fprintf - threadsafe? - i.e. with process linked against '-pthread'?

[Konstantin Belousov]

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.