// By running this program under strace -ff, you can see that the
// server’s write() call blocks until the client finishes reading the
// data sent, rather than returning immediately after a partial write,
// unless `O_NONBLOCK` is set.  (Tested on Debian Linux
// 6.1.0-26-amd64.)

// Running with one flag, like ./bigsocketwrite -nb, you activate
// nonblocking behavior.  With two flags, like ./bigsocketwrite -nb
// -sleep, you also make the client sleep for a second before reading
// the data from the server.

#include <netinet/in.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#include <fcntl.h>

enum { PORT = 20331, TOTAL = 100*1024*1024 }; /* 100 MiB */

void die(const char *why)
{
  perror(why);
  exit(1);
}
  
int main(int argc, char **argv)
{
  struct sockaddr_in addr = {
    .sin_family = AF_INET,
    .sin_addr = { .s_addr = htonl(INADDR_LOOPBACK) },
    .sin_port = htons(PORT),
  };
  socklen_t alen = sizeof(addr);

  char *buf = malloc(TOTAL);
  if (!buf) die("malloc");

  memset(buf, 'A', TOTAL);      // Ensure pages are wired and quiet valgrind

  int fd = socket(AF_INET, SOCK_STREAM, 0);
  if (fd < 0) die("socket");

  // Avoid “Address already in use” problems (must be *before* bind())
  uint32_t optval = 1;
  if (0 != setsockopt(fd, SOL_SOCKET, SO_REUSEADDR, &optval, sizeof(optval))) {
    die("SO_REUSEADDR");
  }

  if (bind(fd, (struct sockaddr*)&addr, sizeof(addr)) < 0) die("bind");
  if (listen(fd, 1) < 0) die("listen");

  pid_t pid = fork();
  if (pid < 0) die("fork");

  if (pid == 0) { // child ≡ server
    alarm(5);     // Kill us in case client dies and never connects

    int c = accept(fd, 0, 0);
    if (c < 0) die("accept");

    if (argc > 1) {
      int flags = fcntl(c, F_GETFL);
      if (-1 == flags) die("F_GETFL");
      if (-1 == fcntl(c, F_SETFL, flags | O_NONBLOCK)) die("F_SETFL");
    }    

    write(c, buf, TOTAL);

  } else { // parent ≡ client
    close(fd);

    fd = socket(AF_INET, SOCK_STREAM, 0);
    if (fd < 0) die("client socket");
    if (connect(fd, (struct sockaddr*)&addr, alen) < 0) die("connect");

    if (argc > 2) sleep(1);
    for (;;) {
      ssize_t n = read(fd, buf, TOTAL);
      if (n < 0) die("read");
      if (n == 0) break;
    }
  }

  return 0;
}