Re: [PATCH] gdbsupport: include cstdlib in common-defs.h

[Christian Biesinger <cbiesinger@google.com>]

On Thu, Apr 9, 2020 at 5:50 PM Simon Marchi via Gdb-patches
<gdb-patches@sourceware.org> wrote:
>
> In PR 25731 [1], the following build failure was reported:
>
>     ../../binutils-gdb/gdb/gdbtypes.c:1254:10: error: no member named 'abs' in namespace 'std'; did you mean simply 'abs'?
>                 = ((std::abs (stride) * element_count) + 7) / 8;
>                     ^~~~~~~~
>                     abs
>     /usr/include/stdlib.h:129:6: note: 'abs' declared here
>     int      abs(int) __pure2;
>              ^
> The original report was using:
>
>     $ gcc -v
>     Apple LLVM version 8.0.0 (clang-800.0.42.1)
>     Target: x86_64-apple-darwin15.6.0
>
> Note that I was _not_ able to reproduce using:
>
>     $ g++ --version
>     Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
>     Apple clang version 11.0.0 (clang-1100.0.33.17)
>     Target: x86_64-apple-darwin19.3.0
>
> The proposed fix is to include <cstdlib> in addition to <stdlib.h>.
>
> Here's an excerpt of [2] relevant to this problem:
>
>     These headers [speaking of the .h form] are allowed to also declare
>     the same names in the std namespace, and the corresponding cxxx
>     headers are allowed to also declare the same names in the global
>     namespace: including <cstdlib> definitely provides std::malloc and
>     may also provide ::malloc.  Including <stdlib.h> definitely provides
>     ::malloc and may also provide std::malloc
>
> Since we use std::abs, we should not assume that our include of stdlib.h
> declares an `abs` function in the `std` namespace.
>
> If we replace the include of stdlib.h with cstdlib, then we fall in the
> opposite situation.  A standard C++ library may decide to only put the
> declarations in the std namespace, requiring us to prefix all standard
> functions with `std::`.  I'm not against that, but for the moment I think the
> safest way forward is to just include both.
>
> Note that I don't know what effect this patch can have on any stdlib.h fix
> provided by gnulib.
>
> [1] https://sourceware.org/bugzilla/show_bug.cgi?id=25731
> [2] https://en.cppreference.com/w/cpp/header#C_compatibility_headers
>
> gdbsupport/ChangeLog:
>
>         * common-defs.h: Include cstdlib.h.
> ---
>  gdbsupport/common-defs.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/gdbsupport/common-defs.h b/gdbsupport/common-defs.h
> index e42d2b80c045..257b6279b888 100644
> --- a/gdbsupport/common-defs.h
> +++ b/gdbsupport/common-defs.h
> @@ -84,6 +84,7 @@
>
>  #include <stdarg.h>
>  #include <stdio.h>
> +#include <cstdlib>
>  #include <stdlib.h>
>  #include <stddef.h>
>  #include <stdint.h>

I think this is reasonable, but I would add a comment why both cstdlib
and stdlib.h are included (ie. to get the std:: version with
overloaded functions)

Christian