Add fmtcheck(), a function for checking consistency of format string

arguments where the format string is obtained from user data, or
otherwise difficult to verify statically.

Example usage:

printf(fmtcheck(user_format, standard_format), arg1, arg2);

checks the format string user_format for consistency (same number/order/
type of format operators) with standard_format.  If they differ,
standard_format is used instead to avoid potential crashes or security
violations.

Obtained from:  NetBSD
Reviewed by:    -arch

1 files changed, 2 insertions, 0 deletions

diff --git a/include/stdio.h b/include/stdio.h
index e8c346b..520c622 100644
--- a/include/stdio.h
+++ b/include/stdio.h
@@ -299,6 +299,8 @@ __END_DECLS
 __BEGIN_DECLS
 int	 asprintf __P((char **, const char *, ...)) __printflike(2, 3);
 char	*ctermid_r __P((char *));
+const char *fmtcheck __P((const char *, const char *))
+	__attribute__((__format_arg__(2)));
 char	*fgetln __P((FILE *, size_t *));
 int	 fpurge __P((FILE *));
 int	 fseeko __P((FILE *, _BSD_OFF_T_, int));