получать сообщение из очереди сообщений (РЕАЛЬНОЕ ВРЕМЯ) (receive a message from a message queue (REALTIME))
Пролог (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.
Имя (Name)
mq_receive, mq_timedreceive — receive a message from a message
queue (REALTIME)
Синопсис (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);
Описание (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.
Возвращаемое значение (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.
Ошибки (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.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
mq_open(3p), mq_send(3p), msgctl(3p), msgget(3p), msgrcv(3p),
msgsnd(3p), time(3p)
The Base Definitions volume of POSIX.1‐2017, mqueue.h(0p),
time.h(0p)
