How to check support compile flag in autoconf for clang

Question

We can check support for specific compiler flags in autoconf using:

AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])

These checks are returning true but clang doesn't support them.

AX_CHECK_COMPILE_FLAG([-Woverride-init])
AX_CHECK_COMPILE_FLAG([-Wformat-truncation])

How to check support compile flag in autoconf for clang?

I change my compiler by

export CC=/usr/bin/clang-6.0 
export CXX=/usr/bin/clang++-6.0 

and if echo into script $CC and $CXX, all is ok.

link to configure.ac

Answer 1

If you use clang -Wsome-invalid-option -xc /dev/null, you'll see that clang emits a warning about an unknown option -Wsome-invalid-option. The docs for AX_CHECK_COMPILE_FLAG state that warnings are ignored.

You need to use argument 4 of the macro to include -Werror:

AX_CHECK_COMPILE_FLAG([-Woverride-init], , , [-Werror])

This check also generates a cache variable, in case you need to override it:

ax_cv_cflags__Werror__Woverride_init

If it's set to yes, then the compiler supports the flag, else it does not support the flag. You can use this for other flags as well:

AX_CHECK_COMPILE_FLAG([-Wformat-truncation], , , [-Werror])
AS_VAR_IF([ax_cv_cflags__Werror__Wformat_truncation], [yes],
    ,
    [AC_FATAL([-Wformat-truncation not supported])])

The generic form is ax_cv_cflags_{EXTRA}_{FLAG} for C. Obviously if you're going to use AS_VAR_IF to check the cache variable, however, you may as well use the second and/or third macro arguments as well, and you can use AC_FATAL if the compilation flag is required:

AX_CHECK_COMPILE_FLAG([-Wformat-truncation],
    ,
    [AC_FATAL([-Wformat-truncation is required])],
    [-Werror])