09-27-2009
Probably the srand/rand functions use some kind of internal state which is bound to the process. When you call fork, the whole process is duplicated, including the state of the randomizer. Either seed it for each process seperately, or use some independent random source (like /dev/random).
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rand(3C) Standard C Library Functions rand(3C)
NAME
rand, srand, rand_r - simple random-number generator
SYNOPSIS
#include <stdlib.h>
int rand(void);
void srand(unsigned int seed);
int rand_r(unsigned int *seed);
DESCRIPTION
The rand() function uses a multiplicative congruential random-number generator with period 2**32 that returns successive pseudo-random num-
bers in the range of 0 to RAND_MAX (defined in <stdlib.h>).
The srand() function uses the argument seed as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to
rand(). If srand() is then called with the same seed value, the sequence of pseudo-random numbers will be repeated. If rand() is called
before any calls to srand() have been made, the same sequence will be generated as when srand() is first called with a seed value of 1.
The rand_r() function has the same functionality as rand() except that a pointer to a seed seed must be supplied by the caller. If
rand_r() is called with the same initial value for the object pointed to by seed and that object is not modified between successive calls
to rand_r(), the same sequence as that produced by calls to rand() will be generated.
The rand() and srand() functions provide per-process pseudo-random streams shared by all threads. The same effect can be achieved if all
threads call rand_r() with a pointer to the same seed object. The rand_r() function allows a thread to generate a private pseudo-random
stream by having the seed object be private to the thread.
USAGE
The spectral properties of rand() are limited. The drand48(3C) function provides a better, more elaborate random-number generator.
When compiling multithreaded applications, the _REENTRANT flag must be defined on the compile line. This flag should be used only in mul-
tithreaded applications.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
+-----------------------------+-----------------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+-----------------------------+-----------------------------+
|Interface Stability |Standard |
+-----------------------------+-----------------------------+
|MT-Level |Safe |
+-----------------------------+-----------------------------+
SEE ALSO
drand48(3C), attributes(5), standards(5)
SunOS 5.10 19 May 2004 rand(3C)