Matthew, On 2/23/09, Matthew Wilcox <matthew@xxxxxx> wrote: > On Sun, Feb 22, 2009 at 09:43:36PM -0500, Carlos O'Donell wrote: >> flock is not POSIX, it's an interface invented by 4.2BSD, and was >> previously emulated by glibc. The glibc wrapper implemented flock with >> fcntl and made sure to return EWOULDBLOCK. > > glibc's emulation of flock() using fcntl() really belongs to the Dark > Ages of Linux. According to the comments in the kernel, flock(2) was > added in June 1995. That's four years before parisc-linux really got > going. > > The semantics of flock-locks and fcntl-locks are really, really > different. Emulating one with the other was a really bad idea. Quite. >> > + ret = sys_flock(fd, cmd); >> > + if (ret == -EAGAIN) >> > + ret = -EWOULDBLOCK; /* fuck you HPUX */ >> >> A more robust solution would be? >> >> if ((ret == -EAGAIN) || (ret == -EWOULDBLOK)) >> ret = -EWOULDBLOCK > > How would that differ from what Kyle wrote? Why would you want to > assign -EWOULDBLOCK to ret if ret is already -EWOULDBLOCK? > >> This covers our ass since POSIX says that EAGAIN and EWOULDBLOCK *may* >> be the same. > > That's irrelevant. The only thing that matters is what the kernel does. > >> > but somehow I suspect this interchangeable use of EAGAIN and EWOULDBLOCK >> > is going to reveal latent problems in this part of the kernel I would >> > rather not delve into... >> >> The ABI is fixed, so all we can do is cleanup the uses in the kernel, >> and make sure we adhere to the documented APIs. > > We actually can do better than this ... > > #ifdef __KERNEL__ > #define EWOULDBLOCK EAGAIN > #else > #define EWOULDBLOCK /* whatever the fuck HPUX uses */ > #endif > > Now our kernel never returns -EWOULDBLOCK, only -EAGAIN. Correct > applications must check for both. Incorrect applications tend to only > check for AGAIN, not WOULDBLOCK. Problem solved. I'm not sure how you define "correct" here. An application that follows the BSD implementation standard need only check for EWOULDBLOCK from flock(), I believe. (I can find no system that documents otherwise for flock().) If you are talking more generally, POSIX.1-2001, which doesn't specify flock(), only refers to the use of EWOULDBLOCK in the context of socket-related calls, where it says that a non-blocking call can fail with either EAGAIN or EWOULDBLOCK, so I suppose that a portable sockets application must check for both. (In all other cases of non-blocking calls (e.g., read()/write() from files other than sockets, sigtimedwait(), lockf(), mq_send(), mq_receive()) that fail in circumstances where they would otherwise block, the error is always EAGAIN, with the exception noted earlier for fcntl(F_SETLK) (EACCES or EWOULDBLOCK).) Cheers, Michael -- Michael Kerrisk Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/ git://git.kernel.org/pub/scm/docs/man-pages/man-pages.git man-pages online: http://www.kernel.org/doc/man-pages/online_pages.html Found a bug? http://www.kernel.org/doc/man-pages/reporting_bugs.html -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
