MQ_RECEIVE(3P)

PROLOG
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.

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

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 abstime);

DESCRIPTION
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.

RETURN VALUE
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.

ERRORS
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.

EXAMPLES
None.

APPLICATION USAGE
None.

RATIONALE
None.

FUTURE DIRECTIONS
None.

SEE ALSO
mq_open(), mq_send(), msgctl(), msgget(), msgrcv(), msgsnd(), time()

The Base Definitions volume of POSIX.1-2017, <mqueue.h>, <time.h>

COPYRIGHT
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 http://www.opengroup.org/unix/online.html .

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