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Editorial Team
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Asked: 2026-06-08T00:46:19+00:00

Consider the output of the below program: int main() { int ret; ret=fork(); ret=fork();

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Consider the output of the below program:

int main()
{
    int ret;
    ret=fork();
    ret=fork();
    ret=fork();
    ret=fork();

    if(!ret)
            printf("one\n");
    else
            printf("two\n");
    return 0;
}

I am getting the output as: 

two
one
two
two

http://ideone.com/omgKm

AFAIT, the output should be 8 times one & 8 times two.

Where are the rest one's & two's?

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1 Answer

  1. Editorial Team

    Consider this alternative code which tracks what goes on somewhat better:

    #include <stdio.h>
#include <unistd.h>

int main(void)
{
    int ret1 = fork();
    int ret2 = fork();
    int ret3 = fork();
    int ret4 = fork();

    if (ret4 == 0)
        printf("one: (%d: %d, %d, %d, %d)\n", (int)getpid(), ret1, ret2, ret3, ret4);
    else
        printf("two: (%d: %d, %d, %d, %d)\n", (int)getpid(), ret1, ret2, ret3, ret4);
    return 0;
}

    Show us the output from this variation and we can see what worked and what failed.

    After seeing the alternative output, I got this on my Mac (where Isis JL: is my prompt and rmk is an alternative implementation of make):

    Isis JL: rmk fb && ./fb
    /usr/bin/gcc -O3 -g -std=c99 -Wall -Wextra fb.c -o fb  
two: (38068: 38069, 38070, 38071, 38072)
one: (38072: 38069, 38070, 38071, 0)
two: (38071: 38069, 38070, 0, 38075)
two: (38070: 38069, 0, 38074, 38077)
two: (38073: 0, 0, 38078, 38079)
two: (38069: 0, 38073, 38076, 38080)
one: (38075: 38069, 38070, 0, 0)
one: (38077: 38069, 0, 38074, 0)
Isis JL: two: (38074: 38069, 0, 0, 38081)
two: (38078: 0, 0, 0, 38082)
one: (38079: 0, 0, 38078, 0)
one: (38081: 38069, 0, 0, 0)
two: (38076: 0, 38073, 0, 38083)
one: (38080: 0, 38073, 38076, 0)
one: (38083: 0, 38073, 0, 0)
one: (38082: 0, 0, 0, 0)

Isis JL:

    Note the interleaved prompt — the blank line at the end is where I hit return after the output completed.

    Hypothesis:

    The output on ideone is not captured after the initial process stops.

    Try this alternative, which waits for children to die before exiting:

    #include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>

int main(void)
{
    int ret1 = fork();
    int ret2 = fork();
    int ret3 = fork();
    int ret4 = fork();

    if (ret4 == 0)
        printf("one: (%d: %d, %d, %d, %d)\n", (int)getpid(), ret1, ret2, ret3, ret4);
    else
        printf("two: (%d: %d, %d, %d, %d)\n", (int)getpid(), ret1, ret2, ret3, ret4);
    while (wait(0) > 0)
        ;
    return 0;
}

    Output on Mac, once more:

    Isis JL: rmk fb && ./fb
    /usr/bin/gcc -O3 -g -std=c99 -Wall -Wextra fb.c -o fb  
two: (38111: 38112, 38113, 38114, 38115)
one: (38115: 38112, 38113, 38114, 0)
two: (38114: 38112, 38113, 0, 38119)
two: (38113: 38112, 0, 38117, 38121)
two: (38117: 38112, 0, 0, 38123)
one: (38119: 38112, 38113, 0, 0)
two: (38118: 0, 38116, 0, 38124)
one: (38121: 38112, 0, 38117, 0)
one: (38125: 0, 0, 38122, 0)
two: (38116: 0, 0, 38122, 38125)
two: (38122: 0, 0, 0, 38126)
two: (38112: 0, 38116, 38118, 38120)
one: (38120: 0, 38116, 38118, 0)
one: (38123: 38112, 0, 0, 0)
one: (38124: 0, 38116, 0, 0)
one: (38126: 0, 0, 0, 0)
Isis JL:

    Hypothesis Proven

    To the extent you can prove anything… http://ideone.com/zFoLn …

    This shows all 16 lines of output. The problem must have been ‘premature termination’.

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