problem with creating multiple child with fork() in C

This is a discussion on problem with creating multiple child with fork() in C within the C Programming forums, part of the General Programming Boards category; heres the code i have, the number of child is base on the user input, here i just put 5, ...

  1. #1
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    problem with creating multiple child with fork() in C

    heres the code i have, the number of child is base on the user input, here i just put 5, my intention is to create multiple child to process individual data and then pipe it back to parent where it will store in an array, and print it out after collecting all data

    Code:
    pipe(fd);
    pid=fork();
    for(i=1; i<5; i++){
    
    if(pid<0){
    exit(-1);
    }
    else if (pid==0){ close(fd[0]); //process data and write() to parent exit(1);
    } else {
    wait(NULL); close(fd[1]); //read() from child then store in integer array[i] pipe(fd); pid=fork();
    }
    } // this is outside the for loop for(a=1; a<5;a++){
    printf("%d", array[a]);
    }
    everything works except that the print for-loop executes twice and give me 2 sets of inputs, i tried to put exit() and wait() in parent but no luck, anyone know what my problem is?

    thank you in advance
    Last edited by Gnivol; 02-06-2010 at 10:55 AM.

  2. #2
    Registered User matrixx333's Avatar
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    In the third line of your code you have:

    Code:
    for(i=1; i=5; i++){
    Did you intend to write

    i=5; or i < 5?

  3. #3
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    i meant i<5, sry, my bad

  4. #4
    Registered User matrixx333's Avatar
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    When I'm trying to figure out a loop problem, I normally use DEBUG statements to help me see where the problem is occuring, for example

    Code:
    printf("DEBUG_1\n");
    
    // this is outside the for loop
    for(a=1; a<5;a++){
    
        printf("DEBUG_2\n");
        printf("%d", array[a]);
        printf("DEBUG_3\n");
    }
    
    printf("DEBUG_4\n");
    Then, depending on the output...it normally points me in the right direction to look for the problem. What happens when you implement something like this?

  5. #5
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    Fork

    Your problem is not the loop. When you fork a program the child created starts execution right after the fork call. You are basically duplicating the parent. The reason your print statement executes twice is because it executes in the parent and in the child. In order to solve this put it in an if statement to dectect it pid is greater than 0. I believe this will solve your problem.
    I think you will also need to put a wait command before this to assure that all child processes complete before executing the print loop.

    Code:
    if(pid > 0) 
    {
       for(a=1; a<5; ++a)
       {
              printf("%d", array[a]);
       }
    }
    Last edited by jtmurphree; 02-18-2010 at 11:44 AM.

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