[Bug math/14561] New: srand() initializing seed for random() function

[Bug math/14561] New: srand() initializing seed for random() function

[jm3dev at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

             Bug #: 14561
           Summary: srand() initializing seed for random() function
           Product: glibc
           Version: 2.12
            Status: NEW
          Severity: normal
          Priority: P2
         Component: math
        AssignedTo: unassigned@sourceware.org
        ReportedBy: jm3dev@gmail.com
    Classification: Unclassified


DESCRIPTION: This problem arises if you mix the use of this two ways of
generating random number: the srand()/rand() way and the srandom()/random()
function family. The issue is that srand() is somehow initializing the seed for
random() function, but it should be initialized *only* with srandom(). The
expected behavior is random() generator not modifying its seed until done by
srandom(), the current behavior is srand() changing that seed.

IMPACT: As happened to me with a scientific code, it could imply a portability
problem as this is working properly (from my humble point of view) in other
implementations of the library (I tested the different behavior between GNU
glibc 2.12 packaged in a Scientific Linux and the BSD/MacOS-Darwin one). I
detected also the problem in GNU implementation of cstdlib with C++.

HOW TO REPRODUCE: It's very easy, this is a test program:

---- START CODE ----
// Test random()
#include <stdio.h>
#include <stdlib.h>

int main() {

  int i;

  srand((unsigned)time(NULL));

  for (i=1;i<11;i++) {
    printf("%g \n",(random() / (double)0x7fffffff));
  }
}
---- CODE END ----

This is what you get with glibc-headers-2.12-1.80:

$ ./rndtest 
0.0940072
0.269949
0.729428
0.48224
0.0592921
0.4699
0.132521
0.973561
0.532405
0.167931
$ ./rndtest 
0.954241
0.686682
0.0374202
0.304717
0.497409
0.197191
0.496588
0.273147
0.345674
0.756533

This is the expected (what you get in the mentioned other implementation of
stdlib.c and cstdlib):

> ./rndtest 
0.840188 
0.394383 
0.783099 
0.79844 
0.911647 
0.197551 
0.335223 
0.76823 
0.277775 
0.55397 
> ./rndtest
0.840188 
0.394383 
0.783099 
0.79844 
0.911647 
0.197551 
0.335223 
0.76823 
0.277775 
0.55397 

Many thanks.

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[Bug math/14561] srand() initializing seed for random() function

[jm3dev at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

JM <jm3dev at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |jm3dev at gmail dot com

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[Bug math/14561] srand() initializes seed for random() function

[jm3dev at gmail dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

JM <jm3dev at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|srand() initializing seed   |srand() initializes seed
                   |for random() function       |for random() function

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[Bug libc/14561] srand() initializes seed for random() function

[jsm28 at gcc dot gnu.org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

Joseph Myers <jsm28 at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |drepper.fsp at gmail dot
                   |                            |com
          Component|math                        |libc

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[Bug libc/14561] srand() initializes seed for random() function

[neleai at seznam dot cz]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

Ondrej Bilka <neleai at seznam dot cz> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         AssignedTo|unassigned at sourceware    |neleai at seznam dot cz
                   |dot org                     |

--- Comment #1 from Ondrej Bilka <neleai at seznam dot cz> 2012-12-19 20:40:27 UTC ---
Created attachment 6784
  --> http://sourceware.org/bugzilla/attachment.cgi?id=6784
split random seeds

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[Bug libc/14561] srand() initializes seed for random() function

[roland at gnu dot org]

http://sourceware.org/bugzilla/show_bug.cgi?id=14561

Roland McGrath <roland at gnu dot org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |ASSIGNED
                 CC|                            |roland at gnu dot org

--- Comment #2 from Roland McGrath <roland at gnu dot org> ---
Originally the existing behavior was correct (srand/rand are just aliases for
srandom/random).  Later versions of 1003.1 started specifying srandom as well
as srand, so this has become nonconforming.

http://pubs.opengroup.org/onlinepubs/9699919799/functions/rand.html             

    The implementation shall behave as if no function defined in this           
    volume of POSIX.1-2008 calls rand() or srand().                             

Note that srandom and random are defined in this volume:                        

http://pubs.opengroup.org/onlinepubs/9699919799/functions/random.html

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[Bug libc/14561] srand() initializes seed for random() function

[fweimer at redhat dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=14561

Florian Weimer <fweimer at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |fweimer at redhat dot com
              Flags|                            |security-

--- Comment #3 from Florian Weimer <fweimer at redhat dot com> ---
Not marking as security because these random number generators should not be
used for anything security-related.

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