mq_timedreceive(), mq_timedreceive_monotonic()

Updated: October 26, 2022

Receive a message from a message queue

Synopsis:

#include <mqueue.h>
#include <time.h>

ssize_t mq_timedreceive( 
                mqd_t mqdes,
                char * msg_ptr, 
                size_t msg_len,
                unsigned int * msg_prio,
                const struct timespec * abs_timeout );

ssize_t mq_timedreceive_monotonic( 
                mqd_t mqdes,
                char * msg_ptr, 
                size_t msg_len,
                unsigned int * msg_prio,
                const struct timespec * abs_timeout );

Arguments:

mqdes
The descriptor of the message queue you want to receive a message from, returned by mq_open().
msg_ptr
A pointer to a buffer where the function can store the message data.
msg_len
The size of the buffer, in bytes.
msg_prio
NULL, or a pointer to a location where the function can store the priority of the message that it removed from the queue.
abs_timeout
A pointer to a timespec structure that specifies the absolute time (not the relative time to the current time) to wait before the function stops trying to receive messages.

Library:

Description:

The mq_timedreceive() function receives the oldest of the highest priority messages in the queue specified by mqdes. The mq_timedreceive_monotonic() function is a QNX Neutrino extension; it's similar to mq_timedreceive(), but it uses CLOCK_MONOTONIC, so the timeout isn't affected by changes to the system time.

If you want to choose the clock against which the timeout is measured, you can instead use mq_clockreceive(), which is similar to mq_timedreceive() and mq_timedreceive_monotonic() and differs only because of its clock parameter (clk). Choosing the clock prevents the timeout from being affected by changes to the system time.

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.

If you call mq_timedreceive() with a msg_len of anything other than the mq_msgsize of the specified queue, then mq_timedreceive() 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 the oflag argument to mq_open()), then the mq_timedreceive() call blocks. If multiple mq_timedreceive() calls are blocked on a single queue, then they're unblocked in FIFO order as messages arrive.

Returns:

The size of the message removed from the queue, or -1 if an error occurred (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
One of the following:
  • 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.
  • The process or thread would have blocked, and the abs_timeout parameter specified a nanoseconds field value less than zero or greater than or equal to 1000 million.
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.
ETIMEDOUT
The timeout value was exceeded.

Examples:

See the example for mq_open().

Classification:

mq_timedreceive() is POSIX 1003.1; mq_timedreceive_monotonic() is QNX Neutrino

Safety:  
Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes