sub string in c

#include<stdio.h>
int sub_string(char *,char *);
int main()
{
    char s1[]="my name is programmer";
    char s2[]="my";
   

if(sub_string("my name is abdullah","my")!=NULL)
{
    printf("copy successfully\n");

}
system("pause");
return 0;
}
int sub_string(char *s1,char *s2)
{
    char *p;
    char *p2;
    int i;
    for(i=0;s1[i];i++)
    {

    p=&s1[i];
    p2=s2;
    while(*p2&&*p2==*p)
    {
    p++;
    p2++;
}
return -1;
}
}