Date: Wed, 14 Feb 2018 23:20:42 -0500 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: fwrite() - possible division by zero On Wed, Feb 14, 2018 at 04:47:46PM -0500, Andrew Bell wrote: > On Wed, Feb 14, 2018 at 4:15 PM, Szabolcs Nagy <nsz@...t70.net> wrote: > > > * Andrew Bell <andrew.bell.ia@...il.com> [2018-02-14 15:11:34 -0500]: > > > On Wed, Feb 14, 2018 at 3:07 PM, Markus Wichmann <nullplan@....net> > > wrote: > > > > > > > On Wed, Feb 14, 2018 at 02:48:14PM -0500, Andrew Bell wrote: > > > > > Why not early return if size == 0 and avoid the call to __fwritex > > > > > altogether? > > > > > > > > > > > > > Because it's a rare corner case? Here, there's also locking correctness > > > > to consider: fwrite() has to block until f is unlocked, irrespective of > > > > parameters. So there's no real benefit to doing an early return. > > > > > > > > > > But it's already being checked to set nmemb to 0. Couldn't you just > > return > > > 0 and avoid the lock as well? > > > > the lock must not be avoided. > > > > otherwise fwrite would make progress on a FILE locked by > > another thread which is non-conforming. > > > That's not how I read this: http://port70.net/~nsz/c/c11/n1570.html#7.21.2p8 > > "All functions that read, write, position, or query the position of a > stream lock the stream before accessing it. > They release the lock associated with the stream when the access is > complete." > > When size == 0, the FILE doesn't need to be accessed so no lock should be > necessary. > Perhaps language of this document has been superseded? > > But it doesn't much matter. It just seemed to make the code more clear to > me and would have avoided the initial question. It's a POSIX requirement: "All functions that reference (FILE *) objects, except those with names ending in _unlocked, shall behave as if they use flockfile() and funlockfile() internally to obtain ownership of these (FILE *) objects." http://pubs.opengroup.org/onlinepubs/9699919799/functions/flockfile.html Rich
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.