memmove

一、函数的引出

首先是这个函数在笔试的时候经常会被问到，另一个就是C库实现的一些技巧以及这个函数本身的一些保证。

二、说明

memmove的说明：将src拷贝到dst，其语义等价于首先将源拷贝到一个和目的、源都不搭的空间中，然后将这个中间空间的内存拷贝到目的空间。

这里最重要的就是当源和目的之间有重合的时候，memmove保证目的是源的一个完整的重复，这一点在源和目的在长度范围内有重合的时候就比较有意义。

三、GLIBC的实现

rettype
memmove (a1, a2, len)
     a1const void *a1;
     a2const void *a2;
     size_t len;
{
  unsigned long int dstp = (long int) dest;
  unsigned long int srcp = (long int) src;

  /* This test makes the forward copying code be used whenever possible.
     Reduces the working set.  */
  if (dstp - srcp >= len) /* *Unsigned* compare!  */ 这里非常诡异而巧妙，是两个无符号数的比较。
    {
      /* Copy from the beginning to the end.  */

      /* If there not too few bytes to copy, use word copy.  */
      if (len >= OP_T_THRES)
 {
   /* Copy just a few bytes to make DSTP aligned.  */
   len -= (-dstp) % OPSIZ;
   BYTE_COPY_FWD (dstp, srcp, (-dstp) % OPSIZ);

   /* Copy whole pages from SRCP to DSTP by virtual address
      manipulation, as much as possible.  */

   PAGE_COPY_FWD_MAYBE (dstp, srcp, len, len);

   /* Copy from SRCP to DSTP taking advantage of the known
      alignment of DSTP.  Number of bytes remaining is put
      in the third argument, i.e. in LEN.  This number may
      vary from machine to machine.  */

   WORD_COPY_FWD (dstp, srcp, len, len);

   /* Fall out and copy the tail.  */
 }

      /* There are just a few bytes to copy.  Use byte memory operations.  */
      BYTE_COPY_FWD (dstp, srcp, len);
    }
  else
    {
      /* Copy from the end to the beginning.  */
      srcp += len;
      dstp += len;

      /* If there not too few bytes to copy, use word copy.  */
      if (len >= OP_T_THRES)
 {
   /* Copy just a few bytes to make DSTP aligned.  */
   len -= dstp % OPSIZ;
   BYTE_COPY_BWD (dstp, srcp, dstp % OPSIZ);

   /* Copy from SRCP to DSTP taking advantage of the known
      alignment of DSTP.  Number of bytes remaining is put
      in the third argument, i.e. in LEN.  This number may
      vary from machine to machine.  */

   WORD_COPY_BWD (dstp, srcp, len, len);

   /* Fall out and copy the tail.  */
 }

      /* There are just a few bytes to copy.  Use byte memory operations.  */
      BYTE_COPY_BWD (dstp, srcp, len);
    }

  RETURN (dest);
}