create and listen to a process

This is a discussion on create and listen to a process within the Linux Programming forums, part of the Platform Specific Boards category; (Using C on ubuntu) In my program I need to fork a process and execute a .bin in the child ...

  1. #1
    frs
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    create and listen to a process

    (Using C on ubuntu)

    In my program I need to fork a process and execute a .bin in the child process. Then I want to create a pipe that "connects" the stdout of the child to the stdout of the parent. I know basically how to do this using write() and read() in the pipes.
    The problem is that the parent should wait until the child sends a specific message (a string), or until a timeout occurs. In the first case both processes should run in "parallel" ( I don't know if its the correct term..), in the second case the child must abort and the parent go on. And I don't have a clue how to accomplish any of this things. Any suggestions ?

    I made a very basic skeleton, comments?


    Code:
     int status;
        int fd[2]; /* pipe */
    
        if( pipe(fd) !=0 )
        {
          perror("Failed to create pipe");
          exit(EXIT_FAILURE);
        }
    
        pid_t pid=fork();
    
        if( pid<0)
        {
          perror("Failed to fork");
          exit(EXIT_FAILURE);
        }
    
        else if(pid == 0) /* child */
        {
          if(execl("/usr/bin/sixad","sixad","-s",NULL)==-1);
          perror("Child unabled to start sixad");
    
          exit(EXIT_FAILURE);
        }

  2. #2
    and the hat of wrongness Salem's Avatar
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    Well reading a descriptor with a timeout is achieved by using the select() or poll() calls. These allow you to wait for something to arrive, or timeout if you choose.

    As for detecting the demise of the child (with it's exit status), then look at the waitpid() call. This too has a "poll" mechanism.
    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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  3. #3
    frs
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    if I use waitpid(), the calling process suspends its course until the child has terminated, isn't that correct?

  4. #4
    frs
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    I've the following question.

    Code:
     
       pid_t pid = fork();
      
       if(pid == 0) /* child */
        {
          if(execl("/usr/bin/sixad","sixad","-s",NULL)==-1);
          {   
          perror("Child unabled to start sixad");
          exit(EXIT_FAILURE);
          }
          printf("here");
        }
    The message "here" is only printed when execl() has finished?

  5. #5
    and the hat of wrongness Salem's Avatar
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    There is NO return is execl() is successful. You're running a different program after that point.

    Besides, your if statement has a very telling ; at the end of it.

    And how much manual page of waitpid did you actually read to miss WNOHANG ?
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  6. #6
    frs
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    Thank you for the answers. WNOHANG returns immediately if no child has exited, I don't want that. The idea is forking, suspend the parent and exec( a program I can't modify ).
    The exec program sends some messages to stdout and the parent should only continue after one specific message is received.

    I apologize for some naive questions but I'm an electronics student, not informatics. I don't have much knowledge about processes and OS's.

  7. #7
    frs
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    When I fork() , the stdout and stdin are shared by both parent and child?

  8. #8
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    Quote Originally Posted by frs View Post
    When I fork() , the stdout and stdin are shared by both parent and child?
    yes. when you fork, the child process gets a copy of *ALL* open file descriptors, including sockets, message queues, shared memory segments, etc (some of these may be platform-dependent). you can close the child's stdout descriptor, and then dup() it to use your pipe, and then you can listen to that file descriptor for output. I have actually written std::iostream-derived classes that do exactly this with stdin and stdout, so you can read/write a child process just like an ordinary file.

  9. #9
    frs
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    here's my effort. I've redirected stdout of the child to a pipe. The resulting output is on below.I still have some problems..

    1- Can anyone explain why the parent process only reads from the pipe after the child has terminated?

    2- Also, why "Connected PLAYSTATION(R)3 Controller (00:23:06:A4:E6:AA)" isn't sent into the pipe? this message is sent with syslog() instead of printf().


    Code:
     int fd[2];
    
        if( pipe(fd) !=0 )
        {
          perror("Failed to create pipe");
          exit(EXIT_FAILURE);
        }
    
        switch ( fork() )
        {
          case -1:
    	perror("Failed to fork");
    	exit(EXIT_FAILURE);
    	break;
    
          case 0:
    
    	if( dup2(fd[1],STDOUT_FILENO) ==-1 )
    	{
    	  perror("Fail redirecting stdout");
    	  exit(EXIT_FAILURE);
    	}
    	close(fd[0]);
    
    	if(execl("/usr/bin/sixad","sixad","-s",NULL)==-1)
    	  perror("Child unable to execute sixad");
    	exit(EXIT_FAILURE);
    	break;
    
          default:
    	printf("parent\n");
    	close(fd[1]);
    	char buf[500];
    
    	while(1)
    	{
    	  printf("while\n");
    	  if ( read(fd[0], buf, sizeof(buf)) <= 0)
    	  {
    	    perror("Fail reading from pipe:\n ");
    	    exit(EXIT_FAILURE);
    	  }
    
    	  printf("Read from pipe:\n %s",buf);
    	}
        }
    Output:
    parent
    while
    (sixad source code: syslog(LOG_INFO, "Connected PLAYSTATION(R)3 Controller (%s)", bda); )

    Output Continuation:
    sixad[7953]: sixad started, press the PS button now
    sixad[7969]: Connected PLAYSTATION(R)3 Controller (00:23:06:A4:E6:AA)
    (I pressed ctrl+c in the console)

    Output Continuation
    ^C
    sixad[7953]: Exit
    Read from pipe:
    sixad settings:
    Enable LEDs: 1
    js# as LED #: 1
    Start LED #: 1
    LED # increase: 1
    LED animation: 1
    Buttons: 1
    Sens. buttons: 1
    Axis: 1
    Accelerometers: 1
    Acceleration: 0
    Speed: 0
    Position: 0
    Rumble: 0
    Debug: 0
    while
    Fail reading from pipe:
    : Success
    Note: sixad settings are printed with printf
    Last edited by frs; 07-16-2010 at 11:14 AM.

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