completing GCC required string functions

src/lib/string.c

@@ -1,12 +1,45 @@
 #include <stddef.h>
 #include "string.h"
 
-void *memset(void *s, int c, size_t n)
+void *memset(void *dest, int val, size_t size)
 {
-    unsigned char *p = s;
-    while (n--)
+    unsigned char *d = dest;
+    while (size--)
     {
-        *p++ = (unsigned char)c;
+        *d++ = (unsigned char)val;
     }
-    return s;
+    return dest;
+}
+
+void *memcpy(void* dest, const void* src, size_t size)
+{
+    unsigned char *d = dest;
+    const unsigned char *s = src;
+    while(--size)
+    {
+        *d++ = *s++;
+    }
+    return dest;
+}
+
+int strcmp(const char * str1, const char * str2)
+{    while(*str1 == *str2)
+    {
+        if (*str1 == '\0')
+        {
+            return 0;
+        }
+        str1++;
+        str2++;
+    }
+    return *str1 - *str2;
+}
+
+size_t strlen(const char* str)
+{
+    size_t c = 0;
+    while(*str++ != '\0'){
+        c++;
+    }
+    return c;
 }