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19.5 warnings

The warnings module allows to regularly build a package with more GCC warnings than the default warnings emitted by GCC. It is often used indirectly through the manywarnings module (see manywarnings).

It provides the following functionality:

• You can select some warning options, such as ‘-Wall’, to be enabled whenever building with a GCC version that supports these options. The user can choose to override these warning options by providing the opposite options in the CFLAGS variable at configuration time.
• You can make these warnings apply to selected directories only. In projects where subprojects are maintained by different people, or where parts of the source code are imported from external sources (for example from gnulib), it is useful to apply different warning options to different directories.
• It lets you use ‘-Werror’ at ‘make distcheck’ time, to verify that on the maintainer’s system, no warnings remain. (Note that use of ‘-Werror’ in CFLAGS does not work in general, because it may break autoconfiguration.)
• Similarly, it lets you use ‘-Werror’ when the builder runs configure with an option such as --enable-gcc-warnings.

To use this module, you need the following:

1. In configure.ac, use for example

   gl_WARN_ADD([-Wall], [WARN_CFLAGS])
   gl_WARN_ADD([-Wpointer-arith], [WARN_CFLAGS])
   

2. In the directories which shall use WARN_CFLAGS, use it in the definition of AM_CFLAGS, like this:

   AM_CFLAGS = $(WARN_CFLAGS)
   

   Note that the AM_CFLAGS is used in combination with CFLAGS and before CFLAGS in build rules emitted by Automake. This allows the user to provide CFLAGS that override the WARN_CFLAGS.

‘gl_WARN_ADD([-Werror])’ is intended for developers, and should be avoided in contexts where it would affect ordinary installation builds. The warnings emitted by GCC depend, to some extent, on the contents of the system header files, on the size and signedness of built-in types, etc. Use of ‘-Werror’ would cause frustration to all users on platforms that the maintainer has not tested before the release. It is better if ‘-Werror’ is off by default, and is enabled only by developers. For example, ‘-Werror’ could affect ‘make distcheck’ or ‘configure --enable-gcc-warnings’ as mentioned above.