Are socket options inherited across accept() from the listening socket?

Question

Suppose the listening socket passed to accept has non-default options set on it with setsockopt. Are these options (some or all of them?) inherited

Answer 1

No, they're not necessarily inherited. Try this sample, which sets the receive buffer size (SO_RCVBUF) on the initial socket to a non-default value and then compares the result with the inherited socket. Run this code, which listens on TCP port 12345, and then connect to it from any other program.

#include 
#include 
#include 
#include 
#include 
#include 

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;
  socklen_t optlen = sizeof(rcvbuf);
  if(getsockopt(s, SOL_SOCKET, SO_RCVBUF, &rcvbuf, &optlen) < 0)
    die("getsockopt (1)");
  printf("initial rcvbuf: %d\n", rcvbuf);
  rcvbuf *= 2;
  if(setsockopt(s, SOL_SOCKET, SO_RCVBUF, &rcvbuf, sizeof(rcvbuf)) < 0)
    die("setsockopt");
  printf("set rcvbuf to %d\n", rcvbuf);

  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)");

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

  return 0;
}

Result on a machine running Linux 3.0.0-21-generic:

initial rcvbuf: 87380
set rcvbuf to 174760
accepted connection
new rcvbuf: 262142