个来源目录的项目：

  src / A 
 / B 
 / C 
  

每个Makefile.am包含

  AM_CXXFLAGS = -fPIC -Wall -Wextra 
  

可以避免在每个源文件夹中重复此操作？

我试图修改src / Makefile.am和configure.in，但没有成功。我想我可以使用AC_PROG_CXX全局设置编译标志，但是找不到关于如何使用这些宏的许多文档（你有任何指针这样的文档吗？）。

提前感谢

解决方案

您可以做以下几件事：

<一个解决方案是在所有的 Makefile.am 上包含一个通用的makefile片段：

  include $（top_srcdir）/common.mk 
 ... 
 bin_PROGRAMS = foo 
 foo_SOURCES = ... 
 
  
    AM_CXXFLAGS = -fpic -Wall -Wextra 
  
到 common.mk ，并且将来更容易通过编辑此文件向所有 Makefile.am 添加更多的宏或规则。
 
 
 
（2）另一个解决方案是在 configure.ac 中全局设置这些变量（名称 configure.in 很久以前已被弃用），如：
  ... 
 AC_SUBST [AM_CXXFLAGS]，[-fpic -Wall -Wextra]）
 ... 
  
你甚至不必在你的 Makefile.am 中说什么，他们会自动继承这个全局定义。缺点是你不能轻松地选择退出（使用第一个解决方案很容易决定不包括 common.mk ），依赖关系不是真的明确的第三（当他们读取 Makefile.am 时，他们没有提示关于标记可能来自哪里）。
 
 
 
（3）第三种解决方案是做 orsogufo 建议：覆盖<$ c $中的用户变量CXXFLAGS c> configure.ac 。我建议反对它，因为它违背了GNU构建系统的一个特性：用户可以覆盖 make  -time的这个变量。例如，您可能需要键入
  make CXXFLAGS =' -  O0 -ggdb'
  
，这将覆盖 CXXFLAGS 的任何定义b $ b不是 AM_CXXFLAGS 中的那些）。说实话，大多数项目不能正确支持这个，因为他们玩 CXXFLAGS 。
 
 
 
最后，提到 -fpic ， -Wall 和 -Werror 不是便携式选项。 
根据项目的范围，您可能需要添加对这些（ gnulib ）最近获得的新的配置检查宏用于测试警告标志， libtool 可用于构建共享库）。
 
I have a project with several sources directories : 
src/A
   /B
   /C
In each, the Makefile.am contains 
AM_CXXFLAGS = -fPIC -Wall -Wextra
How can avoid repeating this in each source folder ? 

I tried to modifiy src/Makefile.am and the configure.in, but without success. I thought I could use AC_PROG_CXX to set the compilation flags globally but can't find much documentation on how to use those macro (do you have any pointer to such a documentation ?).

Thanks in advance
解决方案
You can do several things:

(1) One solution is to include a common makefile fragment on all your Makefile.ams:
include $(top_srcdir)/common.mk
...
bin_PROGRAMS = foo
foo_SOURCES = ...
in that case you would write
AM_CXXFLAGS = -fpic -Wall -Wextra
to common.mk and in the future it will be easier to add more macros or rules to all Makefile.ams by just editing this file.

(2) Another solution would be to set these variables globally in your configure.ac (the name configure.in has been deprecated long ago), as in :
...
AC_SUBST([AM_CXXFLAGS], [-fpic -Wall -Wextra])
...
Then you don't even have to say anything in your Makefile.ams, they automatically inherit this global definition.  The drawback is that you can't opt-out easily (with the first solution it's easy to decide not to include common.mk) and the dependency is not really explicit to third-party people (when they read the Makefile.am they have no hint about where the flags may come from).

(3) A third solution would be to do as orsogufo suggested: overwriting the user variable CXXFLAGS in configure.ac.  I would advise against it, because it defeats one of the features of the GNU Build System: users are allowed to override this variable at make-time.   For instance you may want to type
make CXXFLAGS='-O0 -ggdb' 
when debugging a piece of code, and this will overwrite any definition of CXXFLAGS (but
not those in AM_CXXFLAGS).  To be honest, most projects fails to support this correctly because they play tricks with CXXFLAGS.

Finally, I should mention that -fpic, -Wall, and -Werror are not portable options.
Depending on the scope of your project you may want to add configure check for these (gnulib recently acquired new macros to tests for warnings flags, and libtool can be used to build shared libraries).

                        
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