Added more string functions

src/string.c

 #include "string.h"
+#include <stdbool.h>
 
 void *memset(void *s, int c, size_t n){
     unsigned char* p=s;
@@ -48,3 +49,62 @@ size_t strlen(const char* str)
 	ret++;
     return ret;
 }
+
+int strcmp(const char* s1, const char* s2)
+{
+    while(*s1 && (*s1==*s2))
+    {
+        s1++,s2++;
+    }
+    return *(const unsigned char*)s1-*(const unsigned char*)s2;
+}
+
+int strncmp(const char *s1, const char *s2, size_t n)
+{
+    for ( ; n > 0; s1++, s2++, --n)
+	if (*s1 != *s2)
+	    return ((*(unsigned char *)s1 < *(unsigned char *)s2) ? -1 : +1);
+	else if (*s1 == '\0')
+	    return 0;
+    return 0;
+}
+
+int atoi(char *p) 
+{
+    int k = 0;
+    while (*p) {
+        k = (k<<3)+(k<<1)+(*p)-'0';
+        p++;
+     }
+     return k;
+}
+
+bool isNumericChar(char x)
+{
+    return (x >= '0' && x <= '9')? true: false;
+}
+
+int stoi(char *str)
+{
+    if (*str == NULL)
+       return 0;
+ 
+    int res = 0;
+    int sign = 1;
+    int i = 0; 
+ 
+    if (str[0] == '-')
+    {
+        sign = -1;
+        i++;
+    }
+ 
+    for (; str[i] != '\0'; ++i)
+    {
+        if (isNumericChar(str[i]) == false)
+            return 0;
+        res = res*10 + str[i] - '0';
+    }
+ 
+    return sign*res;
+}

src/string.h

@@ -5,5 +5,8 @@
 void* memset(void *s, int c, size_t n);
 void itoa(int n, char s[]);
 size_t strlen(const char* str);
-
+int strcmp(const char* s1, const char* s2);
+int strncmp(const char* s1, const char* s2, size_t n);
+int atoi(char *p);
+int stoi(char* str);
 #endif