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

[emaste]

Summary

This function originated in glibc, and this matches the behaviour of their implementation.

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

Test Plan

kyua test lib/libc/string/memmem:memmem_basic

[ngie]

LGTM:

$ cat /etc/redhat-release 
Fedora release 20 (Heisenbug)
$ ./test_memmem ; echo $?
0
$ cat test_memmem.c 
#define	_GNU_SOURCE
#include <assert.h>
#include <string.h>

char p0[] = "";
int lp0 = 0;
char b0[] = "";
int lb0 = 0;
char b2[] = "0123456789";
int lb2 = 10;

int
main(void)
{

	assert(memmem(b2, lb2, p0, lp0) == b2);
	assert(memmem(b0, lb0, p0, lp0) == b0);

	return 0;
}

[emaste]

For reference, NetBSD, OpenBSD and musl share the "new" behaviour (with this change) with glibc
OS X and DragonFly have the "old" behaviour

[emaste]

Update cast as in OpenBSD

[emaste]

update man page too