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Message-ID:	<51671EAA.8030901@net-b.de>
Date:	Thu, 11 Apr 2013 22:35:54 +0200
From:	Tobias Burnus <burnus AT net-b DOT de>
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To:	cygwin AT cygwin DOT com, fortran AT gcc DOT gnu DOT org
Subject:	Re: Cygwin with clock_gettime and CLOCK_MONOTONIC - gives always 0
References:	<5166A0B3 DOT 70801 AT net-b DOT de> <5166BCA1 DOT 5040204 AT net-b DOT de> <20130411134835 DOT GD18333 AT calimero DOT vinschen DOT de> <51670B6B DOT 3000503 AT net-b DOT de> <20130411195200 DOT GJ18333 AT calimero DOT vinschen DOT de>
In-Reply-To:	<20130411195200.GJ18333@calimero.vinschen.de>

Corinna Vinschen wrote:
> On Apr 11 21:13, Tobias Burnus wrote:
>> Where is actually the source code of the clock_gettime, which Cygwin
>> uses? I thought that it was newlib. But looking at
>> http://sourceware.org/cgi-bin/cvsweb.cgi/src/newlib/libc/sys/linux/clock_gettime.c?cvsroot=src
>> the code should return -1 for "clock_gettime (CLOCK_MONOTONIC" as
>> CLOCK_MONOTONIC (= 4) is not handled, only CLOCK_REALTIME (= 1,
>> according to src/newlib/libc/include/time.h).
>>
>> However, as both you and Angelo get a zero return value, I must look
>> at the wrong file.
> Yes, for Cygwin it's
> http://cygwin.com/cgi-bin/cvsweb.cgi/src/winsup/cygwin/timer.cc?cvsroot=src

Well, it's "times.cc" not "timer.cc". I think the problem with Cygwin 
was a red herring. But there are a bunch of problems:


a) Angelo's code to initialize the random seed assumes that Fortran's 
"system_clock" and the "clock_gettime(CLOCK_MONOTONIC" it calls give a 
result where 0 is the beginning of the epoch (like on Linux/glibc) or 
the system start up (as Windows' GetTickCount/GetTickCount64). However, 
QueryPerformanceCounter (probably intrinsically, but at least as used by 
Cygwin) returns some time based on the startup time - and as the 
frequency seems to be slow such that the result is 0 or (nearly) the 
same number for the startup. Thus, the random_seed initialization 
doesn't work as expected on Cygwin/Windows.

b) Newlib is broken. For clock_gettime, CLOCK_MONOTONIC is not 
implemented. But both CLOCK_MONOTONIC and even _POSIX_MONOTONIC_CLOCK 
(with value 200112L) are defined. However, POSIX states: "If the 
Monotonic Clock option is supported, all implementations shall support a 
clock_id of CLOCK_MONOTONIC defined in <time.h>."


Thus:

* Newlib should be fixed to not define CLOCK_MONOTONIC and 
_POSIX_MONOTONIC_CLOCK - or it should implement it, otherwise it doesn't 
follow POSIX.

* Angelo shouldn't use system_clock to set the random seed.

* libgfortran should fall back to gf_gettime (= gettimeofday or 
clock_gettime CLOCK_REALTIME or time) if clock_gettime returns nonzero 
to work around the broken netlib (and possibly other implementations of 
clock_gettime)

* gfortran's example for random_see should be change to not use 
system_clock for the random seed.


Sorry for misinterpreting the results of Cygwin. And thanks for the 
reply and testing.

Tobias

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