How to turn off buffering on write() system call?

Question

I used to think that write() system call is unbuffered and that fwrite and fread are used for buffered IO. However I wrote simple programs to establish that some buffering was still going on when using write(). I am using write() and read() on sockets. Due to buffering, it is possible for the client to lag behind while server keeps sending packets. I do not want that. I want that the client must consume the record before the server sends more records.

How can I make that happen without adding network load of acknowledgments etc !

I am using gcc on linux

server.c :

#include <stdio.h>
#include <errno.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <netinet/tcp.h>

int remote_rr_port=2000; // Server will send RR logs using connection on this port.
char const *remote_server_ip="127.0.0.1";
int connFD_rr;


static void startTcpServer(int *sd, const int port) {
  *sd= socket(AF_INET, SOCK_STREAM, 0);

  // Set socket option so that port can be reused
  int enable = 1;
  setsockopt(*sd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(int));
  struct sockaddr_in a;
  memset(&a,0,sizeof(a));
  a.sin_family = AF_INET;
  a.sin_port = port;
  a.sin_addr.s_addr = INADDR_ANY;
  int bindResult = bind(*sd, (struct sockaddr *) &a, sizeof(a));
  listen(*sd,2);
}


// Wait for connection from client
static int getTcpConnection(int sd) {
  char buf[100];
  socklen_t len;
  struct sockaddr_in clientAddress;
  printf("\nWaiting for connection from remote client\n");
  len = sizeof(clientAddress);
  int connFD = accept(sd, (struct sockaddr*) &clientAddress, &len);
  setsockopt(connFD_rr, SOL_SOCKET, SO_SNDBUF, (int[]){0}, sizeof(int));
  printf("\n Connection from : %s:%d\n",inet_ntop(AF_INET, &clientAddress.sin_addr, buf, sizeof(buf)),clientAddress.sin_port);
  fflush(stdout);
  return connFD;
}

FILE* rdrr_server_start(void) {

  // Socket Descriptors for the two connections
  int rr_sd;
  int input_sd;

  startTcpServer(&rr_sd, remote_rr_port);

  connFD_rr = getTcpConnection(rr_sd);

  return fdopen(connFD_rr, "w");
}

int main() {
  int i = 0;
  rdrr_server_start();

  for(i=0;i<10000000; i++) {
    write(connFD_rr, &i, sizeof (int));
    printf("%d\n", i);
  }
  return 0;
}

client.c :

#include <sys/socket.h>
#include <arpa/inet.h>
#include <stdio.h>
#include <netinet/in.h>
#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <unistd.h>
#include <netinet/tcp.h>



int remote_rr_port=2000; // Server will send RR logs using connection on this port.
char const *remote_server_ip="127.0.0.1";
int connFD_rr;

FILE* rdrr_client_start(void) {

  connFD_rr = socket(AF_INET, SOCK_STREAM, 0);

  struct sockaddr_in a;
  memset(&a,0,sizeof(a));
  a.sin_family = AF_INET;
  a.sin_port = remote_rr_port;
  a.sin_addr.s_addr = inet_addr(remote_server_ip);

  printf("\nConnecting to Server on RR port");
  int CONNECT_TO_SERVER= connect(connFD_rr,(struct sockaddr *)  &a, sizeof(a));
  printf("\nConnected to server on RR port");
  setsockopt(connFD_rr, SOL_SOCKET, SO_RCVBUF, (int[]){0}, sizeof(int));
  return fdopen(connFD_rr, "r");
}  

int main() {
  int i = 0;
  rdrr_client_start();
  getrchar();
  while(1) {
    read(connFD_rr, &i, sizeof (int));
    printf("%d\n", i);
  }
  return 0;
} 

Answer 1

Perhaps what you mean is that you want to disable Nagle's Algorithm in which case the solution is:

setsockopt(sock, IPPROTO_TCP, TCP_NODELAY, (int[]){1}, sizeof(int));

Edit: Hmm, it looks like you want more than this, and I doubt what you want is possible without designing your own protocol on top of UDP.

Edit 2: You may be able to get an effect similar to what you want by limiting the send and receive buffer sizes. The server (sender) should do:

setsockopt(sock, SOL_SOCKET, SO_SNDBUF, (int[]){YOUR_BUF_LIMIT}, sizeof(int));

and the client (receiver) should do:

setsockopt(sock, SOL_SOCKET, SO_RCVBUF, (int[]){YOUR_BUF_LIMIT}, sizeof(int));