Send a message to a queue


#include <mqueue.h>

int mq_send( mqd_t mqdes, 
             const char * msg_ptr, 
             size_t msg_len, 
             unsigned int msg_prio );


The message-queue descriptor, returned by mq_open(), of the message queue that you want to send a message to.
A pointer to the message that you want to send.
The size of the message.
The priority of the message, in the range from 0 to (MQ_PRIO_MAX-1).



The mq_send() function puts a message of size msg_len and pointed to by msg_ptr into the queue indicated by mqdes. The new message has a priority of msg_prio.

Note: The message queue manager needs to be running. QNX Neutrino supports two implementations of message queues: a traditional implementation, and an alternate one that uses the mq server and a queue in kernel space. For more information, see the entries for mq and mqueue in the Utilities Reference, as well as the POSIX Message Queues: Two Implementations technote.

The queue is maintained in priority order, and in FIFO order within the same priority.

If the number of elements on the specified queue is equal to its mq_maxmsg, and O_NONBLOCK wasn't set (in the oflag argument to mq_open()), the call to mq_send() blocks. It becomes unblocked when there's room on the queue to send the given message. If more than one mq_send() is blocked on a given queue, and space becomes available in that queue to send, then the mq_send() with the highest priority message is unblocked.

In the traditional (mqueue) implementation, calling write() with mqdes is analogous to calling mq_send() with a msg_prio of 0.


-1 if an error occurred (errno is set). Any other value indicates success.


The O_NONBLOCK flag was set when opening the queue, and the specified queue is full.
The mqdes argument doesn't represent a valid message queue descriptor, or mqdes wasn't opened for writing.
The call was interrupted by a signal.
One of the following cases is true:
  • msg_len was negative
  • msg_prio was greater than (MQ_PRIO_MAX-1)
  • msg_prio was less than 0
The msg_len argument was greater than the msgsize associated with the specified queue.


POSIX 1003.1 MSG

Cancellation point Yes
Interrupt handler No
Signal handler No
Thread Yes