On 01/26/2012 10:07 AM, Bruno Haible wrote: > Eric Blake wrote: >> Thread one calls pipe2(O_CLOEXEC), but gets the gnulib version, which >> first calls pipe(), then we context switch. >> Thread two calls fork(), and in the child, calls exec() > > Right, this is why pipe2 should better be atomic. > > But this goes beyond of what is normally understood by multithread-safety: > it affects only programs which use threads *and* fork child processes. > 1) There are programs (like 'sort' or 'msgmerge') which use threads but > no child processes.
True - at which point pipe2() is great for syntactic sugar. > 2) IIRC, in some past POSIX version fork() was declared incompatible with > threads (or was is exec()?). Actually, the current version of POSIX requires fork() to be thread-safe, but glibc's implementation is not thread-safe [1], and the _next_ version of POSIX will be recommending posix_spawn() and a new _Fork() as the only thread-safe functions [2] (where _Fork() is like fork() except that pthread_atfork() can't interfere). Which is kind of sad - the whole reason pthread_atfork() was introduced was to attempt to make fork() thread-safe; but in the process of introducing it, pthread_atfork() became the reason why fork() is not threadsafe. [1] http://sourceware.org/bugzilla/show_bug.cgi?id=4737 [2] http://austingroupbugs.net/view.php?id=62 > +++ doc/glibc-functions/accept4.texi Thu Jan 26 17:59:28 2012 > @@ -10,7 +10,8 @@ > This function is missing on many non-glibc platforms: > MacOS X 10.5, FreeBSD 6.0, NetBSD 5.0, OpenBSD 3.8, Minix 3.1.8, AIX 5.1, > HP-UX 11, > IRIX 6.5, OSF/1 5.1, Solaris 11 2011-11, Cygwin 1.7.1, mingw, MSVC 9, > Interix 3.5, BeOS. > -But the replacement function is not atomic in multi-threaded programs. > +But the replacement function is not atomic; this matters in multi-threaded > +programs that spawn child processes. I like it. That's a definite improvement and accurate characterization of the issue. -- Eric Blake ebl...@redhat.com +1-919-301-3266 Libvirt virtualization library http://libvirt.org
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