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

[Jed Brown]

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
-- 
1.8.2.3