Add macros for diagnostic control, use for scanf %a tests.

In <https://sourceware.org/ml/libc-alpha/2014-11/msg00326.html>,
Roland requested internal macros for use of "#pragma GCC diagnostic".

This patch adds such macros and uses them to disable -Wformat warnings
for some code testing GNU scanf %as where GCC expects C99 scanf %a
(several other stdio tests currently use -Wno-format to disable
warnings).  Limitations in GCC's diagnostic pragmas require separate
macros before and after the code generating the warnings, rather than
a single macro taking that code as an argument.

The macros are named DIAG_*_NEEDS_COMMENT to emphasise to reviewers
the need for a comment accompanying any use of them (such comments may
however just appear once for several uses of the macros for the same
issue in the same file).  I put a GCC version in the arguments to
DIAG_IGNORE_NEEDS_COMMENT, as that seems something useful to grep for
when obsoleting support for an old GCC version and needing to decide
if warning-disabling code is still relevant.

These macros should be usable for replacing existing -Wno-* use in
makefiles (as also suggested by Roland), though I have no plans to
work on that (only on use of the macros in cases where warnings are
currently present that need disabling to use -Werror).

Tested for x86_64.

	* include/libc-internal.h (DIAG_PUSH_NEEDS_COMMENT): New macro.
	(DIAG_POP_NEEDS_COMMENT): Likewise.
	(_DIAG_STR1): Likewise.
	(_DIAG_STR): Likewise.
	(DIAG_IGNORE_NEEDS_COMMENT): Likewise.
	* stdio-common/bug21.c: Include <libc-internal.h>.
	(do_test): Disable -Wformat around call to sscanf.
	* stdio-common/scanf14.c: Include <libc-internal.h>.
	(main): Disable -Wformat around some calls to scanf functions.

2 files changed, 30 insertions, 0 deletions

diff --git a/stdio-common/bug21.c b/stdio-common/bug21.c
index d22b9c1a97..ca27272ba1 100644
--- a/stdio-common/bug21.c
+++ b/stdio-common/bug21.c
@@ -1,4 +1,5 @@
 #include <stdio.h>
+#include <libc-internal.h>
 
 static int
 do_test (void)
@@ -6,7 +7,15 @@ do_test (void)
   static const char buf[] = " ";
   char *str;
 
+  /* GCC in C99 mode treats %a as the C99 format expecting float *,
+     but glibc with _GNU_SOURCE treats %as as the GNU allocation
+     extension, so resulting in "warning: format '%a' expects argument
+     of type 'float *', but argument 3 has type 'char **'".  This
+     applies to the other %as, %aS and %a[] formats below as well.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   int r = sscanf (buf, "%as", &str);
+  DIAG_POP_NEEDS_COMMENT;
   printf ("%d %p\n", r, str);
 
   return r != -1 || str != NULL;
diff --git a/stdio-common/scanf14.c b/stdio-common/scanf14.c
index 6ca5c7c567..cffccb0b19 100644
--- a/stdio-common/scanf14.c
+++ b/stdio-common/scanf14.c
@@ -2,6 +2,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <wchar.h>
+#include <libc-internal.h>
 
 #define FAIL() \
   do {							\
@@ -23,6 +24,13 @@ main (void)
     FAIL ();
   else if (f != 0.25 || memcmp (c, "s x", 3) != 0)
     FAIL ();
+  /* GCC in C99 mode treats %a as the C99 format expecting float *,
+     but glibc with _GNU_SOURCE treats %as as the GNU allocation
+     extension, so resulting in "warning: format '%a' expects argument
+     of type 'float *', but argument 3 has type 'char **'".  This
+     applies to the other %as, %aS and %a[] formats below as well.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   if (sscanf (" 1.25s x", "%as%2c", &sp, c) != 2)
     FAIL ();
   else
@@ -32,10 +40,14 @@ main (void)
       memset (sp, 'x', sizeof "1.25s");
       free (sp);
     }
+  DIAG_POP_NEEDS_COMMENT;
   if (sscanf (" 2.25s x", "%las%2c", &d, c) != 2)
     FAIL ();
   else if (d != 2.25 || memcmp (c, " x", 2) != 0)
     FAIL ();
+  /* See explanation above.  */
+  DIAG_PUSH_NEEDS_COMMENT;
+  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
   if (sscanf (" 3.25S x", "%4aS%3c", &lsp, c) != 2)
     FAIL ();
   else
@@ -54,6 +66,7 @@ main (void)
       memset (sp, 'x', sizeof "4.25");
       free (sp);
     }
+  DIAG_POP_NEEDS_COMMENT;
   if (sscanf ("5.25[0-9.] x", "%la[0-9.]%2c", &d, c) != 2)
     FAIL ();
   else if (d != 5.25 || memcmp (c, " x", 2) != 0)
@@ -82,6 +95,9 @@ main (void)
 	FAIL ();
       if (fseek (fp, 0, SEEK_SET) != 0)
 	FAIL ();
+      /* See explanation above.  */
+      DIAG_PUSH_NEEDS_COMMENT;
+      DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
       if (fscanf (fp, "%as%2c", &sp, c) != 2)
 	FAIL ();
       else
@@ -91,11 +107,15 @@ main (void)
 	  memset (sp, 'x', sizeof "1.25s");
 	  free (sp);
 	}
+      DIAG_POP_NEEDS_COMMENT;
 
       if (freopen (fname, "r", stdin) == NULL)
 	FAIL ();
       else
 	{
+	  /* See explanation above.  */
+	  DIAG_PUSH_NEEDS_COMMENT;
+	  DIAG_IGNORE_NEEDS_COMMENT (4.9, "-Wformat");
 	  if (scanf ("%as%2c", &sp, c) != 2)
 	    FAIL ();
 	  else
@@ -105,6 +125,7 @@ main (void)
 	      memset (sp, 'x', sizeof "1.25s");
 	      free (sp);
 	    }
+	  DIAG_POP_NEEDS_COMMENT;
 	}
 
       fclose (fp);