[PATCH] libdwfl: use GNU strerror_r only when available.

[Mark Wielaard]

Hi Érico,

On Mon, Feb 01, 2021 at 03:56:26PM -0300, Érico Nogueira via Elfutils-devel wrote:
> Some C libraries don't provide the GNU version of strerror_r, only the
> XSI-compliant one. We use the GNU version when available, since it fits
> the code better, and otherwise use the XSI-compliant one.

Could you also mention this bug in the commit message?
https://sourceware.org/bugzilla/show_bug.cgi?id=21010

> If possible, I'd like to get this patch in before the release, otherwise
> it's an easy one to carry locally.

One other comment, but this looks good to go otherwise.

> diff --git a/configure.ac b/configure.ac
> index 346ab800..baf6faf5 100644
> --- a/configure.ac
> +++ b/configure.ac
> @@ -428,6 +428,18 @@ AC_CHECK_DECLS([mempcpy],[],[],
>  
>  AC_CHECK_FUNCS([process_vm_readv])
>  
> +AC_CACHE_CHECK([whether C library provides GNU strerror_r], ac_cv_gnu_strerror_r, [dnl
> +old_CFLAGS="$CFLAGS"
> +CFLAGS="$CFLAGS -Werror"
> +AC_COMPILE_IFELSE([AC_LANG_SOURCE([dnl
> +#define _GNU_SOURCE
> +#include <string.h>
> +char * (*s)(int, char*, size_t) = strerror_r;
> +])], ac_cv_gnu_strerror_r=yes, ac_cv_gnu_strerror_r=no)
> +CFLAGS="$old_CFLAGS"])
> +AS_IF([test "x$ac_cv_gnu_strerror_r" = "xyes"],
> +      [AC_DEFINE([HAVE_GNU_STRERROR_R], [1], [Defined if libc has GNU style strerror_r])])
> +

autoconf comes with the following check:

 - Macro: AC_FUNC_STRERROR_R

    If strerror_r is available, define HAVE_STRERROR_R, and if it is
    declared, define HAVE_DECL_STRERROR_R. If it returns a char *
    message, define STRERROR_R_CHAR_P; otherwise it returns an int
    error number. The Thread-Safe Functions option of Posix requires
    strerror_r to return int, but many systems (including, for
    example, version 2.2.4 of the GNU C Library) return a char * value
    that is not necessarily equal to the buffer argument.

Can we use that instead?

> +static const char *
> +errnomsg(int error)
> +{
> +  /* Won't be changed by strerror_r, but not const so compiler doesn't throw warning */
> +  static char unknown[] = "unknown error";
> +
> +#ifdef HAVE_GNU_STRERROR_R

And use #ifdef STRERROR_R_CHAR_P here?

Thanks,

Mark