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

From: Dave Dribin (dave-ml_at_[hidden])
Date: 2006-05-22 17:33:58

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 don't have a copy of the C++ standard handy, but my Stroustrup book  
just says that iostream::read(Ch *p, streamsize n) "returns at most n  
char".  Thus it heavily implies that reading less than "n char" is  
not only possible, but not an error or exceptional condition.  But  
the book really doesn't say anything more than that.

I am just curious because I am looking into using Boost Iostreams as  
a wrapper around some socket I/O that uses standard Unix read()  
semantics.  I don't use the socket in non-blocking mode, but since  
the Unix read() may return less than "n char", do I need to write a  
loop that ensures exactly "n char" is read, unless an EOF occurs?

Thanks,

-Dave