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 abs_timeout);
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 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.
If the Timers option is supported, the timeout shall be based on the
CLOCK_REALTIME clock; if the Timers option is not supported, the
timeout shall be based on the system clock as returned by the time()
function.
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 abs_timeout 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
The mq_receive() and mq_timedreceive() 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 abs_timeout
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.
The mq_receive() and mq_timedreceive() 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() , mq_timedsend() , msgctl() , msgget() , msgrcv()
, msgsnd() , time() , the Base Definitions volume of
IEEE Std 1003.1-2001, <mqueue.h>, <time.h>
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 .