Date: Thu, 12 Oct 2017 00:54:17 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: freebsd-hackers@freebsd.org Subject: Re: fprintf - threadsafe? - i.e. with process linked against '-pthread'? Message-ID: <20171011215417.GH95911@kib.kiev.ua> In-Reply-To: <20171011213452.GB32399@britannica.bec.de> References: <03DA6274A199550235DC7351@[10.12.30.106]> <20171011213452.GB32399@britannica.bec.de>
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On Wed, Oct 11, 2017 at 11:34:52PM +0200, Joerg Sonnenberger wrote: > On Wed, Oct 11, 2017 at 10:13:35AM +0100, Karl Pielorz wrote: > > Presumably (and from what I can see) fprintf is 'thread safe'? - And it also > > appears multiple threads could write to a single file using it (i.e. it > > provides for atomic writes so lines won't intermingle - the lines written > > don't seem to intermingle). > > stdio does not provide atomic IO. If you have two file handles or even > just two separate FILE instances sharing the same file handle and two > threads or processes are writing to them concurrently, buffering will > result in seemingly random interwoven output. Even using non-buffered IO > can trigger the same behavior as there is internal blocking going on. > Try a printf that created i.e. 16K output or so in one way, it should > increase the chance of seeing this a lot. Indeed stdio does not provide atomic IO, but it is thread-safe. That is, each isolated stdio operation, except the ones with explicit _unlocked suffix, are required to take an exclusive ownership on the FILE for the duration of the operation. See the description of flockfile().
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