mq_send

put a message into a message queue

Synopsis:

#include <mqueue.h>
int mq_send (mqd_t mqdes, 
             const char *msg_ptr, 
             size_t msg_len, 
             unsigned int msg_prio);

Description:

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

The queue is maintained in priority order (priorities may range from 0 to MQ_PRIO_MAX), and in FIFO order within the same priority.

If the QNX extension MQ_PRIO_RESTRICT has been set for the queue, then an attempt to send a message at a msg_prio higher than the priority of the sending process results in an error, and errno is set to EINVAL. If the QNX extension MQ_PRIO_BOOST has been set, and msg_prio is 0, then the priority of the message is boosted to that of the calling process.

If the number of elements on the specified queue is equal to its mq_maxmsg (and mq_maxmsg doesn't equal 0), and neither O_NONBLOCK (in oflag of mq_open()) nor MQ_NONBLOCK (in the queue's mq_flags) has been set, the call to mq_send() blocks. It becomes unblocked when there is 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.

Calling write() on a file descriptor that represents a message queue is analogous to calling mq_send() with a msg_prio of 0.

Returns:

If the mq_send() call was successful, then it returns the number of bytes actually sent. If not, it returns -1, and errno is set.

Errors:

EAGAIN

The O_NONBLOCK flag was set when opening the queue, or the MQ_NONBLOCK flag was set in its attributes, and the specified queue is full.

EBADF

mq_des doesn't represent a valid message queue descriptor, or mq_des wasn't opened for writing.

EINTR

The call was interrupted by a signal.

EINVAL

This value indicates one of the following:

• msg_len is negative
• msg_prio is greater than MQ_PRIO_MAX
• msg_prio is less than 0
• MQ_PRIO_RESTRICT is set in the mq_attr of mq_des, and msg_prio is greater than the priority of the calling process.

EMSGSIZE

The msg_len is greater than the msgsize associated with the specified queue.

See also:

errno, mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_setattr(), mq_unlink()

Classification:

POSIX 1003.4

Systems:

QNX