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mq_receive()

Receive a message from a queue

Synopsis:

#include <mqueue.h>

ssize_t mq_receive( mqd_t mqdes, 
                    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 get a message from.

msg_ptr

A pointer to a buffer where the function can store the message received.

msg_len

The message size of the given queue.

msg_prio

NULL, or a pointer to a location where the function can store the priority of the message received.

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_receive() function is used to receive the oldest of the highest priority messages in the queue specified by mqdes. The priority of the message received is put in the location pointed to by msg_prio, the data itself in the location pointed to by msg_ptr, and the size received is be returned.

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.

If you call mq_receive() with a msg_len of anything other than the mq_msgsize of the specified queue, then mq_receive() returns an error, and errno is set to EINVAL.

If there are no messages on the queue specified, and O_NONBLOCK wasn't set in oflag during mq_open(), then the mq_receive() call blocks. If multiple mq_receive() calls are blocked on a single queue, then they're unblocked in FIFO order as messages arrive.

In the traditional (mqueue) implementation, calling read() with mqdes is analogous to calling mq_receive() with a NULL msg_prio.

Returns:

The size of the message removed from the queue. If the call fails, -1 is returned as the size, no message is removed from the queue, and errno is set.

Errors:

EAGAIN

The O_NONBLOCK flag was set and there are no messages currently on the specified queue.

EBADF

The mqdes argument doesn't represent a valid queue open for reading.

EINTR

The operation was interrupted by a signal.

EINVAL

The msg_ptr argument isn't a valid pointer, or msg_len is less than 0, or msg_len is less than the message size specified in mq_open(). The default message size is 4096 bytes for the traditional (mqueue) implementation, and 256 bytes for the alternate (mq) implementation.

EMSGSIZE

The given msg_len is shorter than the mq_msgsize for the given queue or the given msg_len is too short for the message that would have been received.

Classification:

POSIX 1003.1 MSG

See also:

mq_close(), mq_open(), mq_send(), mq_timedreceive(), read()

mq, mqueue in the Utilities Reference