Re: Random seed initialization

[Tobias Burnus <burnus at net-b dot de>]

Ciao Angelo,

Angelo Graziosi wrote:
> $ ./a.exe
> gettimeofday:  err=0, secs=1365678623, usecs=437500
> clock_gettime: err=0, secs=1365678623, usecs=437500
> time:          err=0, secs=1365678623, usecs=0
> clock_gettime: err=0, secs=0, nsecs=0, tck=1000000000
>
> $ ./a.exe
> gettimeofday:  err=0, secs=1365678667, usecs=484375
> clock_gettime: err=0, secs=1365678667, usecs=484375
> time:          err=0, secs=1365678667, usecs=0
> clock_gettime: err=0, secs=0, nsecs=0, tck=1000000000

It seems as if Cygwin's clock_gettime correctly works for CLOCK_REALTIME 
but not for CLOCK_MONOTONIC.

In libgfortran:
/* POSIX states that CLOCK_REALTIME must be present if clock_gettime
   is available, others are optional.  */
#ifdef CLOCK_MONOTONIC
#define GF_CLOCK_MONOTONIC CLOCK_MONOTONIC
#else
#define GF_CLOCK_MONOTONIC CLOCK_REALTIME
#endif

But that doesn't help if CLOCK_MONOTONIC is defined but doesn't work. 
The question is whether checking additionally for _POSIX_MONOTONIC_CLOCK 
[1] will correctly detect this.

However, it seems as if Cygwin sets it - possibly without even 
supporting CLOCK_MONOTONIC, 
http://sourceware.org/ml/newlib/2010/msg00454.html  - In any case, the 
patch was approved before MONOTONIC_CLOCK was submitted. I couldn't find 
the actual patch, but I might have missed it.

And if I do a grep on my Linux system, I only find in 
/usr/include/bits/posix_opt.h:
  /* The monotonic clock might be available.  */
  #define _POSIX_MONOTONIC_CLOCK    0

That's better than the "-1" for not available but is not the, e.g., 
200809L which states that the support is indeed available.

Does anyone have a good suggestion how to detect the availability?

Tobias

[1] http://pubs.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap02.html