Test if setsockopt(SO_PRIORITY) sets the IP type-of-service (TOS) field or not.

test_socket_prio_and_tos.c

#include <assert.h>
#include <netinet/ip.h>
#include <stdio.h>
#include <sys/socket.h>
#include <sys/types.h>


void test_setsockopt()
{
  int priority = 6;
  int iptos = IPTOS_CLASS_CS6;

  int fd = socket(AF_INET, SOCK_STREAM, 0);

  if(setsockopt(fd, SOL_SOCKET, SO_PRIORITY, &priority,
                sizeof(priority)) < 0){
    printf("Oh no\n");
  }

  int tos = -1;
  int optlen = sizeof(tos);
  if (getsockopt(fd, IPPROTO_IP, IP_TOS, &tos, &optlen) < 0){
    printf("Oh no\n");
  }

  if (tos == 0){
    printf("The man page lies or the kernel is buggy\n");
    printf("setsockopt(SO_PRIORITY) didn't set the IP type-of-service (TOS) field.\n");
  } else {
    printf("the man page doesn't lie\n");
  }

  close(fd);
}

int main(void)
{
  // background: https://ocrete.ca/2009/07/24/when-a-man-page-lies/
  test_setsockopt();

  return 0;
}