Two processes reading the same stdin

问题

In a C program, I have a menu, with some options to choose from, represented by characters. There's an option that forks the process, and runs a function (we can say it runs in the background). This function that runs in the background, under some conditions, can ask the user to enter data.

My problem is: when the main process is asking for data (or option) and the child process is asking for data too, I can't send the data properly.

Do you have any idea on how to deal with this?

I'll add a bit of the structure of the code (I can't post it all, because is around 600 lines of code):

int main(int argc, char *argv[])
{
    // Some initialization here
    while(1)
    {
        scanf("\n%c", &opt);

        switch(opt)
        {
            // Some options here
            case 'r':
                send_command(PM_RUN, fiforfd, fifowfd, pid);
                break;
            // Some more options here
        }
     }
 }

 void send_command(unsigned char command, int fiforfd, int fifowfd, int pid)
 {
     // Some operations here
     if(command == PM_RUN)
     {
         time = microtime();                
         childpid = fork();
         if(childpid == -1)
         {
             // Error here
         }   
         else if(childpid == 0)
         {
             // Here is the child
             // Some operations and conditions here
             // In some condition, appears this (expected a short in hexadecimal)
             for(i = 0; i < 7; ++i)
                 scanf("%hx\n",&data[i]));
             // More operations
             exit(0);
          }
      }
  }

回答1:

It's not working but it can be a beginning of solution

void    send_command(int *p)
{
  pid_t pid;

  pid = fork(); // check -1
  if (pid == 0)
    {
      int       i = 0;
      int       h;
      int       ret;
      char      buffer[128] = {0};

      dup2(p[0], 0);
      while (i < 2)
        {
          if ((ret = scanf("%s\n", buffer)))
            {
              //get your value from the buffer
              i++;
            }
        }
      printf("done\n");
      exit(1);
    }
}

in the child process you are going to read everything from the input and then find the value you need inside.

int     main()
{
  char  opt;
  int   p[2];

  pipe(p);
  while(1)
    {
      scanf("\n%c", &opt);


      write(p[1], &opt, 1);
      write(p[1], "\n", 1);

      switch(opt)
        {
          // Some options here
        case 'r':
          {
            send_command(p);
            break;
          }
        default:
          break;
          // Some more options here
        }
    }
}

In the current case, each character that you read will be written to the child process who read on p[0].

Of course if you have many child process you must have a list of file descriptor and write to each of them (and call pipe for each child).

Good luck

edit: maybe you should look at namedpipe where the parent process write everything in it and all the child read in the same

来源：https://stackoverflow.com/questions/17880157/two-processes-reading-the-same-stdin

标签

fork

stdin