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[libc] Add strstr()

--- a/libc/src/Makefile

+++ b/libc/src/Makefile

@@ -3,7 +3,7 @@

 LIBCOBJ = assert.o strcpy.o strcmp.o strlen.o strchr.o \

           strrchr.o strcat.o strncmp.o strncpy.o strncat.o strcoll.o \

-          strxfrm.o strtok.o \

+          strxfrm.o strtok.o strstr.o \

           memset.o memcpy.o memmove.o memcmp.o memchr.o \

           isalnum.o isalpha.o isascii.o isblank.o iscntrl.o isdigit.o \

           isgraph.o islower.o isprint.o ispunct.o isspace.o isupper.o \

--- /dev/null

+++ b/libc/src/strstr.c

@@ -1,0 +1,27 @@

+/* See LICENSE file for copyright and license details. */

+

+#include <string.h>

+

+char *

+strstr(const char *s1, const char *s2)

+{

+	const char *p, *q;

+	int c;

+

+	c = *s2++;

+	if (c == '\0')

+		return (char *) s1;

+

+	while (*s1) {

+		if (*s1 != c) {

+			++s1;

+		} else {

+			p = s1++;

+			for (q = s2; *q && *s1 == *q; ++s1, ++q)

+				/* nothing */;

+			if (*q == '\0')

+				return (char *) p;

+		}

+	}

+	return NULL;

+}