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


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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