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MQ_RECEIVE(3P) POSIX Programmer's Manual MQ_RECEIVE(3P)
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
mq_receive, mq_timedreceive — receive a message from a message queue
(REALTIME)
#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 abstime);
The mq_receive() function shall receive 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 shall fail and return an error. Otherwise, the selected
message shall be removed from the queue and copied to the buffer
pointed to by the msg_ptr argument.
If the value of msg_len is greater than {SSIZE_MAX}, the result is
implementation-defined.
If the argument msg_prio is not NULL, the priority of the selected
message shall 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()
shall block 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
and the Priority Scheduling option is supported, then the thread of
highest priority that has been waiting the longest shall be selected
to receive the message. Otherwise, it is unspecified which waiting
thread receives the message. If the specified message queue is empty
and O_NONBLOCK is set in the message queue description associated
with mqdes, no message shall be removed from the queue, and
mq_receive() shall return an error.
The mq_timedreceive() function shall receive the oldest of the
highest priority messages from the message queue specified by mqdes
as described for the mq_receive() function. However, if O_NONBLOCK
was not specified when the message queue was opened via the mq_open()
function, and no message exists on the queue to satisfy the receive,
the wait for such a message shall be terminated when the specified
timeout expires. If O_NONBLOCK is set, this function is equivalent to
mq_receive().
The timeout expires when the absolute time specified by abstime
passes, as measured by the clock on which timeouts are based (that
is, when the value of that clock equals or exceeds abstime), or if
the absolute time specified by abstime has already been passed at the
time of the call.
The timeout shall be based on the CLOCK_REALTIME clock. The
resolution of the timeout shall be the resolution of the clock on
which it is based. The timespec argument is defined in the <time.h>
header.
Under no circumstance shall the operation fail with a timeout if a
message can be removed from the message queue immediately. The
validity of the abstime parameter need not be checked if a message
can be removed from the message queue immediately.
Upon successful completion, the mq_receive() and mq_timedreceive()
functions shall return the length of the selected message in bytes
and the message shall be removed from the queue. Otherwise, no
message shall be removed from the queue, the functions shall return a
value of −1, and set errno to indicate the error.
These functions shall 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.
EMSGSIZE
The specified message buffer size, msg_len, is less than the
message size attribute of the message queue.
EINTR The mq_receive() or mq_timedreceive() operation was
interrupted by a signal.
EINVAL The process or thread would have blocked, and the abstime
parameter specified a nanoseconds field value less than zero
or greater than or equal to 1000 million.
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.
These functions may fail if:
EBADMSG
The implementation has detected a data corruption problem with
the message.
The following sections are informative.
None.
None.
None.
None.
mq_open(3p), mq_send(3p), msgctl(3p), msgget(3p), msgrcv(3p),
msgsnd(3p), time(3p)
The Base Definitions volume of POSIX.1‐2008, mqueue.h(0p), time.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2013 MQ_RECEIVE(3P)
Pages that refer to this page: mqueue.h(0p), time.h(0p), clock_getres(3p), mq_notify(3p), mq_open(3p), mq_send(3p), mq_timedreceive(3p), msgctl(3p), msgget(3p), msgrcv(3p), msgsnd(3p)