mystrstr.c

#include "mystrstr.h"
#include <string.h>
#include <stdio.h>

char big_string[] = "Hello";
char sub_string[] = "ell";

char *str = big_string;
char *substr = sub_string;

char* mystrstr(char *s, char *subs) {
  char *a = s;
  char *b = subs;

  for( ; *s != '\0'; *s++) {
    if(*s != *b) {
      continue;
    }

    a = s;
    while(1) {
      if(*b == '\0') {
        return s;
      } else if(*a++ != *b++) {
        break;
      }
    }
    b = subs;
  }

  return (char *) NULL;
}

int main(void) {
  char *res = mystrstr(str, substr);
  if(res != (char *) NULL) {
    printf("Substring match starting at: %c\n", *mystrstr(str, substr));
  } else {
    printf("There was no match found.\n");
  }
  return 1;
}

mystrstr.h

#define NULL -1

char* mystrstr(char *s, char *subs);