Re: [Boost-users] [iostreams] blocking read() confusion

From: Pedro Lamarão (pedro.lamarao_at_[hidden])
Date: 2006-05-23 11:01:21

Dave Dribin escreveu:
> According to the documentation I/O Streams defines blocking I/O as:
> 
>    <http://www.boost.org/libs/iostreams/doc/concepts/blocking.html>
> 
>> A Device is Blocking if a read request never produces fewer  
>> characters than requested except at end-of-stream, and if a write  
>> request never consumes fewer characters than requested under any  
>> circumstances.
> 
> So, if I issue an iostream::read() with 10 characters, and only 5  
> come back, this is considered non-blocking.  To me, this is  
> inconsistent with at least Unix read() semantics.  As long as  
> iostream::read() blocks for at least 1 character, it should be  
> considered blocking, no?

I think you're confusing nomenclature.

"Blocking" means your process is kept out of the ready queue until the
system call is "satisfied". In this context, "satisfied" means you get
the data you asked for, or that became impossible, either because EOF
was reached, or because an error happened.

"Non-blocking" means your process is never kept out of the ready queue
like this; to achieve that, the first "satisfaction" requirement is
alleviated: a "non-blocking" call _may_ return less data than you asked for.

"Blocking" in this context does not mean "spending some time in a
function call (because it takes time to execute)", it means "being taken
out of the ready queue (because an I/O device takes time to answer a
request)".

--
 Pedro Lamarão