why out put is blank....

This is a discussion on why out put is blank.... within the C Programming forums, part of the General Programming Boards category; hi friends, in this program, i m creating a pipe and creating two child process of main which will write ...

  1. #1
    Registered User
    Join Date
    Aug 2006
    Posts
    17

    Question why out put is blank....

    hi friends,

    in this program, i m creating a pipe and creating two child process of main which will write and read in this pipe.
    no error no warning, but when i m running the a.out, screen is blank, and waiting for somthing, for infinite time. ( its a gcc code)
    Code:
          1 // create pipe between two processes
          2 #include<stdio.h>
          3 #include<unistd.h>
          4
          5 main()
          6     {
          7         int fd[2];
          8         int report[2];
          9         int freturn ;
         10         freturn = pipe(fd);
         11         if(freturn < 0)
         12             {
         13                 fprintf(stderr,"Cant create pipe..\n");
         14                 exit(1) ;
         15             }
         16         switch(fork())
         17             {
         18                 case -1 : fprintf(stderr,"Cant fork..\n") ;
         19                           exit(2) ;
         20                 case 0 : // fork is successfull
         21                         close(1) ; //1 is writing fd
         22                         if(dup(fd[1]) == -1)
         23                             {     
         24                                 fprintf(stdout,"Can't dup..\n") ;
         25                                 perror("dup");
         26                                 exit(2);
         27                             }
         28                         if(execlp("ls","ls", "-l","-a", NULL) == -1)
         29                             {
         30                                 fprintf(stderr,"Cant exec..\n") ;
         31                                 exit(3) ;
         32                             }
         33                         break ;
         34             }// end of switch1
         35
         36         switch(fork())
         37             {
         38                 case -1 :fprintf(stderr,"Cant fork..\n") ;
         39                          exit(4) ;
         40                 case 0 : //fork is successfull
         41                          close(0);
         42                          if(dup(fd[0]) == -1)
         43                             {
         44                                 fprintf(stderr,"Cant dup..\n") ;
         45                                 perror("2nd dup") ;
         46                                 exit(5) ;
         47                             }
         48                         if(execlp("wc","wc", NULL) == -1 )
         49                             {
         50                                 fprintf(stderr,"Cant exec..\n") ;
         51                                 exit(6) ;
         52                             }
         53                         break ;
         54             } // end of switch 2
         55
         56         wait(&report[0]) ;
         57         wait(&report[1]) ;
         58     } // end of main
         59

  2. #2
    Registered User
    Join Date
    Sep 2003
    Posts
    224
    You are creating a parent with two child, or sibling, processes. I don't think siblings can communicateover a pipe. The childs can only communicate to the parent over a pipe. I assume you are trying to implement "pipe" functionality. What you should do is have the parent fork a child, have the parent exec ls, the child exec wc, and redirect the parent's output to the child's input. Therefore, you should have something like:
    Code:
    rc = fork();
    
    if(rc)
    {
      //parent
      exec("ls");
    }
    else if(!rc)
    {
      exec(wc);
    }
    Last edited by Yasir_Malik; 08-23-2006 at 08:46 PM.

  3. #3
    Just kidding.... fnoyan's Avatar
    Join Date
    Jun 2003
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    Still in the egg
    Posts
    269
    Well, are you sure (or does the system guarantee) which process is the one that will be run next?! If I were you, I would use some mutex mechanisms to synchonize the processes.

  4. #4
    Registered User
    Join Date
    Sep 2003
    Posts
    224
    I thought about that too. However, if you run wc, you'll notice that it waits for input, so synchronization is done. For example:
    Code:
    #include <unistd.h>
    
    int main()
    {
     int fds[2];
     int pid;
    
     pipe(fds);
    
     if((pid = fork()) < 0)
     {
      perror("fork");
      exit(1);
     }
     else if(pid)
     {
      /* parent */
      close(fds[0]);
    
      /* made standard output the same as pipe's write end */
      dup2(fds[1], 1);
    
      close(fds[1]);
      execlp("ls","ls", "-l","-a", NULL);
     }
     else
     {
      /* child */
      close(fds[1]);
    
      /* made standard input the same as pipe's read end */
      dup2(fds[0], 0);
    
      close(fds[0]);
      execlp("wc","wc", NULL);
     }
    }

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