How to write a TCP server with the pthread API

I’ve described several ways to write a TCP server:

• using the socket system calls, serving one client at a time
• using fork system call to serve multiple clients (one client per process)
• using select system call to serve multiple clients
• using kqueue system calls to serve multiple clients

Today I’ll describe another: using the pthread library to serve multiple clients. The pthread functions are not system calls, but they are part of the standard POSIX API. (In future, I’ll describe how pthreads are implemented on top of system calls.)

The library in pthread.h includes many functions, but today we’ll just use the most fundamental one:

#include <pthread.h>
int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void *(*start_routine)(void *), void *arg);

The pthread_create function is somewhat like the fork system call: it creates two threads of control where previously there was one. The important differences today are:

• The fork system call starts the new process at the same program counter as the old process. But the pthread_create function takes a function pointer, and starts the new thread at that position.
• The fork system call clones the file descriptor table for the new process. The pthread_create function does not; instead, the thread shares the same file descriptor table.

Here’s the program, which runs an “echo” server for every TCP client, using pthread_create for each new TCP connection:

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

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

void * thread_func(void * arg) {
  intptr_t conn_fd = (int) arg;
  printf("thread: serving fd %ld\n", conn_fd);
  char buf[1024];
  for (;;) {
    int bytes_received = guard(recv(conn_fd, buf, sizeof(buf), 0), "Could not recv");
    if (bytes_received == 0) { goto stop_serving; }
    int bytes_sent = 0;
    while (bytes_sent < bytes_received) {
      ssize_t bytes_sent_this_call = send(conn_fd, buf+bytes_sent, bytes_received-bytes_sent, 0);
      if (bytes_sent_this_call == -1) { goto stop_serving; }
      bytes_sent += bytes_sent_this_call;
    }
  }
  stop_serving:
  guard(close(conn_fd), "Could not close socket");
  printf("thread: finished serving %ld\n", conn_fd);
  return NULL;
}

int main(void) {
  int listen_fd = guard(socket(AF_INET, SOCK_STREAM, 0), "Could not create TCP listening socket");
  guard(listen(listen_fd, 100), "Could not listen");
  struct sockaddr_in addr;
  socklen_t addr_bytes = sizeof(addr);
  guard(getsockname(listen_fd, (struct sockaddr *) &addr, &addr_bytes), "Could not get sock name");
  printf("Listening on port %d\n", ntohs(addr.sin_port));
  for (;;) {
    intptr_t conn_fd = guard(accept(listen_fd, NULL, NULL), "Could not accept");
    pthread_t thread_id;
    int ret = pthread_create(&thread_id, NULL, thread_func, (void*) conn_fd);
    if (ret != 0) { printf("Error from pthread: %d\n", ret); exit(1); }
    printf("main: created thread to handle connection %ld\n", conn_fd);
  }
  return 0;
}

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