c - Socket recv() hang on large message with MSG_WAITALL

Question

I have an application that reads large files from a server and hangs frequently on a particular machine. It has worked successfully under RHEL5.2 for a long time. We have recently upgraded to RHEL6.1 and it now hangs regularly.

I have created a test app that reproduces the problem. It hangs approx 98 times out of 100.

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/param.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <netdb.h>
#include <sys/socket.h>
#include <sys/time.h>

int mFD = 0;

void open_socket()
{
  struct addrinfo hints, *res;
  memset(&hints, 0, sizeof(hints));
  hints.ai_socktype = SOCK_STREAM;
  hints.ai_family = AF_INET;

  if (getaddrinfo("localhost", "60000", &hints, &res) != 0)
  {
    fprintf(stderr, "Exit %d
", __LINE__);
    exit(1);
  }

  mFD = socket(res->ai_family, res->ai_socktype, res->ai_protocol);

  if (mFD == -1)
  {
    fprintf(stderr, "Exit %d
", __LINE__);
    exit(1);
  }

  if (connect(mFD, res->ai_addr, res->ai_addrlen) < 0)
  {
    fprintf(stderr, "Exit %d
", __LINE__);
    exit(1);
  }

  freeaddrinfo(res);
}

void read_message(int size, void* data)
{
  int bytesLeft = size;
  int numRd = 0;

  while (bytesLeft != 0)
  {
    fprintf(stderr, "reading %d bytes
", bytesLeft);

    /* Replacing MSG_WAITALL with 0 works fine */
    int num = recv(mFD, data, bytesLeft, MSG_WAITALL);

    if (num == 0)
    {
      break;
    }
    else if (num < 0 && errno != EINTR)
    {
      fprintf(stderr, "Exit %d
", __LINE__);
      exit(1);
    }
    else if (num > 0)
    {
      numRd += num;
      data += num;
      bytesLeft -= num;
      fprintf(stderr, "read %d bytes - remaining = %d
", num, bytesLeft);
    }
  }

  fprintf(stderr, "read total of %d bytes
", numRd);
}

int main(int argc, char **argv)
{
  open_socket();

  uint32_t raw_len = atoi(argv[1]);
  char raw[raw_len];

  read_message(raw_len, raw);

  return 0;
}

Some notes from my testing:

• If "localhost" maps to the loopback address 127.0.0.1, the app hangs on the call to recv() and NEVER returns.
• If "localhost" maps to the ip of the machine, thus routing the packets via the ethernet interface, the app completes successfully.
• When I experience a hang, the server sends a "TCP Window Full" message, and the client responds with a "TCP ZeroWindow" message (see image and attached tcpdump capture). From this point, it hangs forever with the server sending keep-alives and the client sending ZeroWindow messages. The client never seems to expand its window, allowing the transfer to complete.
• During the hang, if I examine the output of "netstat -a", there is data in the servers send queue but the clients receive queue is empty.
• If I remove the MSG_WAITALL flag from the recv() call, the app completes successfully.
• The hanging issue only arises using the loopback interface on 1 particular machine. I suspect this may all be related to timing dependencies.
• As I drop the size of the 'file', the likelihood of the hang occurring is reduced

The source for the test app can be found here:

Socket test source

The tcpdump capture from the loopback interface can be found here:

tcpdump capture

I reproduce the issue by issuing the following commands:

>  gcc socket_test.c -o socket_test
>  perl -e 'for (1..6000000){ print "a" }' | nc -l 60000
>  ./socket_test 6000000

This sees 6000000 bytes sent to the test app which tries to read the data using a single call to recv().

I would love to hear any suggestions on what I might be doing wrong or any further ways to debug the issue.

Answer 1

MSG_WAITALL should block until all data has been received. From the manual page on recv:

This flag requests that the operation block until the full request is satisfied.

However, the buffers in the network stack probably are not large enough to contain everything, which is the reason for the error messages on the server. The client network stack simply can't hold that much data.

The solution is either to increase the buffer sizes (SO_RCVBUF option to setsockopt), split the message into smaller pieces, or receiving smaller chunks putting it into your own buffer. The last is what I would recommend.

Edit: I see in your code that you already do what I suggested (read smaller chunks with own buffering,) so just remove the MSG_WAITALL flag and it should work.

Oh, and when recv returns zero, that means the other end have closed the connection, and that you should do it too.