mq_receive, mq_timedreceive - receive a message from a message queue
Real-time library (
librt,
-lrt)
#include <mqueue.h>
ssize_t mq_receive(mqd_t mqdes, char msg_ptr[.msg_len],
size_t msg_len, unsigned int *msg_prio);
#include <time.h>
#include <mqueue.h>
ssize_t mq_timedreceive(mqd_t mqdes, char *restrict msg_ptr[.msg_len],
size_t msg_len, unsigned int *restrict msg_prio,
const struct timespec *restrict abs_timeout);
mq_timedreceive():
_POSIX_C_SOURCE >= 200112L
mq_receive() removes the oldest message with the highest priority from
the message queue referred to by the message queue descriptor
mqdes,
and places it in the buffer pointed to by
msg_ptr. The
msg_len
argument specifies the size of the buffer pointed to by
msg_ptr; this
must be greater than or equal to the
mq_msgsize attribute of the queue
(see
mq_getattr(3)). If
msg_prio is not NULL, then the buffer to
which it points is used to return the priority associated with the received
message.
If the queue is empty, then, by default,
mq_receive() blocks until a
message becomes available, or the call is interrupted by a signal handler. If
the
O_NONBLOCK flag is enabled for the message queue description, then
the call instead fails immediately with the error
EAGAIN.
mq_timedreceive() behaves just like
mq_receive(), except that if
the queue is empty and the
O_NONBLOCK flag is not enabled for the
message queue description, then
abs_timeout points to a structure which
specifies how long the call will block. This value is an absolute timeout in
seconds and nanoseconds since the Epoch, 1970-01-01 00:00:00 +0000 (UTC),
specified in a
timespec(3) structure.
If no message is available, and the timeout has already expired by the time of
the call,
mq_timedreceive() returns immediately.
On success,
mq_receive() and
mq_timedreceive() return the number
of bytes in the received message; on error, -1 is returned, with
errno
set to indicate the error.
- EAGAIN
- The queue was empty, and the O_NONBLOCK flag was set
for the message queue description referred to by mqdes.
- EBADF
- The descriptor specified in mqdes was invalid or not
opened for reading.
- EINTR
- The call was interrupted by a signal handler; see
signal(7).
- EINVAL
- The call would have blocked, and abs_timeout was
invalid, either because tv_sec was less than zero, or because
tv_nsec was less than zero or greater than 1000 million.
- EMSGSIZE
-
msg_len was less than the mq_msgsize
attribute of the message queue.
- ETIMEDOUT
- The call timed out before a message could be
transferred.
For an explanation of the terms used in this section, see
attributes(7).
Interface |
Attribute |
Value |
mq_receive (), mq_timedreceive () |
Thread safety |
MT-Safe |
POSIX.1-2001, POSIX.1-2008.
On Linux,
mq_timedreceive() is a system call, and
mq_receive() is
a library function layered on top of that system call.
mq_close(3),
mq_getattr(3),
mq_notify(3),
mq_open(3),
mq_send(3),
mq_unlink(3),
timespec(3),
mq_overview(7),
time(7)