strcpy(s1, s2); //Copies string s2 into string s1.
strcat(s1, s2); //Concatenates string s2 onto the end of string s1
strlen(s1); //Returns the length of string s1.
strcmp(s1, s2); //Returns 0 if s1 and s2 are the same; less than 0 if s1<s2; greater than 0 if s1>s2.
strchr(s1, ch); //Returns a pointer to the first occurrence of character ch in string s1.
strstr(s1, s2); //Returns a pointer to the first occurrence of string s2 in string s1.

#include <stdio.h>
#include <string.h>
int main () {
  char str1[12] = "Hello";
   char str2[12] = "World";
   char str3[12];
   int  len ;

   /* copy str1 into str3 */
   strcpy(str3, str1);
   printf("strcpy( str3, str1) :  %s\n", str3 );

/* concatenates str1 and str2 */
   strcat( str1, str2);
   printf("strcat( str1, str2):   %s\n", str1 );

   /* total lenghth of str1 after concatenation */
   len = strlen(str1);
   printf("strlen(str1) :  %d\n", len );

   return 0;
}