Section: POSIX Programmer's Manual (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.
--- receive a message from a message queue (REALTIME
ssize_t mq_receive(mqd_t mqdes, char *msg_ptr, size_t msg_len,
ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr,
size_t msg_len, unsigned *restrict msg_prio,
const struct timespec *restrict abstime);
function shall receive the oldest of the highest priority
message(s) from the message queue specified by
If the size of the buffer in bytes, specified by the
argument, is less than the
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
If the value of
is greater than
the result is implementation-defined.
If the argument
is not NULL, the priority of the selected message shall be stored in the
location referenced by
If the specified message queue is empty and O_NONBLOCK
is not set in the message queue description associated with
shall block until a message is enqueued on the message queue or until
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
no message shall be removed from the queue, and
shall return an error.
function shall receive the oldest of the highest priority messages
from the message queue specified by
as described for the
function. However, if O_NONBLOCK was not specified when the message
queue was opened via the
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
The timeout expires when the absolute time specified by
passes, as measured by the clock on which timeouts are based (that is,
when the value of that clock equals or exceeds
or if the absolute time specified by
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
argument is defined in the
Under no circumstance shall the operation fail with a timeout if a
message can be removed from the message queue immediately. The validity
parameter need not be checked if a message can be removed from the
message queue immediately.
Upon successful completion, the
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
to indicate the error.
These functions shall fail if:
O_NONBLOCK was set in the message description associated with
and the specified message queue is empty.
argument is not a valid message queue descriptor open for reading.
The specified message buffer size,
is less than the message size attribute of the message queue.
operation was interrupted by a signal.
The process or thread would have blocked, and the
parameter specified a nanoseconds field value less than zero or greater
than or equal to 1000 million.
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:
The implementation has detected a data corruption problem with the
The following sections are informative.
The Base Definitions volume of POSIX.1-2017,
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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
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