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 < 0)
{
error (0, errno, _("reading %s"), quote (input_file));
if (conversions_mask & C_NOERROR)
{
print_stats ();
/* Seek past the bad block if possible. */
if (!advance_input_after_read_error (input_blocksize - partread))
{
exit_status = EXIT_FAILURE;
/* Suppress duplicate diagnostics. */
input_seekable = false;
input_seek_errno = ESPIPE;
}
if ((conversions_mask & C_SYNC) && !partread)
/* Replace the missing input with null bytes and
proceed normally. */
nread = 0;
else
continue;
}
else
{
/* Write any partial block. */
exit_status = EXIT_FAILURE;
break;
}
}
n_bytes_read = nread;
advance_input_offset (nread);
if (n_bytes_read < input_blocksize)
{
r_partial++;
partread = n_bytes_read;
if (conversions_mask & C_SYNC)
{
if (!(conversions_mask & C_NOERROR))
/* If C_NOERROR, we zeroed the block before reading. */
memset (ibuf + n_bytes_read,
(conversions_mask & (C_BLOCK | C_UNBLOCK)) ? ' ' : '\0',
input_blocksize - n_bytes_read);
n_bytes_read = input_blocksize;
}
}
else
{
r_full++;
partread = 0;
}
if (ibuf == obuf) /* If not C_TWOBUFS. */
{
size_t nwritten = iwrite (STDOUT_FILENO, obuf, n_bytes_read);
w_bytes += nwritten;
if (nwritten != n_bytes_read)
{
error (0, errno, _("writing %s"), quote (output_file));
return EXIT_FAILURE;
}
else if (n_bytes_read == input_blocksize)
w_full++;
else
w_partial++;
continue;
}
static size_t
iwrite (int fd, char const *buf, size_t size)
{
size_t total_written = 0;
while (total_written < size)
{
ssize_t nwritten;
process_signals ();
nwritten = write (fd, buf + total_written, size - total_written);
if (nwritten < 0)
{
if (errno != EINTR)
break;
}
else if (nwritten == 0)
{
/* Some buggy drivers return 0 when one tries to write beyond
a device's end. (Example: 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 < 0 && errno == EINTR))
return nread;
}
}
dd_copy函数
dd_copy (void)
{
char *ibuf, *bufstart; /* Input buffer. */
/* These are declared static so that even though we don't free the
buffers, valgrind will recognize that there is no "real" leak. */
static char *real_buf; /* real buffer address before alignment */
static char *real_obuf;
ssize_t nread; /* Bytes read in the current block. */
/* If nonzero, then the previously read block was partial and
PARTREAD was its size. */
size_t partread = 0;
int exit_status = EXIT_SUCCESS;
size_t n_bytes_read;
/* Leave at least one extra byte at the beginning and end of `ibuf'
for conv=swab, but keep the buffer address even. But some peculiar
device drivers work only with word-aligned buffers, so leave an
extra two bytes. */
/* Some devices require alignment on a sector or page boundary
(e.g. character disk devices). Align the input buffer to a
page boundary to cover all bases. Note that due to the swab
algorithm, we must have at least one byte in the page before
the input buffer; thus we allocate 2 pages of slop in the
real buffer. 8k above the blocksize shouldn't bother anyone.
The page alignment is necessary on any linux system that supports
either the SGI raw I/O patch or Steven Tweedies raw I/O patch.
