mq_send()

Send a message to a queue

Synopsis:

#include <mqueue.h>

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

Arguments:

mqdes

The message-queue descriptor, returned by mq_open(), of the message queue that you want to send a message to.

msg_ptr

A pointer to the message that you want to send.

msg_len

The size of the message.

msg_prio

The priority of the message, in the range from 0 to (MQ_PRIO_MAX-1).

Library:

• For the traditional implementation, libc

  Use the -l c option to qcc to link against this library. This library is usually included automatically.

• For the alternate implementation using asynchronous messages, libmq

  Use the -l mq option to qcc to link against this library.

Description:

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.

---

Neutrino supports two implementations of message queues: a traditional implementation, and an alternate one that uses asynchronous messages. For more information, see the entry for mq and mqueue in the Utilities Reference.

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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.

Returns:

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

Errors:

EAGAIN

The O_NONBLOCK flag was set when opening the queue, and the specified queue is full.

EBADF

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

EINTR

The call was interrupted by a signal.

EINVAL

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

EMSGSIZE

The msg_len argument was greater than the msgsize associated with the specified queue.

Classification:

POSIX 1003.1 MSG

See also:

mq_close(), mq_open(), mq_receive(), mq_timedsend()

mq, mqueue in the Utilities Reference