How do I correctly shut down a connection to force the client to disconnect after a tty disconnects?

Question

I apologize for the length, but I suspect the bug might be in the error handling and there's a bit of necessary boilerplate for networking.

I start the following implementation of a single-file forking tcp server.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/select.h>
#include <sys/socket.h>
#include <sys/wait.h>
#include <netinet/in.h>
#include <netdb.h>
#include <string.h>
#include <arpa/inet.h>
#include <pthread.h>
#include <errno.h>
#include <pty.h>

#define PORT 9600
#define BUF_SIZE 128
#define SERVER_BACKLOG 10

void handle_conn(int conn_fd) {
  // Create a TTY / PTY pair and hook it up to the connection.
  int master_fd;
  char slave_name[100];
  pid_t pid = forkpty(&master_fd, slave_name, NULL, NULL);
  if (pid == -1) {
    fprintf(stderr, "Error on first fork: %d %s",  errno, strerror(errno));
    return;
  } else if (pid == 0) { // Child process attached to slave pty
    execl("/bin/bash", "bash", (char*) NULL);
  }

  // Set both fds to be non-blocking
  int flags = fcntl(master_fd, F_GETFL, 0);
  fcntl(master_fd, F_SETFL, flags | O_NONBLOCK);
  flags = fcntl(conn_fd, F_GETFL, 0);
  fcntl(conn_fd, F_SETFL, flags | O_NONBLOCK);

  // Parent continues here, shuffling data back and forth from the socket to
  // the master fd.
  int n;
  char c;
  while(1) {
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(conn_fd, &fds);
    FD_SET(master_fd, &fds);
    int numFds = select((conn_fd > master_fd ? conn_fd : master_fd) + 1, &fds, NULL, NULL, NULL);

    if (numFds == -1) {
      fprintf(stderr, "Error on select: %d %s",  errno, strerror(errno));
      return;
    }
    if (numFds == 0) {
      continue;
    }
    if (FD_ISSET(conn_fd, &fds)) {
      // Read from socket and write to tty.
      while ((n = read(conn_fd, &c, 1)) > 0) {
        write(master_fd, &c, 1);
      }

      if (n == 0) {
        // Reached EOF
        break;
      }
      // Presumed n is -1
      if (errno != EAGAIN || errno != EWOULDBLOCK) {
        fprintf(stderr, "Error reading socket %d %s\n", errno, strerror(errno));
        break;
      }
    }

    if (FD_ISSET(master_fd, &fds)) {
      // Read from tty and write to socket as long as things are still
      // readable.
      while ((n = read(master_fd, &c, 1)) > 0) {
        write(conn_fd, &c, 1);
      }

      if (n == 0) {
        // Reached EOF
        break;
      }

      // Presumed n is -1
      if (errno != EAGAIN && errno != EWOULDBLOCK) {
        fprintf(stderr, "Error reading master FD %d %s\n", errno, strerror(errno));
        break;
      }
    }
  }
  printf("Closing connection [%d]\n", conn_fd);
  int err = close(conn_fd);
  if (err != 0) {
    fprintf(stderr, "Error while closing connection [%d]\n", conn_fd);
  }

  err = close(master_fd);
  if (err != 0) {
    fprintf(stderr, "Error while closing master fd [%d]\n", conn_fd);
  }
}

int main(int argc, char *argv[]) {
  int server_fd = socket(PF_INET, SOCK_STREAM, 0);
  if (server_fd < 0) {
    fprintf(stderr, "Cannot create socket\n");
    return 1;
  }

  struct sockaddr_in server_addr;
  server_addr.sin_family = AF_INET;
  server_addr.sin_port = htons(PORT);
  int ok = inet_pton(AF_INET, "127.0.0.1", &server_addr.sin_addr);
  if (!ok) {
    fprintf(stderr, "Cannot parse IP address\n");
    return 1;
  }

  // Allow reuse of port.
  int optval = 1;
  setsockopt(server_fd, SOL_SOCKET, SO_REUSEPORT, &optval, sizeof(optval));

  int err = bind(server_fd, (struct sockaddr *) &server_addr, sizeof(server_addr));
  if (err != 0) {
    fprintf(stderr, "Cannot bind server: Error %d %s\n", errno, strerror(errno));
    return 1;
  }

  err = listen(server_fd, SERVER_BACKLOG);
  if (err != 0) {
    fprintf(stderr, "Cannot listen\n");
    return 1;
  }

  printf("Server listening on port %d\n", PORT);

  int client_fd;
  struct sockaddr_in client_addr;
  socklen_t client_addr_len;
  while (1) {
    client_fd = accept(server_fd, (struct sockaddr *) &client_addr, &client_addr_len);
    if (client_fd < 0) {
      fprintf(stderr, "Cannot accept: Error %d %s\n", errno, strerror(errno));
      return 1;
    }

    printf("New client [%d]\n", client_fd);

    // Using fork instead of threads here to avoid pitfalls involved with
    // mixing forking and threads, since this use case case of a tty host is
    // not expected to involve a great many requests.
    int pid = fork();
    if (pid == -1) {
      fprintf(stderr, "Error on forking to handle new connection: %d %s",  errno, strerror(errno));
      // Server does not need to die here.
      continue;
    } else if (pid == 0) { // Child will handle the connection and then return.
      handle_conn(client_fd);
      return 0;
    }
    // Parent continues to accept
  }
}

Then on another terminal, I run the following netcat client.

stty -icanon -echo; nc localhost 9600

At this point, I can type commands like ls or cat on the terminal with nc.

However, when I type exit, the server prints Error reading master FD 5 Input/output error\nClosing connection [4], and no more errors, so I assume that the server has successfully closed the connection, but the client continues to hang until I press Control-C on the client.

How can I modify the server code to force the client to disconnect?

Answer 1

I have discovered the error! It turns out that I had forgotten to close the socket in the parent process. Once I did that, the close() worked as expected in the child.

A simple modification of the above code, as follows, fixes the problem.

    ...
    // Parent continues to accept
    close(client_fd);