dd命令的实质

copyking

1、dd命令的源代码：
while (1)
  {
   if (r_partial + r_full >= max_records)
  break;
   /* Zero the buffer before reading, so that if we get a read error,
  whatever data we are able to read is followed by zeros.
  This minimizes data loss. */
   if ((conversions_mask & C_SYNC) && (conversions_mask & C_NOERROR))
  memset (ibuf,
    (conversions_mask & (C_BLOCK | C_UNBLOCK)) ? ' ' : '\0',   设定内存空间为0
    input_blocksize);
   nread = iread (STDIN_FILENO, ibuf, input_blocksize);
   if (nread == 0)
  break;      /* EOF. */
   if (nread Linux 1.2.13 on /dev/fd0.)
    Set errno to ENOSPC so they get a sensible diagnostic. */
   errno = ENOSPC;
   break;
  }
   else
  total_written += nwritten;
  }
return total_written;
}
static ssize_t
iread (int fd, char *buf, size_t size)
{
for (;;)
  {
   ssize_t nread;
   process_signals ();
   nread = read (fd, buf, size);
   if (! (nread = max_records)
  break;
   /* Zero the buffer before reading, so that if we get a read error,
  whatever data we are able to read is followed by zeros.
  This minimizes data loss. */
   if ((conversions_mask & C_SYNC) && (conversions_mask & C_NOERROR))
  memset (ibuf,
    (conversions_mask & (C_BLOCK | C_UNBLOCK)) ? ' ' : '\0',
    input_blocksize);
   nread = iread (STDIN_FILENO, ibuf, input_blocksize);
   if (nread == 0)
  break;      /* EOF. */
   if (nread  0)
  {
   /* If the final input line didn't end with a '\n', pad
  the output block to `conversion_blocksize' chars. */
   size_t i;
   for (i = col; i

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