fix aliasing-based undefined behavior in string functions

use the GNU C may_alias attribute if available, and fallback to naive
byte-by-byte loops if __GNUC__ is not defined.

this patch has been written to minimize changes so that history
remains reviewable; it does not attempt to bring the affected code
into a more consistent or elegant form.

1 files changed, 6 insertions, 3 deletions

diff --git a/src/string/memccpy.c b/src/string/memccpy.c
index 7c233d5e..f515581c 100644
--- a/src/string/memccpy.c
+++ b/src/string/memccpy.c
@@ -11,19 +11,22 @@ void *memccpy(void *restrict dest, const void *restrict src, int c, size_t n)
 {
 	unsigned char *d = dest;
 	const unsigned char *s = src;
-	size_t *wd, k;
-	const size_t *ws;
 
 	c = (unsigned char)c;
+#ifdef __GNUC__
+	typedef size_t __attribute__((__may_alias__)) word;
+	word *wd;
+	const word *ws;
 	if (((uintptr_t)s & ALIGN) == ((uintptr_t)d & ALIGN)) {
 		for (; ((uintptr_t)s & ALIGN) && n && (*d=*s)!=c; n--, s++, d++);
 		if ((uintptr_t)s & ALIGN) goto tail;
-		k = ONES * c;
+		size_t k = ONES * c;
 		wd=(void *)d; ws=(const void *)s;
 		for (; n>=sizeof(size_t) && !HASZERO(*ws^k);
 		       n-=sizeof(size_t), ws++, wd++) *wd = *ws;
 		d=(void *)wd; s=(const void *)ws;
 	}
+#endif
 	for (; n && (*d=*s)!=c; n--, s++, d++);
 tail:
 	if (*s==c) return d+1;