How to write a TCP server using the fork syscall

I’ve previously described some simple TCP servers. First, a program which uses a single process, serving one client at a time. Usually, this is unacceptable: we want to multiplex multiple clients’ connections, so that each client is responded to promptly. I described a couple of ways to do this which also use a single process. One uses the select system call, which blocks waiting for one of many possible events. Another uses the kqueue system calls. But I omitted a more obvious way to achieve multiplexing, which uses one process per client connection.

The process-per-connection approach uses the parent process to accept new connections, and uses the fork system call to spawn a new child process for each accepted connection. When the connection is closed, the child process exits.

#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>

int guard(int n, char * err) { if (n == -1) { perror(err); exit(1); } return n; }

int main(void) {
  int listen_fd = guard(socket(PF_INET, SOCK_STREAM, 0), "Could not create TCP socket");
  printf("Created new socket %d\n", listen_fd);
  guard(listen(listen_fd, 100), "Could not listen on TCP socket");
  struct sockaddr_in listen_addr;
  socklen_t addr_len = sizeof(listen_addr);
  guard(getsockname(listen_fd, (struct sockaddr *) &listen_addr, &addr_len), "Could not get socket name");
  printf("Listening for connections on port %d\n", ntohs(listen_addr.sin_port));
  for (;;) {
    int conn_fd = accept(listen_fd, NULL, NULL);
    printf("Got new connection %d\n", conn_fd);
    if (guard(fork(), "Could not fork") == 0) {
      pid_t my_pid = getpid();
      printf("%d: forked\n", my_pid);
      char buf[100];
      for (;;) {
        ssize_t num_bytes_received = guard(recv(conn_fd, buf, sizeof(buf), 0), "Could not recv on TCP connection");
        if (num_bytes_received == 0) {
          printf("%d: received end-of-connection; closing connection and exiting\n", my_pid);
          guard(shutdown(conn_fd, SHUT_WR), "Could not shutdown TCP connection");
          guard(close(conn_fd), "Could not close TCP connection");
          exit(0);
        }
        printf("%d: received bytes; echoing\n", my_pid);
        guard(send(conn_fd, buf, num_bytes_received, 0), "Could not send to TCP connection");
        printf("%d: echoed bytes; receiving more\n", my_pid);
      }
    } else {
      // Child takes over connection; close it in parent
      close(conn_fd);
    }
  }
  return 0;
}

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