Re: Problem with socketpair , AF_UNIX and select call - can anybody through any light on this!

[Howard Wilkinson <howard@xxxxxxxxxxx>]

Title: Signature

Luciano Rocha wrote:

......

Is there a reason for using SOCK_DGRAM instead of the usual SOCK_STREAM?
The latter will make the recv return 0, indicating the other end closed
the connection, as it did happen, as one peers exits before the other
select(2)s:

$ ./socketpair 
server - Socket is ready to write
server - Socket is not ready to read
exited: 14520
master - Socket is ready to write
master - Socket is ready to read
master - recv - Resource temporarily unavailable(11)

This could be a bug, or standard behaviour for disconnected datagram
sockets (I don't know enough of the standards).

  

I attach a slightly improved version. This sleeps for an hour on both sides. I have tried removing the MSG_DONTWAIT but still get the same response.

The SOCK_DGRAM is because I have small packets flowing that I do not want fragmented but do not mind if they get lost, .... but then inside a machine that does not matter. However with SOCK_STREAM I get zero length messages but still ready to read continuously!!! See attached!

--

Howard Wilkinson	Phone:	+44(20)76907075
Coherent Technology Limited	Fax:
23 Northampton Square,	Mobile:	+44(7980)639379
United Kingdom, EC1V 0HL	Email:	howard@xxxxxxxxxxx

 

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/select.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>

int socktest(int fd, char *prefix) {
  fd_set 		rfd, wfd, efd;
  int 			rv;
  char 			buf[1024];
  struct timeval	timeout = { 10, 0 };

  while (1) {
    FD_ZERO(&rfd);
    FD_ZERO(&wfd);
    FD_ZERO(&efd);
    FD_SET(fd, &rfd);
    FD_SET(fd, &wfd);
    FD_SET(fd, &efd);

    rv = select(fd+1, &rfd, &wfd, &efd, &timeout);
    if (rv < 0) {
      printf("%s - select %s", prefix, strerror(errno));
      exit(1);
    }

    if (FD_ISSET(fd, &wfd)) printf("%s - Socket is ready to write\n", prefix);
    if (FD_ISSET(fd, &efd)) printf("%s - Socket is in except\n", prefix);

    if (FD_ISSET(fd, &rfd)) {
      printf("%s - Socket is ready to read\n", prefix);
      rv = recv(fd, buf, sizeof(buf), MSG_DONTWAIT);
      if (rv < 0) {
	printf("%s - recv - %s(%d)\n", prefix, strerror(errno), errno);
	sleep (10);
	exit (1);
      }

      printf("%s - Socket returned %d bytes\n", prefix, rv);
    } else {
      printf("%s - Socket is not ready to read\n", prefix);
    }

    sleep(10);
  }
  exit (0);
}

int main() {
  int 			fd[2] = { -1, -1 };
  int			rv;
  pid_t			pid;

  rv = socketpair(PF_UNIX, SOCK_STREAM, 0, fd);
  if (rv < 0) printf("socketpair");

  fcntl(fd[0], F_SETFL, fcntl(fd[0], F_GETFL, 0) | O_NONBLOCK);
  fcntl(fd[1], F_SETFL, fcntl(fd[1], F_GETFL, 0) | O_NONBLOCK);

  if ((pid = fork()) > 0) {
    shutdown(fd[1], 2);
    close(fd[1]);
    socktest(fd[0], "master");
  } else {
    shutdown(fd[0], 2);
    close(fd[0]);
    socktest(fd[1], "server");
  }
}