Is TCP_NODELAY inherited from listening sockets on your platform?

tcp-nodelay-test.c

#include <errno.h>
#include <fcntl.h>
#include <netdb.h>
#include <netinet/in.h>
#include <netinet/tcp.h>
#include <stdlib.h>
#include <stdio.h>
#include <strings.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

int main() {
  const int portnumber = 9233;
  int lsock = 0;

  if ((lsock = socket(AF_INET, SOCK_STREAM, IPPROTO_IP)) < 0) {
    close(lsock);
    exit(EXIT_FAILURE);
  }

  struct hostent* server = gethostbyname("127.0.0.1");
  if (server == NULL) {
    perror("gethostbyname");
    exit(EXIT_FAILURE);
  }

  int nodelay_flag = 1;
  setsockopt(lsock, IPPROTO_TCP, TCP_NODELAY, (void*) &nodelay_flag, sizeof(int));
  int flags = fcntl(lsock, F_GETFL, 0);
  fcntl(lsock, F_SETFL, flags | O_NONBLOCK);

  struct sockaddr_in addr;
  bzero(&addr, sizeof(struct sockaddr_in));
  addr.sin_family = AF_INET;
  addr.sin_port = htons(portnumber);
  bcopy((char *)server->h_addr,
        (char *)&addr.sin_addr.s_addr,
        server->h_length);

  if (bind(lsock, (struct sockaddr *) &addr, sizeof(struct sockaddr_in)) < 0) {
    perror("bind");
    exit(EXIT_FAILURE);
  }

  printf("Listening\n");
  listen(lsock, 1);

  sleep(1);
  printf("Connecting\n");

  int fd = 0;
  if ((fd = socket(AF_INET, SOCK_STREAM, IPPROTO_IP)) < 0) {
    perror("socket");
    exit(EXIT_FAILURE);
  }

  int err_code = 0;
  connect(fd, (struct sockaddr*) &addr, sizeof(struct sockaddr_in));

  sleep(1);

  printf("Accepting\n");

  int sfd = accept(lsock, NULL, NULL);
  if (sfd < 0) {
    perror("accept");
    exit(1);
  }

  socklen_t dummy;
  nodelay_flag = -100;
  getsockopt(sfd, IPPROTO_TCP, TCP_NODELAY, (void*) &nodelay_flag, &dummy);
  printf("TCP_NODELAY on inherited socket is %d.\n", nodelay_flag);

  flags = fcntl(sfd, F_GETFL, 0);
  printf("O_NONBLOCK on inherited socket is %d.\n",
         ((flags & O_NONBLOCK) == 0));

  return 0;
}

Suggested change:

nodelay_flag = -100;
socklen_t dummy = sizeof(nodelay_flag);

For getsockopt(), optlen is a value-result argument, initially containing the size of the buffer pointed to by optval, and modified on return to indicate the actual size of the value returned.