[mpich-discuss] [PATCH] config: change PAC_FUNC_NEEDS_DECL to check by assignment

[Pavan Balaji]

Hi Jed,

Thanks.  This looks OK to me.  Can you send the patch as an attachment,
so I can apply it?

There's also the autoconf macro AC_CHECK_DECLS, which I was planning to
migrate to after some testing.  I haven't investigated what it does
internally, though.

 -- Pavan

On 05/17/2013 10:41 PM US Central Time, Jed Brown wrote:
> The old method attempts to create a failed compile when the function
> *is* already declared, by creating an incompatible declaration.  That is
> insufficient with clang-3.2, which only warns in case of incompatible
> redeclaration of a library function:
> 
>   conftest.c:44:5: warning: incompatible redeclaration of library function 'strncmp'
>   int strncmp(double, int, double, const char *);
>       ^
>   /usr/include/string.h:143:12: note: 'strncmp' is a builtin with type 'int (const char *, const char *, size_t)'
>   extern int strncmp (const char *__s1, const char *__s2, size_t __n)
>              ^
>   1 warning generated.
> 
> Instead, we take the opposite approach, generating a failed compile when
> the library function has *not* been declared in the header.
> 
>   void (*fptr)(void) = (void(*)(void))strncmp;
> 
> Signed-off-by: Jed Brown <jedbrown at mcs.anl.gov>
> ---
>  confdb/aclocal_cc.m4 | 21 +++++++++------------
>  1 file changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/confdb/aclocal_cc.m4 b/confdb/aclocal_cc.m4
> index 7a92a8a..10964e1 100644
> --- a/confdb/aclocal_cc.m4
> +++ b/confdb/aclocal_cc.m4
> @@ -1205,25 +1205,22 @@ dnl Sets 'NEEDS_<funcname>_DECL' if 'funcname' is not declared by the
>  dnl headerfiles.
>  dnl
>  dnl Approach:
> -dnl Try to compile a program with the function, but passed with an incorrect
> -dnl calling sequence.  If the compilation fails, then the declaration
> -dnl is provided within the header files.  If the compilation succeeds,
> -dnl the declaration is required.
> +dnl Attempt to assign library function to function pointer.  If the function
> +dnl is not declared in a header, this will fail.  Use a non-static global so
> +dnl the compiler does not warn about an unused variable.
>  dnl
> -dnl We use a 'double' as the first argument to try and catch varargs
> -dnl routines that may use an int or pointer as the first argument.
> +dnl Simply calling the function is not enough because C89 compilers allow
> +dnl calls to implicitly-defined functions.  Re-declaring a library function
> +dnl with an incompatible prototype is also not sufficient because some
> +dnl compilers (notably clang-3.2) only produce a warning in this case.
>  dnl
> -dnl There is one difficulty - if the compiler has been instructed to
> -dnl fail on implicitly defined functions, then this test will always
> -dnl fail.
> -dnl 
>  dnl D*/
>  AC_DEFUN([PAC_FUNC_NEEDS_DECL],[
>  AC_CACHE_CHECK([whether $2 needs a declaration],
>  pac_cv_func_decl_$2,[
>  AC_TRY_COMPILE([$1
> -int $2(double, int, double, const char *);],[int a=$2(1.0,27,1.0,"foo");],
> -pac_cv_func_decl_$2=yes,pac_cv_func_decl_$2=no)])
> +void (*fptr)(void) = (void(*)(void))$2;],[],
> +pac_cv_func_decl_$2=no,pac_cv_func_decl_$2=yes)])
>  if test "$pac_cv_func_decl_$2" = "yes" ; then
>  changequote(<<,>>)dnl
>  define(<<PAC_FUNC_NAME>>, translit(NEEDS_$2_DECL, [a-z *], [A-Z__]))dnl
> 

-- 
Pavan Balaji
http://www.mcs.anl.gov/~balaji