memmem(3): empty little string matches the beginning of the big string

This function originated in glibc, and this matches their behaviour
(and NetBSD, OpenBSD, and musl).

An empty big string (arg "l") is handled by the existing
l_len < s_len test.

Reviewed by:	bapt, ngie
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D2657

contrib/netbsd-tests/lib/libc/string/t_memmem.c

@@ -75,7 +75,7 @@ ATF_TC_HEAD(memmem_basic, tc)
 ATF_TC_BODY(memmem_basic, tc)
 {
 
-#if defined(__darwin__) || defined(__FreeBSD__)
+#if defined(__darwin__)
 	expect(memmem(b2, lb2, p0, lp0) == NULL);
 	expect(memmem(b0, lb0, p0, lp0) == NULL);
 #else

lib/libc/string/memmem.3

@@ -26,7 +26,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd August 24, 2005
+.Dd May 26, 2015
 .Dt MEMMEM 3
 .Os
 .Sh NAME
@@ -51,14 +51,12 @@ in the byte string
 .Fa big .
 .Sh RETURN VALUES
 If
-.Fa big_len
-is smaller than
-.Fa little_len ,
-if
 .Fa little_len
-is 0, if
-.Fa big_len
-is 0 or if
+is zero
+.Fa big
+is returned (that is, an empty little is deemed to match at the beginning of
+big);
+if
 .Fa little
 occurs nowhere in
 .Fa big ,
@@ -84,3 +82,11 @@ function first appeared in
 .Sh BUGS
 This function was broken in Linux libc up to and including version 5.0.9
 and in GNU libc prior to version 2.1.
+Prior to
+.Fx 11.0
+.Nm
+returned
+.Dv NULL
+when
+.Fa little_len
+equals 0.

lib/libc/string/memmem.c

@@ -42,9 +42,9 @@ memmem(const void *l, size_t l_len, const void *s, size_t s_len)
 	const char *cl = (const char *)l;
 	const char *cs = (const char *)s;
 
-	/* we need something to compare */
-	if (l_len == 0 || s_len == 0)
-		return NULL;
+	/* empty "s" matches the beginning of "l" */
+	if (s_len == 0)
+		return (void *)cl;
 
 	/* "s" must be smaller or equal to "l" */
 	if (l_len < s_len)