Date: Wed, 11 Oct 2017 17:48:09 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: Karl Pielorz <kpielorz_lst@tdx.co.uk> Cc: freebsd-hackers@freebsd.org Subject: Re: fprintf - threadsafe? - i.e. with process linked against '-pthread'? Message-ID: <20171011144809.GA95911@kib.kiev.ua> In-Reply-To: <03DA6274A199550235DC7351@[10.12.30.106]> References: <03DA6274A199550235DC7351@[10.12.30.106]>
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On Wed, Oct 11, 2017 at 10:13:35AM +0100, Karl Pielorz wrote: > > Hi, > > I have a number of 10.3-R amd64 boxes which runs a heavily threaded > process. This is linked against '-pthread' - and compiles / runs fine. > > Using 'fprintf' to log data to a file - it sometimes doesn't complete > writing the line - e.g. literally in code: > > fprintf( fd, "The quick brown %s jumped over the slow lazy animal\n", > animal ); > > Will sometimes result in: > > " > The quick brown fox ju" > > Being written to the file. > > > 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). > > The process doesn't crash - but I can't understand why / how frpintf could > either stop, or get stopped 'mid way' through? > > e.g. If a signal occurred would it complete the write to file? > > This only happens very, very occasionally (one fprintf out of many > millions, with hundreds of threads running). > > Just a bit stumped as to what to try looking at to fix / debug the problem > - if anyone has any suggestions, or further reading I can look at. Does the program use cancellation ? If yes, it might be an issue solved by the r321074. Otherwise, you need to debug the program. I usually use ktrace for start, but if the event is rare and program intensively issues syscalls, you would need to develop some ad-hoc tecnhique.
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