Forking

This is a discussion on Forking within the Linux Programming forums, part of the Platform Specific Boards category; When you fork and modify a variable, is it modified in the parent process too?...

  1. #1
    C++ Developer XSquared's Avatar
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    Forking

    When you fork and modify a variable, is it modified in the parent process too?
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

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  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    IIRC no. When the child process is created it gets it's own memory area.

    Test program:
    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main (void)
    {
      int fork_return;
      int x = 5;
    
      x++;
    
      fork_return = fork();
    
      if ( fork_return == 0)
      {
        sleep(10);
        printf("In parent: ");
        x+=10;
      }
      else
      {
        x*= 10;
        printf("In child: ");
      }
    
      printf(" %d\n", x);
    
      return 0;
    }
    Result:
    In child: 60
    In parent: 16

  3. #3
    C++ Developer XSquared's Avatar
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    Well, in that case, is there any way to start a new thread sharing the memory with its parent?
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

    You. Fetch me my copy of the Wall Street Journal. You two, fight to the death - Stewie

  4. #4
    C++ Developer XSquared's Avatar
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    Nevermind. Just started reading up on pthreads.
    Naturally I didn't feel inspired enough to read all the links for you, since I already slaved away for long hours under a blistering sun pressing the search button after typing four whole words! - Quzah

    You. Fetch me my copy of the Wall Street Journal. You two, fight to the death - Stewie

  5. #5
    & the hat of GPL slaying Thantos's Avatar
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    Yea I find multi threading more useful the multi process.
    I personally use the pth library from GNU.

  6. #6
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    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  7. #7
    Registered User gandalf_bar's Avatar
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    Quote Originally Posted by Thantos
    IIRC no. When the child process is created it gets it's own memory area.

    Test program:
    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main (void)
    {
      int fork_return;
      int x = 5;
    
      x++;
    
      fork_return = fork();
    
      if ( fork_return == 0)
      {
        sleep(10);
        printf("In parent: ");
        x+=10;
      }
      else
      {
        x*= 10;
        printf("In child: ");
      }
    
      printf(" %d\n", x);
    
      return 0;
    }
    Result:
    In child: 60
    In parent: 16

    Shouldn't be like this:

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main (void)
    {
      int fork_return;
      int x = 5;
    
      x++;
    
      fork_return = fork();
    
      if ( fork_return == 0)
      {
        sleep(10);
        printf("In child: ");
        x+=10;
      }
      else
      {
        x*= 10;
        printf("In parent: ");
      }
    
      printf(" %d\n", x);
    
      return 0;
    }
    A man asked, "Who are you?"
    Buddha answered, "I am awaked."

  8. #8
    & the hat of GPL slaying Thantos's Avatar
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    Meh small mistake. Basic idea lives on.

  9. #9
    Registered User gandalf_bar's Avatar
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    Just a simple question:

    Consider this code:

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main(void) {
        int fork_return;
        fork_return = fork();
    
        if( fork_return == 0 ) {
            sleep(5);
            printf("Child only ");
        }
        else {
            printf("Parent only! ");
        }
    
        printf("I am touchabled by parent and child");
    
        return 0;
    }
    Then this:

    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main(void) {
        int fork_return;
        fork_return = fork();
    
        if( fork_return == 0 ) {
            sleep(5);
            printf("Child only ");
            return 0;
        }
        else {
            printf("Parent only! ");
        }
    
        printf("Now I am  touchabled by parent only");
    
        return 0;
    }
    Ok, I understand this two code. But look at this one:
    Code:
    #include <stdio.h>
    #include <unistd.h>
    #include <sys/wait.h>
    
    int main(void) {
        int fork_return;
        fork_return = fork();
    
        if( fork_return == 0 ) {
            sleep(5);
    
    
            //Why does this not show up in console
            printf("Child only ");
            //End of Why
    
    
            char *arg[] = {
                  "ls",
                  NULL
            };
            execvp("ls",arg);
        }
        else {
            printf("Parent only! ");
        }
    
        printf("Now I am touchabled by parent only");
    
        return 0;
    }
    Why?????

    Just as reference, I use Slackware 9.1 and GNU C++ compiler version 3.3.3.
    A man asked, "Who are you?"
    Buddha answered, "I am awaked."

  10. #10
    and the hat of wrongness Salem's Avatar
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    > Why?????
    1) Try printing a newline with that as well
    2) Try calling fflush(stdout) before the exec call

    Output is buffered, so my guess is the buffers get trashed along with everything else owned by the program, when the exec happens.
    Incomplete (ie unflushed) output is lost.
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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  11. #11
    & the hat of GPL slaying Thantos's Avatar
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    Also
    Code:
           char *arg[] = {
                  "ls",
                  NULL
            };
    needs to go before the sleep(). You can only declare variables at the beginning of a block in C (which is what I'm guessing you are using since you are using printf() and linux)

  12. #12
    End Of Line Hammer's Avatar
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    >>You can only declare variables at the beginning of a block in C
    It's a C99 extension that allows this, maybe his compiler has it too. Or maybe it's being compiled as c++
    When all else fails, read the instructions.
    If you're posting code, use code tags: [code] /* insert code here */ [/code]

  13. #13
    & the hat of GPL slaying Thantos's Avatar
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    Most likely the later then the former...

  14. #14
    Registered User gandalf_bar's Avatar
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    Quote Originally Posted by Salem
    > Why?????
    1) Try printing a newline with that as well
    2) Try calling fflush(stdout) before the exec call

    Output is buffered, so my guess is the buffers get trashed along with everything else owned by the program, when the exec happens.
    Incomplete (ie unflushed) output is lost.
    That works. Thank you.....
    A man asked, "Who are you?"
    Buddha answered, "I am awaked."

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