Q: What happens to socket send/recv buffer sizes after an accept()?

socket_buffer_sizes

#include <errno.h>
#include <netinet/in.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>

void die(const char *f)
{
  printf("%s: %s\n", f, strerror(errno));
  exit(1);
}

int main(void)
{
  int s = socket(AF_INET, SOCK_STREAM, 0);
  if(s < 0)
    die("socket");

  int rcvbuf, sndbuf;
  socklen_t optlen = sizeof(rcvbuf);
  if(getsockopt(s, SOL_SOCKET, SO_RCVBUF, &rcvbuf, &optlen) < 0)
    die("getsockopt (1)");
  if(getsockopt(s, SOL_SOCKET, SO_SNDBUF, &sndbuf, &optlen) < 0)
    die("getsockopt (1)");
  printf("initial rcvbuf: %d\n", rcvbuf);
  printf("initial sndbuf: %d\n", sndbuf);
  rcvbuf *= 20;
  sndbuf *= 20;
  if(setsockopt(s, SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf)) < 0)
    die("setsockopt");
  if(setsockopt(s, SOL_SOCKET, SO_SNDBUF, &sndbuf, sizeof(sndbuf)) < 0)
    die("setsockopt");
  printf("set rcvbuf to %d\n", rcvbuf);
  printf("set sndbuf to %d\n", sndbuf);

  struct sockaddr_in sin;
  memset(&sin, 0, sizeof(sin));
  sin.sin_family = AF_INET;
  sin.sin_port = htons(12345);
  sin.sin_addr.s_addr = INADDR_ANY;
  if(bind(s, (struct sockaddr *)&sin, sizeof(sin)) < 0)
    die("bind");

  if(listen(s, 10) < 0)
    die("listen");

  struct sockaddr_in client_addr;
  socklen_t addr_len = sizeof(client_addr);
  int s2 = accept(s, (struct sockaddr *)&client_addr, &addr_len);
  if(s2 < 0)
    die("accept");
  printf("accepted connection\n");
  optlen = sizeof(rcvbuf);
  if(getsockopt(s2, SOL_SOCKET, SO_RCVBUF, &rcvbuf, &optlen) < 0)
    die("getsockopt (2)");
  if(getsockopt(s2, SOL_SOCKET, SO_SNDBUF, &sndbuf, &optlen) < 0)
    die("getsockopt (2)");

  printf("new rcvbuf: %d\n", rcvbuf);
  printf("new sndbuf: %d\n", sndbuf);

  return 0;
}

Linux:
initial rcvbuf: 87380
initial sndbuf: 16384
set rcvbuf to 1747600
set sndbuf to 327680
accepted connection
new rcvbuf: 425984
new sndbuf: 425984

OS X:
initial rcvbuf: 131072
initial sndbuf: 131072
set rcvbuf to 2621440
set sndbuf to 2621440
accepted connection
new rcvbuf: 2629452
new sndbuf: 2629452

Windows 7:
initial rcvbuf: 8192
initial sndbuf: 8192
set rcvbuf to 163840
set sndbuf to 163840
accepted connection
new rcvbuf: 163840
new sndvbuf: 163840