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1.7.8: mq_timedreceive() blocks, mq_receive() with O_NONBLOCK blocks, too


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



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"

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 */
  attrQueue.mq_flags = 0;                      /* no exceptional behavior (just O_NONBLOCK currently available)  */
  attrQueue.mq_maxmsg = 10;                    /* room for at most 10 messages in the queue */
  attrQueue.mq_msgsize = MAX_QUEUE_ENTRY_SIZE; /* maximum size of a message */
  attrQueue.mq_curmsgs = 0;                    /* this (current number of messages) will be ignored */

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

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

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

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

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


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