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19.6 manywarnings

The manywarnings module enables many GCC warnings for your package. Here is an example use:

    [control generation of GCC warnings.  The TYPE 'no' disables
     warnings; 'yes' (default) generates cheap warnings;
     'expensive' in addition generates expensive warnings.])])

AS_IF([test "$enable_gcc_warnings" != no],
   # Set up the list of unwanted warning options.
   if test "$enable_gcc_warnings" != expensive; then
     nw="$nw -fanalyzer"
   nw="$nw -Wbad-function-cast" # Casting a function's result is not more
                                # dangerous than casting any other value.
   nw="$nw -Winline"            # It's OK to not inline.
   nw="$nw -Wsign-compare"      # Too many false alarms.
   nw="$nw -Wstrict-overflow"   # It's OK to optimize strictly.
   nw="$nw -Wsystem-headers"    # Don't warn in system headers.

   # Setup the list of meaningful warning options for the C compiler.
   # The list comes from manywarnings.m4. Warning options that are not
   # generally meaningful have already been filtered out (cf.
   # build-aux/gcc-warning.spec).

   # Compute the list of warning options that are desired.
                          [$possible_warning_options], [$nw])
   # Compute the list of remaining undesired warning options.
   # Namely those, that were not in manywarnings.m4 because they were
   # already listed in build-aux/gcc-warning.spec; this includes those
   # that are implied by -Wall.
                          [$nw], [$possible_warning_options])

   # Add the desired warning options to WARN_CFLAGS.
   for w in $desired_warning_options; do

   # Add the opposites of the remaining undesired warning options to
   for w in `echo "$remaining_undesired_warning_options" | sed -e 's/-W/-Wno-/g'`; do

This module sets up many GCC warning options.

When you use it for the first time, it is common practice to do it as follows:

When a new major version of GCC is released, the Gnulib maintainers add the newly available warning options into the gl_MANYWARN_ALL_GCC macro. You will then enjoy the benefits of the new warnings, simply by updating to the newest Gnulib. If some of the new warnings are undesired, you can add them to the ‘nw’ variable, as described above.

Comments on particular warning flags:


The manywarnings module by default uses GCC’s -fanalyzer option, as this issues some useful warnings. (It can also help GCC generate better code.) However, -fanalyzer can greatly slow down compilation, and in programs with large modules it can be so slow as to be unusable, so it is common for configure to disable it unless configure is given an option like --enable-gcc-warnings=expensive.


Although the manywarnings module does not enable GCC’s -fstrict-aliasing option, it is enabled by default if you compile with -O2 or higher optimization, and can help GCC generate better warnings.


The -fanalyzer option generates many false alarms about malloc leaks, which manywarnings suppresses by also using -Wno-analyzer-malloc-leak.


The manywarnings module by default uses GCC’s -fstrict-flex-arrays option if available, so that GCC can warn about nonportable usage of flexible array members. In a few cases this can help GCC generate better code, so it is not strictly a warning option.


GCC and Clang generate too many false alarms with -Wsign-compare, and we don’t recommend that warning. You can disable it by using gl_WARN_ADD([-Wno-sign-compare]) as illustrated above. Programs using Gnulib generally don’t enable that warning when compiling Gnulib code. If you happen to find a real bug with that warning we’d like to know it.

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