| MQ_RECEIVE(3) | Library Functions Manual | MQ_RECEIVE(3) | 
mq_receive, mq_timedreceive —
#include <mqueue.h>
ssize_t
  
  mq_receive(mqd_t mqdes,
    char *msg_ptr, size_t msg_len,
    unsigned *msg_prio);
#include
    <mqueue.h>
  
  #include <time.h>
ssize_t
  
  mq_timedreceive(mqd_t mqdes,
    char *restrict msg_ptr, size_t
    msg_len, unsigned *restrict msg_prio,
    const struct timespec *restrict abs_timeout);
mq_receive() function receives the oldest of the
  highest priority message(s) from the message queue specified by
  mqdes. If the size of the buffer in bytes, specified by
  the msg_len argument, is less than the
  mq_msgsize attribute of the message queue, the function
  fails and returns an error. Otherwise, the selected message will be removed
  from the queue and copied to the buffer pointed to by the
  msg_ptr argument.
If the argument msg_prio is not
    NULL, the priority of the selected message will be
    stored in the location referenced by msg_prio.
If the specified message queue is empty and
    O_NONBLOCK is not set in the message queue
    description associated with mqdes,
    mq_receive() blocks until a message is enqueued on
    the message queue or until mq_receive() is
    interrupted by a signal. If more than one thread is waiting to receive a
    message when a message arrives at an empty queue, then the thread of highest
    priority that has been waiting the longest will be selected to receive the
    message. If the specified message queue is empty and
    O_NONBLOCK is set in the message queue description
    associated with mqdes, no message will be removed from
    the queue, and mq_receive() returns an error.
The timeout expires when the absolute time specified by abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call.
The resolution of the timeout is based on the CLOCK_REALTIME
    clock. The timespec argument is defined in the
    <time.h> header.
Under no circumstance will the operation fail with a timeout if a message can be removed from the message queue immediately. The validity of the abs_timeout parameter will not be checked if a message can be removed from the message queue immediately.
mq_receive() and
  mq_timedreceive() functions return the length of the
  selected message in bytes, and the message is removed from the queue.
  Otherwise, no message will be removed from the queue, the functions return a
  value of -1, and set the global variable errno to
  indicate the error.
mq_receive() and
  mq_timedreceive() functions fail if:
EAGAIN]O_NONBLOCK
      was set in the message description associated with
      mqdes, and the specified message queue is
    empty.EBADF]EINTR]mq_receive() or
      mq_timedreceive() operation was interrupted by a
      signal.EINVAL]EMSGSIZE]ETIMEDOUT]O_NONBLOCK flag was not set when the message
      queue was opened, but no message arrived on the queue before the specified
      timeout expired.mq_receive() and
  mq_timedreceive() functions first appeared in
  NetBSD 5.0.
| November 18, 2015 | NetBSD 9.3 |