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Top Forums Programming Random numbers in parent/child? Post 302356744 by xyzt on Sunday 27th of September 2009 09:41:06 AM
Old 09-27-2009
Random numbers in parent/child?

Hi

I'm trying to generate random numbers both in parent process and the child process. But I get the same random number in both processes. Why doesn't it generate different numbers altough I seed random number generator?
Here's my code:

Code:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <time.h>

void seedit(void)
{
    struct timeval tv;
    gettimeofday(&tv, NULL);
    srand(tv.tv_sec * tv.tv_usec);
}

int main()
{
        int pid;
        seedit();
        pid = fork();
        if (pid==0)
        {
                int r = rand()%10 + 1;
                printf("[CHILD] sleeping %d sec\n", r);
                sleep(r);
        }
        else if (pid>0)
        {
                int r = rand()%10 + 1;
                printf("[PARENT] sleeping %d sec\n", r);
                sleep(r);
        }
        else
        {
        }
}

 

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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 5.11 19 May 2004 rand(3C)
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