It is necessary when accessing raw (i.e. character special) disk
devices on Unixware or other SVR4-derived system. */
real_buf = xmalloc (input_blocksize + INPUT_BLOCK_SLOP);
ibuf = real_buf;
ibuf += SWAB_ALIGN_OFFSET; /* allow space for swab */
ibuf = ptr_align (ibuf, page_size);
if (conversions_mask & C_TWOBUFS)
{
/* Page-align the output buffer, too. */
real_obuf = xmalloc (output_blocksize + OUTPUT_BLOCK_SLOP);
obuf = ptr_align (real_obuf, page_size);
}
else
{
real_obuf = NULL;
obuf = ibuf;
}
if (skip_records != 0)
{
skip (STDIN_FILENO, input_file, skip_records, input_blocksize, ibuf);
/* POSIX doesn't say what to do when dd detects it has been
asked to skip past EOF, so I assume it's non-fatal if the
call to 'skip' returns nonzero. FIXME: maybe give a warning. */
}
if (seek_records != 0)
{
uintmax_t write_records = skip (STDOUT_FILENO, output_file,
seek_records, output_blocksize, obuf);
if (write_records != 0)
{
memset (obuf, 0, output_blocksize);
do
if (iwrite (STDOUT_FILENO, obuf, output_blocksize)
!= output_blocksize)
{
error (0, errno, _("writing to %s"), quote (output_file));
quit (EXIT_FAILURE);
}
while (--write_records != 0);
}
}
if (max_records == 0)
return exit_status;
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',
input_blocksize);
nread = iread (STDIN_FILENO, ibuf, input_blocksize);
if (nread == 0)
break; /* EOF. */
if (nread < 0)
{
error (0, errno, _("reading %s"), quote (input_file));
if (conversions_mask & C_NOERROR)
{
print_stats ();
/* Seek past the bad block if possible. */
if (!advance_input_after_read_error (input_blocksize - partread))
{
exit_status = EXIT_FAILURE;
/* Suppress duplicate diagnostics. */
input_seekable = false;
input_seek_errno = ESPIPE;
}
if ((conversions_mask & C_SYNC) && !partread)
/* Replace the missing input with null bytes and
proceed normally. */
nread = 0;
else
continue;
}
else
{
/* Write any partial block. */
exit_status = EXIT_FAILURE;
break;
}
}
n_bytes_read = nread;
advance_input_offset (nread);
if (n_bytes_read < input_blocksize)
{
r_partial++;
partread = n_bytes_read;
if (conversions_mask & C_SYNC)
{
if (!(conversions_mask & C_NOERROR))
/* If C_NOERROR, we zeroed the block before reading. */
memset (ibuf + n_bytes_read,
(conversions_mask & (C_BLOCK | C_UNBLOCK)) ? ' ' : '\0',
input_blocksize - n_bytes_read);
n_bytes_read = input_blocksize;
}
}
else
{
r_full++;
partread = 0;
}
if (ibuf == obuf) /* If not C_TWOBUFS. */
{
size_t nwritten = iwrite (STDOUT_FILENO, obuf, n_bytes_read);
w_bytes += nwritten;
if (nwritten != n_bytes_read)
{
error (0, errno, _("writing %s"), quote (output_file));
return EXIT_FAILURE;
}
else if (n_bytes_read == input_blocksize)
w_full++;
else
w_partial++;
continue;
}
/* Do any translations on the whole buffer at once. */
if (translation_needed)
translate_buffer (ibuf, n_bytes_read);
if (conversions_mask & C_SWAB)
bufstart = swab_buffer (ibuf, &n_bytes_read);
else
bufstart = ibuf;
if (conversions_mask & C_BLOCK)
copy_with_block (bufstart, n_bytes_read);
else if (conversions_mask & C_UNBLOCK)
copy_with_unblock (bufstart, n_bytes_read);
else
copy_simple (bufstart, n_bytes_read);
}
/* If we have a char left as a result of conv=swab, output it. */
if (char_is_saved)
{
if (conversions_mask & C_BLOCK)
copy_with_block (&saved_char, 1);
else if (conversions_mask & C_UNBLOCK)
copy_with_unblock (&saved_char, 1);
else
output_char (saved_char);
}
if ((conversions_mask & C_BLOCK) && col > 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 < conversion_blocksize; i++)
output_char (space_character);
}
if ((conversions_mask & C_UNBLOCK) && col == conversion_blocksize)
/* Add a final '\n' if there are exactly `conversion_blocksize'
characters in the final record. */
output_char (newline_character);
/* Write out the last block. */
if (oc != 0)
{
size_t nwritten = iwrite (STDOUT_FILENO, obuf, oc);
w_bytes += nwritten;
if (nwritten != 0)
w_partial++;
if (nwritten != oc)
{
error (0, errno, _("writing %s"), quote (output_file));
return EXIT_FAILURE;
}
}
if ((conversions_mask & C_FDATASYNC) && fdatasync (STDOUT_FILENO) != 0)
{
if (errno != ENOSYS && errno != EINVAL)
{
error (0, errno, _("fdatasync failed for %s"), quote (output_file));
exit_status = EXIT_FAILURE;
}
conversions_mask |= C_FSYNC;
}
if (conversions_mask & C_FSYNC)
while (fsync (STDOUT_FILENO) != 0)
if (errno != EINTR)
{
error (0, errno, _("fsync failed for %s"), quote (output_file));
return EXIT_FAILURE;
}
return exit_status;
}
2、dd的本质
本质就是按照bs的大小作为一次Iread和Iwrite：
size_t nwritten = iwrite (STDOUT_FILENO, obuf, n_bytes_read);
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