Fork Example

This is a discussion on Fork Example within the Linux Programming forums, part of the Platform Specific Boards category; I'm having some problems to understand this code, especially how "(!pid[0])" works in if statements. Can somebody explain me how ...

  1. #1
    Registered User
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    Aug 2009
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    4

    Fork Example

    I'm having some problems to understand this code, especially how "(!pid[0])" works in if statements. Can somebody explain me how it's evaluated?

    Code:
    #include <sys/types.h>
    #include <sys/wait.h>
    #include <stdio.h>
    #include <errno.h>
    #include <unistd.h>
        
        int main(int argc, char *argv[])
        {
          int status;
          int pid[2];
          int pipe_fd[2];
    
          char *prog1_argv[4];
          char *prog2_argv[2];
    
          /* 
           * Build argument list
           */
    
          prog1_argv[0] = "/usr/local/bin/ls";
          prog1_argv[1] = "-l";
          prog1_argv[2] = "/";
          prog1_argv[3] = NULL;
    
    
          prog2_argv[0] = "/usr/ucb/more";
          prog2_argv[1] = NULL;
    
          /*
           * Create the pipe
           */
          if (pipe(pipe_fd) < 0)
          {
            perror ("pipe failed");
            exit (errno);
          }
    
          /*
           * Create a process space for the ls  
           */
          if ((pid[0]=fork()) < 0)
          {
            perror ("Fork failed");
            exit(errno);
          }
    
          if (!pid[0])
          {
              /*
               * Set stdout to pipe     
               */
              close (pipe_fd[0]);
              dup2 (pipe_fd[1], 1);
              close (pipe_fd[1]);
    
            /* Execute the ls */ 
            execvp (prog1_argv[0], prog1_argv);
          }
    
          if (pid[0])
          {
            /* 
             * We're in the parent 
             */
    
            /*
             * Create a process space for the more
             */
            if ((pid[1]=fork()) < 0)
            {
              perror ("Fork failed");
              exit(errno);
            }
      
            if (!pid[1])
            {
              /*
               * We're in the child
               */
    
              /*
               * Set stdin to pipe     
               */
              close (pipe_fd[1]);
              dup2 (pipe_fd[0], 0);
              close (pipe_fd[0]);
    
              /* Execute the more */ 
              execvp (prog2_argv[0], prog2_argv);
            }
    
            /* This is the parent */
            close(pipe_fd[0]);
            close(pipe_fd[1]);
    
            waitpid (pid[1], &status, 0);
            printf ("Done waiting for more.\n");
    
          }
    
        }

  2. #2
    and the hat of wrongness Salem's Avatar
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    > if (!pid[0])
    Well writing
    if ( pid[0] == 0 )
    would have been more meaningful.
    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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