请问c语言如何实现？

/***
*strstr.c - search for one string inside another
*
*       Copyright (c) Microsoft Corporation. All rights reserved.
*
*Purpose:
*       defines strstr() - search for one string inside another
*
*******************************************************************************/
 
#include <cruntime.h>
#include <string.h>
 
/***
*char *strstr(string1, string2) - search for string2 in string1
*
*Purpose:
*       finds the first occurrence of string2 in string1
*
*Entry:
*       char *string1 - string to search in
*       char *string2 - string to search for
*
*Exit:
*       returns a pointer to the first occurrence of string2 in
*       string1, or NULL if string2 does not occur in string1
*
*Uses:
*
*Exceptions:
*
*******************************************************************************/
 
char * __cdecl strstr (
        const char * str1,
        const char * str2
        )
{
        char *cp = (char *) str1;
        char *s1, *s2;
 
        if ( !*str2 )
            return((char *)str1);
 
        while (*cp)
        {
                s1 = cp;
                s2 = (char *) str2;
 
                while ( *s1 && *s2 && !(*s1-*s2) )
                        s1++, s2++;
 
                if (!*s2)
                        return(cp);
 
                cp++;
        }
 
        return(NULL);
 
}