Recv-Q vs Send-Q

Recv-Q
    Established: The count of bytes not copied by the user program connected to this socket.  Listening: Since Kernel 2.6.18 this column contains the current syn backlog.

Send-Q
    Established: The count of bytes not acknowledged by the remote host.  Listening: Since Kernel 2.6.18 this column contains the maximum size of the syn backlog.

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

#define MAXLNE 4096
#define PORT 8080
#define DO_IGNORE_REVC 0
#define DO_FLOOD_SEND 1
#define FLOOD_SEND_CONCURRENCY 100000

int main(int argc, char **argv) {
  int listenfd, connfd, n;
  char buff[MAXLNE];

  if ((listenfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {
    printf("create socket error: %s(errno: %d)\n", strerror(errno), errno);
    return 0;
  }

  printf("create listenfd %d\n", listenfd);

  struct sockaddr_in servaddr;
  memset(&servaddr, 0, sizeof(servaddr));
  servaddr.sin_family = AF_INET;
  servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
  servaddr.sin_port = htons(PORT);

  if (bind(listenfd, (struct sockaddr *)&servaddr, sizeof(servaddr)) == -1) {
    printf("bind socket error: %s(errno: %d)\n", strerror(errno), errno);
    return 0;
  }

  if (listen(listenfd, 10) == -1) {
    printf("listen socket error: %s(errno: %d)\n", strerror(errno), errno);
    return 0;
  }

  struct sockaddr_in client;
  socklen_t len = sizeof(client);
  if ((connfd = accept(listenfd, (struct sockaddr *)&client, &len)) == -1) {
    printf("accept socket error: %s(errno: %d)\n", strerror(errno), errno);
    return 0;
  }

  printf("== waiting for client's request ==\n");

  while (1) {

#if DO_IGNORE_REVC

    sleep(10);
    printf("ignore recv, do nothing");

#else

    n = recv(connfd, buff, MAXLNE, 0);
    if (n > 0) {
      buff[n] = '\0';
      printf("recv msg from client: %s\n", buff);

      if (DO_FLOOD_SEND) {
        printf("flood send\n");
        for (int i = 1; i < FLOOD_SEND_CONCURRENCY; i++) {
          send(connfd, buff, n, 0);
        }
        printf("done\n");
      } else {
        send(connfd, buff, n, 0);
      }

    } else if (n == 0) {
      close(connfd);
    }

#endif
  }

  close(listenfd);
  return 0;
}