From: Manuel Wienand <Manuel DOT Wienand AT ubitronix DOT com>
To: "cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Date: Thu, 3 Mar 2011 10:57:37 +0100
Subject: 1.7.8: mq_timedreceive() blocks, mq_receive() with O_NONBLOCK blocks, too
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Hi,

I have a problem with the mqueues.
If I use mq_timedreceive() and do not set O_NONBLOCK, mq_timedreceive() blo=
cks forever and eats 100% of one cpu core.
If I use mq_receive() and set O_NONBLOCK, mq_receive() blocks forever (0% c=
pu).

Thanks,

Manuel


Here is some test code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <mqueue.h>
#include <errno.h>
#include <fcntl.h>

#define QUEUE_NAME "testQ"
#define SEND_TEST  "SendTest"
#define MAX_QUEUE_ENTRY_SIZE 20

int main(void) {

  int             res;
  mqd_t           msgQueue;
  struct mq_attr  attrQueue;
  char            recBuffer[MAX_QUEUE_ENTRY_SIZE];
  struct timespec abs_timeout;

  /* Message-Queue attribute structure */
  memset(&attrQueue,0x00,sizeof(attrQueue));
  attrQueue.mq_flags =3D 0;                      /* no exceptional behavior=
 (just O_NONBLOCK currently available)  */
  attrQueue.mq_maxmsg =3D 10;                    /* room for at most 10 mes=
sages in the queue */
  attrQueue.mq_msgsize =3D MAX_QUEUE_ENTRY_SIZE; /* maximum size of a messa=
ge */
  attrQueue.mq_curmsgs =3D 0;                    /* this (current number of=
 messages) will be ignored */

  res =3D mq_unlink(QUEUE_NAME);
  if ((res =3D=3D -1) && (errno !=3D ENOENT))
  {
    // Don't print anything if the queue can't be unlinked, when it just do=
esn't exist.
    printf("Failed to unlink msg queue %s: %s %d\n", QUEUE_NAME, sys_errlis=
t[errno],errno); fflush(stdout);
  }

  /* mq_open() for creating a new queue*/
  msgQueue =3D mq_open(QUEUE_NAME, O_RDWR | O_CREAT | O_EXCL, S_IRWXU | S_I=
RWXG, &attrQueue);
  //msgQueue =3D mq_open(QUEUE_NAME, O_RDWR | O_CREAT | O_EXCL, S_IRWXU | S=
_IRWXG | O_NONBLOCK, &attrQueue);
  if( msgQueue =3D=3D (mqd_t)-1 )
  {
    printf("Failed to create msg queue %s: %s %d\n", QUEUE_NAME, sys_errlis=
t[errno],errno); fflush(stdout);
    return -1;
  }
  printf("Queue is open.\n"); fflush(stdout);

#if 0
  // Send data.
  res =3D mq_send(msgQueue, SEND_TEST, strlen(SEND_TEST), 0);
  if (res =3D=3D -1)
  {
    printf("Failed to send request message.\n");
    return -1;
  }
  printf("Sent data.\n"); fflush(stdout);
#endif

  clock_gettime(CLOCK_REALTIME, &abs_timeout);
  abs_timeout.tv_sec +=3D 1; // 1 Second timeout

  // Wait for the response message.
  res =3D mq_timedreceive(msgQueue, recBuffer, MAX_QUEUE_ENTRY_SIZE, NULL, =
&abs_timeout);
  //res =3D mq_receive(msgQueue, recBuffer, MAX_QUEUE_ENTRY_SIZE, NULL);
  if (res =3D=3D -1)
  {
    printf("Failed to receive message from read queue: %s %d.\n", sys_errli=
st[errno],errno); fflush(stdout);
    return -1;
  }
  printf("Received data.\n"); fflush(stdout);

	return EXIT_SUCCESS;
}


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