ssize_t
mq_receive(
mqd_t mqdes
char *msg_ptr
size_t msg_len
unsigned *msg_prio
)
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
)
)
function receives 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 fails and returns an error.
Otherwise, the selected message will be removed from the queue and copied
to the buffer pointed to by the
msg_ptr
argument.
If the argument
msg_prio
is not
NULL
,
the priority of the selected message will 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()
blocks 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, then the thread of highest
priority that has been waiting the longest will be selected to
receive the message.
If the specified message queue is empty and
O_NONBLOCK
is set in the message queue description associated with
mqdes
,
no message will be removed from the queue, and
mq_receive()
returns an error.
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.
The resolution of the timeout is based on the CLOCK_REALTIME clock.
The
timespec
argument is defined in the
<time.h>
header.
Under no circumstance will the operation fail with a timeout if a
message can be removed from the message queue immediately.
The validity of the
abs_timeout
parameter will not be checked if a message can be removed from the
message queue immediately.
)
and
mq_timedreceive(
)
functions return a value of zero.
Otherwise, no message will be removed from the queue,
the functions return a value of
-1, and set the global variable
errno
to indicate the error.
)
and
mq_timedreceive(
)
functions fail if:
EAGAIN
]
O_NONBLOCK
was set in the message description associated with
mqdes
,
and the specified message queue is empty.
EBADF
]
mqdes
argument is not a valid message queue descriptor open for reading.
EINTR
]
)
or
mq_timedreceive(
)
operation was interrupted by a signal.
EINVAL
]
abs_timeout
parameter specified a nanoseconds field value less than zero
or greater than or equal to 1000 million.
EMSGSIZE
]
msg_len
,
is less than the message size attribute of the message queue.
ETIMEDOUT
]
O_NONBLOCK
flag was not set when the message queue was opened,
but no message arrived on the queue before the specified timeout expired.
)
and
mq_timedreceive(
)
functions first appeared in
NetBSD5.0.