libs/libc/string: add memmem

This function is the GNU extension.

include/string.h

@@ -89,6 +89,8 @@ int        memcmp(FAR const void *s1, FAR const void *s2, size_t n);
 FAR void  *memcpy(FAR void *dest, FAR const void *src, size_t n);
 FAR void  *memmove(FAR void *dest, FAR const void *src, size_t count);
 FAR void  *memset(FAR void *s, int c, size_t n);
+FAR void  *memmem(FAR const void *haystack, size_t haystacklen,
+                  FAR const void *needle, size_t needlelen);
 
 void explicit_bzero(FAR void *s, size_t n);
 

libs/libc/string/Make.defs

@@ -21,7 +21,7 @@
 # Add the string C files to the build
 
 CSRCS += lib_ffs.c lib_ffsl.c lib_ffsll.c lib_fls.c lib_flsl.c
-CSRCS += lib_flsll.c lib_isbasedigit.c lib_memccpy.c lib_memrchr.c
+CSRCS += lib_flsll.c lib_isbasedigit.c lib_memccpy.c lib_memrchr.c lib_memmem.c
 CSRCS += lib_popcount.c lib_popcountl.c lib_popcountll.c
 CSRCS += lib_skipspace.c lib_stpcpy.c lib_stpncpy.c lib_strcasecmp.c
 CSRCS += lib_strcat.c lib_strcspn.c lib_strchrnul.c lib_strdup.c

libs/libc/string/lib_memmem.c

@@ -0,0 +1,71 @@
+/****************************************************************************
+ * libs/libc/string/lib_memmem.c
+ *
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.  The
+ * ASF licenses this file to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance with the
+ * License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ *
+ ****************************************************************************/
+
+/****************************************************************************
+ * Included Files
+ ****************************************************************************/
+
+#include <string.h>
+
+/****************************************************************************
+ * Public Functions
+ ****************************************************************************/
+
+/****************************************************************************
+ * Name: memmem
+ *
+ * Description:
+ *   The memmem() function finds the start of the first occurrence of the
+ *   substring needle of length needlelen in the memory area haystack of
+ *   length haystacklen.
+ *
+ * Returned Value:
+ *   The memmem() function returns a pointer to the beginning of the
+ *   substring, or NULL if the substring is not found.
+ *
+ ****************************************************************************/
+
+FAR void *memmem(FAR const void *haystack, size_t haystacklen,
+                 FAR const void *needle, size_t needlelen)
+{
+  FAR const unsigned char *h = haystack;
+  FAR const unsigned char *n = needle;
+  size_t i;
+  size_t y;
+
+  if (needlelen > haystacklen)
+    {
+      return NULL;
+    }
+
+  for (i = 0; i < haystacklen - needlelen; i++)
+    {
+      y = 0;
+      while (h[i + y] == n[y])
+        {
+          if (++y == needlelen)
+            {
+              return (void *)(h + i);
+            }
+        }
+    }
+
+  return NULL;
+}