Sub-optimal libc's read-ahead buffering behaviour

Andrey Chernov ache at FreeBSD.ORG
Fri Aug 5 09:38:52 GMT 2005

On Fri, Aug 05, 2005 at 05:38:48AM +1000, Peter Jeremy wrote:
> 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:

If the underlying lseek() fails, it sounds like _driver_ problem. Ideally 
it should hide all internal I/O granulation from the user and allow seek 
to any position. If the position is in the middle, the driver can transfer 
only needed part of block. So, don't blame _stdio_ for not nice driver 

Looking more practical, aligned seeks and blocked read are enough in most 
situations, so there is no urgent demand to complicate drivers to do nice 


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