Re: [PATCH v3 1/1] git-compat-util: add a test balloon for C99 support

> The C99 standard was released in January 1999, now 22 years ago.  It
> provides a variety of useful features, including variadic arguments for
> macros, declarations after statements, designated initializers, and a
> wide variety of other useful features, many of which we already use.
>
> We'd like to take advantage of these features, but we want to be
> cautious.  As far as we know, all major compilers now support C99 or a
> later C standard, such as C11 or C17.  POSIX has required C99 support as
> a requirement for the 2001 revision, so we can safely assume any POSIX
> system which we are interested in supporting has C99.
>
> Even MSVC, long a holdout against modern C, now supports both C11 and
> C17 with an appropriate update.  Moreover, even if people are using an
> older version of MSVC on these systems, they will generally need some
> implementation of the standard Unix utilities for the testsuite, and GNU
> coreutils, the most common option, has required C99 since 2009.
> Therefore, we can safely assume that a suitable version of GCC or clang
> is available to users even if their version of MSVC is not sufficiently
> capable.
>
> Let's add a test balloon to git-compat-util.h to see if anyone is using
> an older compiler.  We'll add a comment telling people how to enable
> this functionality on GCC and Clang, even though modern versions of both
> will automatically do the right thing, and ask people still experiencing
> a problem to report that to us on the list.
>
> Note that C89 compilers don't provide the __STDC_VERSION__ macro, so we
> use a well-known hack of using "- 0".  On compilers with this macro, it
> doesn't change the value, and on C89 compilers, the macro will be
> replaced with nothing, and our value will be 0.
>
> For sparse, we explicitly request the gnu99 style because we've
> traditionally taken advantage of some GCC- and clang-specific extensions
> when available and we'd like to retain the ability to do that.  sparse
> also defaults to C89 without it, so things will fail for us if we don't.

>
> Update the cmake configuration to require C11 for MSVC.  We do this
> because this will make MSVC to use C11, since it does not explicitly
> support C99.  We do this with a compiler options because setting the
> C_STANDARD option does not work in our CI on MSVC and at the moment, we
> don't want to require C11 for Unix compilers.
>
> In the Makefile, don't set any compiler flags for the compiler itself,
> since on some systems, such as FreeBSD, we actually need C11, and asking
> for C99 causes things to fail to compile.  The error message should make
> it obvious what's going wrong and allow a user to set the appropriate
> option when building in the event they're using a Unix compiler that
> doesn't support it by default.
>
> Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
> ---
>  Makefile                            |  2 +-
>  contrib/buildsystems/CMakeLists.txt |  2 +-
>  git-compat-util.h                   | 13 +++++++++++++
>  3 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 12be39ac49..3d0ce6ddf6 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -1215,7 +1215,7 @@ ARFLAGS = rcs
>  PTHREAD_CFLAGS =
>
>  # For the 'sparse' target
> -SPARSE_FLAGS ?=
> +SPARSE_FLAGS ?= -std=gnu99
>  SP_EXTRA_FLAGS = -Wno-universal-initializer
>
>  # For informing GIT-BUILD-OPTIONS of the SANITIZE=leak target
> diff --git a/contrib/buildsystems/CMakeLists.txt b/contrib/buildsystems/CMakeLists.txt
> index fd1399c440..07b6c5494b 100644
> --- a/contrib/buildsystems/CMakeLists.txt
> +++ b/contrib/buildsystems/CMakeLists.txt
> @@ -208,7 +208,7 @@ endif()
>  if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
>  	set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG ${CMAKE_BINARY_DIR})
>  	set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_RELEASE ${CMAKE_BINARY_DIR})
> -	add_compile_options(/MP)
> +	add_compile_options(/MP /std:c11)
>  endif()
>
>  #default behaviour
> diff --git a/git-compat-util.h b/git-compat-util.h
> index d70ce14286..ffe70b570f 100644
> --- a/git-compat-util.h
> +++ b/git-compat-util.h
> @@ -1,6 +1,19 @@
>  #ifndef GIT_COMPAT_UTIL_H
>  #define GIT_COMPAT_UTIL_H
>
> +#if __STDC_VERSION__ - 0 < 199901L
> +/*
> + * Git is in a testing period for mandatory C99 support in the compiler.  If
> + * your compiler is reasonably recent, you can try to enable C99 support (or,
> + * for MSVC, C11 support).  If you encounter a problem and can't enable C99
> + * support with your compiler (such as with "-std=gnu99") and don't have access
> + * to one with this support, such as GCC or Clang, you can remove this #if
> + * directive, but please report the details of your system to
> + * git@vger.kernel.org.
> + */
> +#error "Required C99 support is in a test phase.  Please see git-compat-util.h for more details."
> +#endif
> +
>  #ifdef USE_MSVC_CRTDBG
>  /*
>   * For these to work they must appear very early in each
>