[Bug libstdc++/21286] [4.0/4.1 Regression] filebuf::xsgetn vs pipes

["ncm-nospam at cantrip dot org" <gcc-bugzilla@gcc.gnu.org>]

------- Additional Comments From ncm-nospam at cantrip dot org  2005-04-30 02:11 -------
Just to add... if sgetn() loops reading until it gets n bytes, but 
underflow() accepts a pipe's short reads, then in_avail() will report 
the size of the short read.  Then, istream::read_some() will 
rdbuf->sgetn(p,rdbuf->in_avail()), thus matching POSIX pipe semantics 
when attached to a pipe.

(For those who don't know: as long as whoever is writing the
other end of the pipe writes no more than PIPE_BUF bytes into 
it, then POSIX requires that read() deliver the bytes on write 
boundaries.  Thus, if you're writing structs, the reader will 
see half a struct.  If you're writing short-enough lines, the 
reader will only ever see complete lines -- although you might
see more than one.  Watch out that PIPE_BUF is 4K or more 
on Linux, but only 512 bytes on BSD!)

It's exciting to finally get this right.  With a bit more work, 
I see now that we might be able to support nonblocking file 
descriptors, and maybe even zero-copy I/O, too!  Thank you, Ralf.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21286