Originální popis anglicky:
mq_receive, mq_timedreceive - receive a message from a message queue (
REALTIME)
Návod, kniha: POSIX Programmer's Manual
#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);
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.
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.
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.
None.
None.
None.
None.
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>
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
.
