多个子进程在同一个管道上读/写

Question

I am currently learning sockets programming using C in a Linux environment. As a project I am attempting to write a basic chat server and client.

The intention is to have the server fork a process for each client that connects.

The problem that I am having is reading data in one child and writing it to all of the connected clients.

I have tried to accomplish this by looping on a call to select in the child that waits for data to arrive on the socket or read end of the pipe. If it arrives on the socket the idea is that it writes to the write end of the pipe which causes select to return the read end of the pipe as ready for reading.

As this pipe is shared between all the children, each child should then read the data on the pipe. This does not work as the data pipe, it appears, cannot be read by each child process at the same time and the children that "miss" the data block in the call to read.

Below is the code in the child process that does this:

for( ; ; )
{
  rset = mset;
  if(select(maxfd+1, &rset, NULL, NULL, NULL) > 0)
  {
    if(FD_ISSET(clientfd, &rset))
    {
      read(clientfd, buf, sizeof(buf));
      write(pipes[1], buf, strlen(buf));
    }
    if(FD_ISSET(pipes[0], &rset))
    {
      read(pipes[0], buf, sizeof(buf));
      write(clientfd, buf, sizeof(buf));
    }
  }
}

I am presuming the method that I am currently using simply will not work. Is it going to be possible for messages received from a client to be written to all of the other connected clients via IPC?

Thanks

Answer 1

To get around the problem of a child reading from the pipe more data than it should (and in-turn making another child get "stuck" trying to read from an empty pipe), you should probably look into using either POSIX message queues or a single pipe between the parent and an individual child processes rather than a single global pipe to communicate between the parent and child processes. As it stands right now, when the server writes to the pipe to communicate with its children, it's not really able to control exactly which child will read from the pipe at any given time, since the scheduling of processes by the OS is non-deterministic. In other words, without some type of synchronizing mechanism or read/write barriers, if the server writes to the pipe, there is nothing in your code stopping one child from "skipping" a read, and a second child from doing a double-read, leaving another child that should have gotten the broadcasted data from the server starved, and therefore blocked.

A simple way around this could again be to have a single private pipe shared between the parent and an individual child. Thus in the server the child processes can read from the client, send that data back to the parent process, and the parent can then, using the entire list of pipe descriptors that it's accumulated for all the children, write back to each individual child process the broadcast message which is then sent back to each client. No child ever gets "starved" of data since there is no possibility of a double-read by another child-process. There is only a single reader/writer on each pipe, and the communication is deterministic.

If you don't want to handle multiple pipes for each child in the server's parent process, you could use a global message queue using POSIX message queues (found in mqueue.h ). With that approach, if a child grabs a message that it's not suppose to have (ie, you would need to pass around a struct that contained some type of ID value), it would place the message back in the queue and attempt to read another message ... that's not quite as efficient speed-wise as the direct pipe approach, but it would allow you to write-back a message that was not designated for the current child without the interleaving complications that would take place with a global pipe or FIFO mechanism.

Answer 2

Each byte of data written to a pipe will be read exactly once. It isn't duplicated to every process with the read end of the pipe open.

If you want the data duplicated to multiple destination processes, you have to explicitly duplicate the data. For example, you could have one "master" process that has a pipe to and from every "slave" process. When a slave wants to broadcast a message to the other slaves, it sends it to the master process, which loops around and writes it once to each pipe going to the other slaves.