Sub-optimal libc's read-ahead buffering behaviour

Peter Jeremy PeterJeremy at optushome.com.au
Thu Aug 4 19:39:01 GMT 2005


On Thu, 2005-Aug-04 14:52:08 +0400, Andrey Chernov wrote:
>But there is no similar user-visible knob to turn on/off fseek's in-buffer 
>seeking, so it is always off for chardev for more safety.
...
>> In both cases above, seek really needs to be intelligent - more so
>> than for regular files.  It needs to lseek() in multiples of the
>> device block size and then adjust the buffer offset to handle any
>> remainder.
>
>I don't understand this statement well enough. Currently fseek always 
>sense in-buffer data for regular files for both SEEK_SET and SEEK_CUR.

Consider buffered stdio to a disk device.  The underlying device
requires I/O to be in multiples of 512 bytes with an offset at
multiples of 512 bytes.  IMHO, these alignment requirements should
be hidden from the user - I should be able to write code like:
	c = getc(disk);
	fseek(disk, 3, SEEK_CUR);
	w = getw(disk);
	fseek(disk, 1, SEEK_CUR);
	c1 = getc(disk);
	fseek(disk, c1, SEEK_CUR);
to work my way through data on the disk.  Currently, I can't do that
because the fseek() is transparent and the underlying lseek() will
fail.  Instead, I need to write code like:
	fread(buf, sizef(buf), 1, disk);
	char *bp = buf;
	...
	/* getc */
	c = *bp++;
	if (bp == buf + sizeof(buf)) {
		fread(buf, sizef(buf), 1, disk);
		bp = buf;
	}
	/* fseek(+3) */
	bp += 3;
	if (bp >= buf + sizeof(buf)) {
		fread(buf, sizef(buf), 1, disk);
		bp = buf;
	}
	/* getw */
	if (buf + sizeof(buf) - bp > sizeof(int)) {
		w = *(int *)bp;
		bp += sizeof(int);
	} else {
		int	i;

		for (w = i = 0; i < sizeof(int); i++)
			w |= *bp++ << (i * 8);
			if (bp == buf + sizeof(buf)) {
				fread(buf, sizef(buf), 1, disk);
				bp = buf;
			}
		}
	}
	/* fseek(+1) */
	bp++;
	if (bp >= buf + sizeof(buf)) {
		fread(buf, sizef(buf), 1, disk);
		bp = buf;
	}
	/* getc */
	c1 = *bp++;
	if (bp == buf + sizeof(buf)) {
		fread(buf, sizef(buf), 1, disk);
		bp = buf;
	}
	/* fseek(+c1) */
	bp += c1;
	if (bp >= buf + sizeof(buf)) {
		fread(buf, sizef(buf), 1, disk);
		bp = buf;
	}
ie, I've had to implement my own buffering which basically negates
the whole purpose of stdio.  (Note that both sets of code examples
need error checking added - which significantly increases the size
of each).

-- 
Peter Jeremy


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