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From:	"James E. King III" <jking AT apache DOT org>
Date:	Fri, 23 Nov 2018 11:27:37 -0500
Message-ID:	<CAOWZHxdJi1E9APtSXm35LkDU33VqGZSwnGtvOfdHA9PTTJgRjQ@mail.gmail.com>
Subject:	No thread safety in clock_gettime (hires_ns::prime)
To:	cygwin AT cygwin DOT com
X-IsSubscribed:	yes

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Using 32-bit cygwin that I set up yesterday.    I found that a call to
clock_gettime(CLOCK_MONOTONIC, ..) has a one-time initialization that
is not thread-safe.  If two threads call this at the same time, they
will race.  The results I am seeing are typically that one of the two
callers get a timespec structure with zero values, and no error return
code from the call.

I prepared a small test that exposes this issue.  Given it is a race
condition, you have to run the test in a loop for it to happen.  I
have attached the Makefile and source code.  If you run "make test" it
will expose the issue:

$ make test
cc    -c -o cyg_hires_clock_race.o cyg_hires_clock_race.c
cc   cyg_hires_clock_race.o   -o cyg_hires_clock_race
ERROR: one of the timespec structures was zero:
main thread: tv_sec =     356242 tv_nsec =  376075900
 2nd thread: tv_sec =          0 tv_nsec =          0
ERROR: one of the timespec structures was zero:
main thread: tv_sec =     356242 tv_nsec =  519016800
 2nd thread: tv_sec =          0 tv_nsec =          0
ERROR: one of the timespec structures was zero:
main thread: tv_sec =     356242 tv_nsec =  734794100
 2nd thread: tv_sec =          0 tv_nsec =          0
ERROR: one of the timespec structures was zero:
main thread: tv_sec =     356243 tv_nsec =  463632400
 2nd thread: tv_sec =          0 tv_nsec =          0
make: *** [Makefile:6: test] Error 1

Relevant cygwin version information:

Cygwin DLL version info:
        DLL version: 2.11.2
        DLL epoch: 19
        DLL old termios: 5
        DLL malloc env: 28
        Cygwin conv: 181
        API major: 0
        API minor: 329
        Shared data: 5
        DLL identifier: cygwin1
        Mount registry: 3
        Cygwin registry name: Cygwin
        Installations name: Installations
        Cygdrive default prefix:
        Build date:
        Shared id: cygwin1S5

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