rand command line concurrency programming

Question

Hi i've got some problems with generating random numbers with concurrency programming, on exit() i would like ti use rand -M 6 but i don't know how, i've tried to use rand() in some ways but the childrens always returns the same number. Thanks a lot.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/sysinfo.h>
#include <sys/wait.h>
#include <errno.h>
#include <time.h>
#include <string.h>
#define NUM_KIDS 5

int i, sum;

int main()
{
    pid_t child_pid;
    int status;

    for (i=0; i<NUM_KIDS; i++) {
        switch (child_pid = fork()) {
        case -1:
            /* Handle error */
                fprintf(stderr,"Error #%03d: %s\n", errno, strerror(errno));
                break;

        case 0:
            /* Perform actions specific to child */

        printf("Hi, my PID is %d\n", getpid());

        exit(/*here i'd like to use rand -M 6*/);
        printf("Hi, my PID is %d and you should never see this message\n", getpid());
        break;

        default:
            /* Perform actions specific to parent */
            printf("I'm the proud parent with PID %d of a child with PID %d\n", getpid(), child_pid);
            break;
        }
    }

    i = 0;

    while ((child_pid = wait(&status)) != -1) {
        printf("PARENT: PID=%d. Got info of child with PID=%d, status=%d\n",
               getpid(), child_pid, status/256);
        sum += status/256;
    }
    if (errno == ECHILD) {
        printf("In PID=%6d, no more child processes, sum: %d\n", getpid(), sum);
        exit(EXIT_SUCCESS);
    }
    return 0;
}

Show the code you tried and which is not working. Not the comments. — Eugene Sh., Nov 08 '17 at 21:12
simply instead of the comment in the exit() functions i've tried to use rand -M 6 as argv[1] but i don't figure out how to do it properly — Zeno Raiser, Nov 08 '17 at 21:15
the point is that i'm searching a way to use rand -M 6 in the code — Zeno Raiser, Nov 08 '17 at 21:16
What's `rand -M 6` anyway? It's not a standard shell command, if it is implied.. — Eugene Sh., Nov 08 '17 at 21:19
@dbush nice one but it wouldn't be random, my pids are always an ordinated sequence of numbers so it would be like something like 2+3+4+5 — Zeno Raiser, Nov 08 '17 at 21:22
See [`srand()` — why call it only once?](https://stackoverflow.com/questions/7343833/), noting that if you want different sequences, you _must_ call it once, with a different value each time. — Jonathan Leffler, Nov 09 '17 at 02:12

score 1 · Answer 1 · answered Nov 08 '17 at 21:18

1

You need to seed the random-number generator with srand(unsigned int).

The seed is just a number that initiates the random-number formula. With the same seed, the same sequence of numbers will always show up. I recommend seeding rand() with either clock() from time.h, or the time since January 1, 1970 in seconds (time in computers is often stored as the number of seconds since January 1, 1970).

answered Nov 08 '17 at 21:18

Cpp plus 1

990
8
25

1

maybe a better idea would be to seed it with pid + time or something like that. – Eugene Sh. Nov 08 '17 at 21:21
so if i understood well i should do something like srand(rand(clock())) ?? – Zeno Raiser Nov 08 '17 at 21:24
I would consider making use of `/dev/random`. – Christian Gibbons Nov 08 '17 at 21:25
@ZenoRaiser But clock() isn't as good as using the system time, in my opinion, since the time since the program started (which is what clock() returns) seems to fluctuate within certain values sometimes. – Cpp plus 1 Nov 08 '17 at 21:26
@Cppplus1 idk if you told me on purpose to learn it but rand() has void arguments so i couldnt do rand(clock) – Zeno Raiser Nov 08 '17 at 21:29
1

@ZenoRaiser The seeding function is srand(). – Cpp plus 1 Nov 08 '17 at 21:30
anyway i've solved it with srand(clock()); exit(rand()%6); – Zeno Raiser Nov 08 '17 at 21:30
apparently `rand` is not thread-safe. So you are going to run into problems using it. – Eugene Sh. Nov 08 '17 at 21:32
@EugeneSh. What about if the random values are pre-caclulated in one core, and sent to the other cores? – Cpp plus 1 Nov 08 '17 at 21:33
@EugeneSh. what would be like something thread-safe in this context? – Zeno Raiser Nov 08 '17 at 21:36
@Cppplus1 As I said and as mentioned in `rand` `man` page, it is not thread-safe. So the whole approach is flawed. The man page is mentioning `rand_r` as a safe alternative. It can work with different seeds. – Eugene Sh. Nov 08 '17 at 21:36
Seeding the simple PRNG has relevance only to producing a different start point (into the same sequence) on each run. Not seeding it is good when you are developing and debugging, because the data is repeatable. – Weather Vane Nov 08 '17 at 21:45

score 1 · Answer 2 · answered Nov 08 '17 at 21:29

The problem with calling rand() from the child process is that it will always be the first call to rand() in that process, so the value will always be the same each time.

What you can do instead is generate the return values in an array in the parent, then the children can grab the value they want from the array.

int main()
{
    pid_t child_pid;
    int status;
    int rvals[NUM_KIDS];

    srand(time(NULL));
    for (i=0; i<NUM_KIDS; i++) {
        rvals[i] = rand();
    }

    for (i=0; i<NUM_KIDS; i++) {
        ...
        case 0:
           printf("Hi, my PID is %d\n", getpid());
           exit(rvals[i]);

rand command line concurrency programming

2 Answers2