Thread: 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");

      }

    }