develooper Front page | perl.perl5.porters | Postings from May 2003

Re: Meaning of sysread()

Thread Previous | Thread Next
Nick Ing-Simmons
May 21, 2003 09:54
Re: Meaning of sysread()
Message ID:
Mark Mielke <> writes:
>Here is another argument:
>sysread() is documented ('man perlfunc') to '[attempt] to read LENGTH
>characters of data ... [bypassing] buffered IO ...'. If Perl sysread()
>is actually implemented using C read(), then it would be impossible to
>read LENGTH characters of data, and only LENGTH characters of data
>without calling C read() once for each byte. Remember - no buffering.

You read LENGTH bytes and see how many characters that is.
You find it is LENGTH-N chars so you read N more bytes and see where 
you are ... 

>You can't put the bytes back. Therefore, the documentation is wrong,
>or sysread() is implemented extremely inefficiently.

It isn't too bad the series converges quite quickly ;-)

>For Perl read(), we don't care, because read() is buffered and is not
>affected by the same performance issues mentioned earlier. Reading one
>byte at a time is not expensive for buffered PerlIO handles.

Nor necessary! as PerlIO can "unread" abitrary amount of data,
and :encoding snoops the buffer a perl-ish manner, and ...

>To validate this entire line of thinking I suggest that the following
>point be seriously considered:
>   - sysread() is supposed to be a more direct system read that avoids
>     the intermediate layers of processing. This definately includes STDIO,
>     and as far as I am concerned, it definatesly includes filtering.

That is certainly my _personal_ view too - but as designer/maintainer
I started this thread to (A) make sure that I was not alone in that view
and (B) to try and collect arguments for the other side as well.

The ticket was caused by Net::FTP script using sysread() and perl5.6.1
on Win32 did CRLF translation on that and it worked. 5.8 does not 
so file becomes CRCRLF on the wire and CRLF when de-ASCIIed at other end.

Nick Ing-Simmons

Thread Previous | Thread Next Perl Programming lists via nntp and http.
Comments to Ask Bjørn Hansen at | Group listing | About