String function (Part 2)

One 、memcpy

void * memcpy ( void * destination, const void * source, size_t num );

• function memcpy from source The position of begins to be copied back num Bytes of data to destination Memory location for .
• This function is encountering '\0' It doesn't stop .
• If source and destination There is any overlap , The results of replication are undefined .

Simulation Implementation ：

void * memcpy ( void * dst, const void * src, size_t count)
{
        void * ret = dst;
 assert(dst);
   assert(src);
        /*
         * copy from lower addresses to higher addresses
         */
        while (count--) {
                *(char *)dst = *(char *)src;
                dst = (char *)dst + 1;
                src = (char *)src + 1;
       }
        return(ret);
}

 

Two 、memmove 

void * memmove ( void * destination, const void * source, size_t num );

• and memcpy The difference is that memmove The source and target memory blocks processed by the function can overlap .
• If the source space and the target space overlap , You have to use memmove Function processing . 

void * memmove ( void * dst, const void * src, size_t count)
{
        void * ret = dst;
        if (dst <= src || (char *)dst >= ((char *)src + count)) {
                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst + 1;
                        src = (char *)src + 1;
               }
       }
        else {
                /*
                 * Overlapping Buffers
                 * copy from higher addresses to lower addresses
                 */
                dst = (char *)dst + count - 1;
                src = (char *)src + count - 1;
                while (count--) {
                        *(char *)dst = *(char *)src;
                        dst = (char *)dst - 1;
                        src = (char *)src - 1;
               }
       }
        return(ret);
}