On Sat, Mar 25, 2017 at 1:34 PM, Florian Weimer <f...@deneb.enyo.de> wrote: > * Yann Ylavic: >> "In the current POSIX.1 specification (POSIX.1-2008), >> readdir() is not required to be thread-safe. However, in modern >> implementations (including the glibc implementation), concurrent calls >> to readdir() that specify different directory streams are thread-safe. >> In cases where multiple threads must read from the same directory >> stream, using readdir() with external synchronization is still >> preferable to the use of the deprecated readdir_r(3) function. It is >> expected that a future version of POSIX.1 will require that readdir() >> be thread-safe when concurrently employed on different directory >> streams." > > It's been a bit of a struggle to get this right.
I think the confusion comes from the term "directory stream", which people (at least me :p ) may read as underlying directory (i.e. filesystem's), though it's the term used to talk about the DIR* in the whole man page... > People are really > concerned about the case where multiple threads read from the same > directory stream. But how often does that happen in practice? I now realize that there is no real thread-safety issue, it indeed shouldn't happen to call readdir() concurrently on the same DIR, IMHO the doc should say that it's simply undefined behaviour (actually I thought it was the case with readdir_r too, so the readdir_r part is the misleading one I think). >> >> Are you thinking of the above editorial? > > I meant this part: > > Preliminary: | MT-Unsafe race:dirstream | AS-Unsafe lock | AC-Unsafe > lock | See POSIX Safety Concepts. > > <https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html> > > “MT-Unsafe race:dirstream” doesn't make much sense because we don't > have this as a category for memcpy because for some reason, it is > “obvious” for memcpy that it's only thread-safe if it is called for > completely separate arguments. OK, got it now, thanks! > > I think the Solaris manual also does not mark readdir as thread-safe, > implicitly suggesting to use readdir_r in multi-threaded programs. > But this suggestion isn't helpful on Solaris, either. So Solaris' readdir() does not return static data either. Any idea of which system keeps doing this?
