ax_compiler_flags
=================

Synopsis
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     AX_COMPILER_FLAGS([CFLAGS-VARIABLE], [LDFLAGS-VARIABLE], [IS-RELEASE], [EXTRA-BASE-CFLAGS], [EXTRA-YES-CFLAGS], [UNUSED], [UNUSED], [UNUSED], [EXTRA-BASE-LDFLAGS], [EXTRA-YES-LDFLAGS], [UNUSED], [UNUSED], [UNUSED])

Description
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Check for the presence of an --enable-compile-warnings option to configure, defaulting to "error" in normal operation, or "yes" if IS-RELEASE is equal to "yes".  Return the value in the variable $ax_enable_compile_warnings.

Depending on the value of --enable-compile-warnings, different compiler warnings are checked to see if they work with the current compiler and, if so, are appended to CFLAGS-VARIABLE and LDFLAGS-VARIABLE.  This allows a consistent set of baseline compiler warnings to be used across a code base, irrespective of any warnings enabled locally by individual developers.  By standardising the warnings used by all developers of a project, the project can commit to a zero-warnings policy, using -Werror to prevent compilation if new warnings are introduced.  This makes catching bugs which are flagged by warnings a lot easier.

By providing a consistent --enable-compile-warnings argument across all projects using this macro, continuous integration systems can easily be configured the same for all projects.  Automated systems or build systems aimed at beginners may want to pass the --disable-Werror argument to unconditionally prevent warnings being fatal.

--enable-compile-warnings can take the values:

      * no:      Base compiler warnings only; not even -Wall.
      * yes:     The above, plus a broad range of useful warnings.
      * error:   The above, plus -Werror so that all warnings are fatal.
                 Use --disable-Werror to override this and disable fatal
                 warnings.

The set of base and enabled flags can be augmented using the EXTRA-*-CFLAGS and EXTRA-*-LDFLAGS variables, which are tested and appended to the output variable if --enable-compile-warnings is not "no". Flags should not be disabled using these arguments, as the entire point of AX_COMPILER_FLAGS is to enforce a consistent set of useful compiler warnings on code, using warnings which have been chosen for low false positive rates.  If a compiler emits false positives for a warning, a #pragma should be used in the code to disable the warning locally. See:

       https://gcc.gnu.org/onlinedocs/gcc-4.9.2/gcc/Diagnostic-Pragmas.html#Diagnostic-Pragmas

The EXTRA-* variables should only be used to supply extra warning flags, and not general purpose compiler flags, as they are controlled by configure options such as --disable-Werror.

IS-RELEASE can be used to disable -Werror when making a release, which is useful for those hairy moments when you just want to get the release done as quickly as possible.  Set it to "yes" to disable -Werror. By default, it uses the value of $ax_is_release, so if you are using the AX_IS_RELEASE macro, there is no need to pass this parameter. For example:

       AX_IS_RELEASE([git-directory])
       AX_COMPILER_FLAGS()

CFLAGS-VARIABLE defaults to WARN_CFLAGS, and LDFLAGS-VARIABLE defaults to WARN_LDFLAGS.  Both variables are AC_SUBST-ed by this macro, but must be manually added to the CFLAGS and LDFLAGS variables for each target in the code base.

If C++ language support is enabled with AC_PROG_CXX, which must occur before this macro in configure.ac, warning flags for the C++ compiler are AC_SUBST-ed as WARN_CXXFLAGS, and must be manually added to the CXXFLAGS variables for each target in the code base.  EXTRA-*-CFLAGS can be used to augment the base and enabled flags.

Warning flags for g-ir-scanner (from GObject Introspection) are AC_SUBST-ed as WARN_SCANNERFLAGS.  This variable must be manually added to the SCANNERFLAGS variable for each GIR target in the code base.  If extra g-ir-scanner flags need to be enabled, the AX_COMPILER_FLAGS_GIR macro must be invoked manually.

AX_COMPILER_FLAGS may add support for other tools in future, in addition to the compiler and linker.  No extra EXTRA-* variables will be added for those tools, and all extra support will still use the single --enable-compile-warnings configure option.  For finer grained control over the flags for individual tools, use AX_COMPILER_FLAGS_CFLAGS, AX_COMPILER_FLAGS_LDFLAGS and AX_COMPILER_FLAGS_* for new tools.

The UNUSED variables date from a previous version of this macro, and are automatically appended to the preceding non-UNUSED variable. They should be left empty in new uses of the macro.

Source Code
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Download the latest version of ax_compiler_flags.m4 (http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_compiler_flags.m4) or browse the macro's revision history (http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=history;f=m4/ax_compiler_flags.m4).

License
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Copyright © 2014, 2015 Philip Withnall philip@tecnocode.co.uk
 Copyright © 2015 David King amigadave@amigadave.com

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved.  This file is offered as-is, without any warranty.