平行叉管

Question

void doWork(){

  int fd[2];
  int pret = pipe(fd);

  close(0);
  close(1);
  int dret = dup2(fd[1], 1);
  close(fd[1]);

  while(1){

    char buf[256];
    system("whoami");
    int rret = read(fd[0], buf, 256);

    if(/* something interesting */){
      return;
    }
  }
}

int main(int argc, char* argv[]){

  int children = 2;

  for(unsigned work = 0; work < children; ++work){

    pid_t pid = fork();

    if(pid == 0){
      doWork();
      break;
    }
  }
  int status;
  wait(&status);

  return 0;
}

What is wrong with this example? I'm trying to have each child process to call an external program, then read the output of that program from the pipe. My code only works when children is set to 1.

EDIT: I'm trying to achieve task-parallelism with fork/pipes. There is no communication between the parent and the child processes. Each child process executes an external program, reads the output, processes the output, and continues till the desired output is found.

Answer 1

You need to create the pipe before you fork() , not after. In your code, only the child process will have the pipe. You need both processes to share it.

Eg.:

int fd[2];
pid_t pid;

if (pipe(fd)) { /* TODO: handle error */ }

pid = fork();
if (pid < 0) { /* TODO: handle error */ }

if (pid == 0)
{
   /* We are the child.  Set fd[1] as stdout. */
   if (dup2(fd[1], 1)) { /* TODO: handle error */ }

   /* Close fd[0]; this process doesn't need it. */
   close(fd[0]);

   do_work();
   exit(0);
} 
else
{
   /* We are the parent... */
   /* Close the other guy's write fd. */
   close(fd[1]);

   /* Now read from fd[0] */
   /* Possibly waitpid() on child pid, etc. */
}

Also: I like to call fflush(stdout); before I fork() . Otherwise you will observe weird behaviors with printf() .

Answer 2

You want to spawn child that will then execute some sub-process, parse it content, interpret it, and then exit ? If so, you need to fork twice for each task you want to create. The first time to create the child that will handle the result of the subprocess, the second time to launch the subprocess.

Something like that :

void ExecuteCommandAndInterpretResult(const char* command)
{
    int fd[2];
    if (pipe(fd))
        exit(1);

    pid_t pid = fork();
    if (pid < 0)
        exit(1);

    if (pid == 0)
    {
        if (dup2(fd[1], 1))
            exit(1);

        close(fd[0]);
        execl(command, basename(command), NULL);
        exit(1);
    }
    else
    {
        int status;

        close(fd[1]);
        // do something with output from command

        wait(&status);
        exit(0);
    }
}

#define CHILDREN 2
int main()
{
    unsigned int i;
    pid_t pids[CHILDREN];

    for (i = 0; i < CHILDREN; i ++)
    {
        pids[i] = fork();
        if (pids[i] < 0)
        {
            // TODO: handle error
        }
        else if (pids[i] == 0)
        {
            ExecuteCommandAndInterpretResult("/usr/bin/ls");
            exit(0);
        }
    }

    for (i = 0; i < CHILDREN; i ++)
    {
        if (pids[i] > 0)
        {
            int status;
            waitpid(pids[0], &status, 0);
        }
    }

    return 0;
}

Answer 3

您是否考虑过在孩子中使用popen()执行whoami ？