MQ_RECEIVE(3)

MQ_RECEIVE(3) | Library Functions Manual | MQ_RECEIVE(3)

# NAME

`mq_receive, mq_timedreceive` —
receive a message from a message queue (REALTIME)

# LIBRARY

POSIX Real-time Library (librt, -lrt)

# SYNOPSIS

`#include <mqueue.h>`

ssize_t
`mq_receive`(mqd_t mqdes, char *msg_ptr, size_t msg_len, unsigned *msg_prio);

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

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

# DESCRIPTION

The `mq_receive`() function receives the oldest of the highest priority message(s) from the message queue specified by mqdes. If the size of the buffer in bytes, specified by the msg_len argument, is less than the mq_msgsize attribute of the message queue, the function fails and returns an error. Otherwise, the selected message will be removed from the queue and copied to the buffer pointed to by the msg_ptr argument.

If the argument msg_prio is not `NULL`, the priority of the selected message will be stored in the location referenced by msg_prio.

If the specified message queue is empty and `O_NONBLOCK` is not set in the message queue description associated with mqdes, `mq_receive`() blocks until a message is enqueued on the message queue or until `mq_receive`() is interrupted by a signal. If more than one thread is waiting to receive a message when a message arrives at an empty queue, then the thread of highest priority that has been waiting the longest will be selected to receive the message. If the specified message queue is empty and `O_NONBLOCK` is set in the message queue description associated with mqdes, no message will be removed from the queue, and `mq_receive`() returns an error.

The timeout expires when the absolute time specified by abs_timeout passes, as measured by the clock on which timeouts are based (that is, when the value of that clock equals or exceeds abs_timeout), or if the absolute time specified by abs_timeout has already been passed at the time of the call.

The resolution of the timeout is based on the CLOCK_REALTIME clock. The timespec argument is defined in the `<time.h>` header.

Under no circumstance will the operation fail with a timeout if a message can be removed from the message queue immediately. The validity of the abs_timeout parameter will not be checked if a message can be removed from the message queue immediately.

# RETURN VALUES

Upon successful completion, the `mq_receive`() and `mq_timedreceive`() functions return the length of the selected message in bytes, and the message is removed from the queue. Otherwise, no message will be removed from the queue, the functions return a value of -1, and set the global variable errno to indicate the error.

# ERRORS

The `mq_receive`() and `mq_timedreceive`() functions fail if:

[`EAGAIN`]
  `O_NONBLOCK` was set in the message description associated with mqdes, and the specified message queue is empty.
[`EBADF`]
  The mqdes argument is not a valid message queue descriptor open for reading.
[`EINTR`]
  The `mq_receive`() or `mq_timedreceive`() operation was interrupted by a signal.
[`EINVAL`]
  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 specified message buffer size, msg_len, is less than the message size attribute of the message queue.
[`ETIMEDOUT`]
  The `O_NONBLOCK` flag was not set when the message queue was opened, but no message arrived on the queue before the specified timeout expired.

# SEE ALSO

mq(3), mq_send(3)

# STANDARDS

These functions are expected to conform to the IEEE Std 1003.1-2001 (“POSIX.1”) standard.

# HISTORY

The `mq_receive`() and `mq_timedreceive`() functions first appeared in NetBSD 5.0.

# COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html.

November 18, 2015 | NetBSD 9.5