The
mq_send() function will add the message pointed to by the argument
msg_ptr to the message queue specified by
mqdes. The
msg_len argument specifies the length of the message, in bytes, pointed to by
msg_ptr. The value of
msg_len will be less than or equal to the
mq_msgsize attribute of the message queue, or
mq_send() will fail.
If the specified message queue is not full,
mq_send() behaves as if the message is inserted into the message queue at the position indicated by the
msg_prio argument. A message with a larger numeric value of
msg_prio will be inserted before messages with lower values of
msg_prio. A message will be inserted after other messages in the queue, if any, with equal
msg_prio. The value of
msg_prio will be less than {
MQ_PRIO_MAX}.
If the specified message queue is full and
O_NONBLOCK is not set in the message queue description associated with
mqdes,
mq_send() blocks until space becomes available to enqueue the message, or until
mq_send() is interrupted by a signal. If more than one thread is waiting to send when space becomes available in the message queue, then the thread of the highest priority that has been waiting the longest will be unblocked to send its message. If the specified message queue is full and
O_NONBLOCK is set in the message queue description associated with
mqdes, the message will not be queued and
mq_send() will return an error.
The
mq_timedsend() function will add a message to the message queue specified by
mqdes in the manner defined for the
mq_send() function. However, if the specified message queue is full and
O_NONBLOCK is not set in the message queue description associated with
mqdes, the wait for sufficient room in the queue will be terminated when the specified timeout expires. If
O_NONBLOCK is set in the message queue description, this function will be equivalent to
mq_send().
The timeout will expire 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 there is sufficient room in the queue to add the message immediately. The validity of the
abs_timeout parameter need not be checked when there is sufficient room in the queue.