strcat, strcpy, strlen, strcmp实现

// 方法strcat

char *strcat(char *strDest, const char *strSrc)

{

         assert((strDest != NULL) && (strSrc != NULL));

         char *address = strDest;

 

         while(*strDest)

                   strDest ++;

         while((*strDest++ = *strSrc++) != '\0')

                   NULL;

 

         return address;

}

// 方法strncat

char *strncat(char *strDest, const char *strSrc, size_t count)

{

         assert((strDest != NULL) && (strSrc != NULL));

         char *address = strDest;

        

        while(*strDest)

                   strDest ++;

 

         while (count --)

                   if ((*strDest++ = *strSrc++) == '\0')

                            return  address;

        

         *strDest = '\0';

         return address;

}

// 方法strcpy

char *strcpy(char *strDest, const char *strSrc)

{

         assert((strDest != NULL) && (strSrc != NULL));

         char *address = strDest;

         while((*strDest++ = *strSrc++) != '\0')

                   NULL;

         return address;

}

// 方法strncpy

char *strcpy(char *strDest, const char *strSrc, size_t count)

{

         assert((strDest != NULL) && (strSrc != NULL));

         char *address = strDest;

        

         while (count && ((*strDest++ = *strSrc++) != '\0'))

                   count --;

         if (count)

                   while(--count)

                            *strDest++ = '\0';

        

         return address;

}

// 方法strlen

int strlen(const char *s)

{

        assert( s != NULL);

         char *p = s;

        

         while(*p != '\0')

                   p++;

         return p - s;

}

// 方法strcmp

int strcmp(const char *src, const char *dst)

{

         int ret = 0;

 

         while( ! (ret = *(unsigned char *)src - *(unsigned char *)dst) && *dst)

             ++src, ++dst;

   

         if ( ret < 0 )

             ret = -1 ;

         else if ( ret > 0 )

            ret = 1 ;

   

         return( ret );

}