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fork and rand()

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# 1  
Old 12-14-2009
fork and rand()
Hi,
I want build 10 processus with fork and that each processus write a value betwen 0 and 9 , but each processus send the same value .

So my code ,you can compile and try .
Code:
#include <time.h> 
#include <stdio.h> 
#include <stdlib.h> 
#include <sys/types.h> 
#include <sys/ipc.h> 
#include <sys/shm.h> 
#include <sys/sem.h> 
#include <sys/wait.h>
#include <signal.h>
#include <unistd.h>

int pid;
main()
{
    srand(time(NULL));
    int i=10;
    while(i>0)
    {
        pid=fork();
        if(pid==0) 
        {
            //srand(time(NULL));
            printf("\n %d ",rand()%10);
            i=-1;
        }
        i--;
    }
exit(0); 
}

Can you tell me how use rand() corectly?
# 2  
Old 12-14-2009
Simple problem, simple solution. You're initializing the PRNG once, and the state of it is copied into each child, giving the same value in each process. Move the initialization into the child, and everything should work as expected.
# 3  
Old 12-14-2009
I am ok with you moreaver is more logic
but i have already try to move srand(time(NULL)) in child .

Code:
int pid;
main()
{
   // srand(time(NULL));
    int i=10;
    while(i>0)
    {
        pid=fork();
        if(pid==0) 
        {
            srand(time(NULL));
            printf("\n %d ",rand()%10);
            i=-1;
        }
        i--;
    }
exit(0); 
}


On my solaris i have
7
7
7
7
...
# 4  
Old 12-14-2009
I'm sorry, my bad. I really should think code through.

Moving the srand() into the child is the first step. The second is to find some way to better initialize it. srand() initializes the PRNG based on the seed value passed, in this case the seconds since the epoch. And unless you're using a computer from way before 1980 (which is very doubtful), you'll spawn the 9 processes in under 1 second.

You could try initializing it using a few bytes from /dev/random, if it's available on the system, or by sleep()ing for a second between each call.
# 5  
Old 12-14-2009
...or just reading bytes from /dev/random instead of using rand() at all.
# 6  
Old 12-14-2009
Good idea with sleep this turn on.
But I don't understand the answer of Corona.

Does it exits a method for initialize srand with µs ?
# 7  
Old 12-14-2009
Quote:
Originally Posted by carton99
But I don't understand the answer of Corona.
Bytes read from /dev/random are random. Having read them, you already have a random number.
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