MQ_RECEIVE(3C)MQ_RECEIVE(3C)NAME
mq_receive, mq_timedreceive, mq_reltimedreceive_np - receive a message
from a message queue
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);
ssize_t mq_reltimedreceive_np(mqd_t mqdes,
char *restrict msg_ptr, size_t msg_len,
unsigned *restrict msg_prio,
const struct timespec *restrict rel_timeout);
DESCRIPTION
The mq_receive() 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 msg_len, is less than the mq_msgsize
member of the message queue, the function fails and returns an error.
Otherwise, the selected message is removed from the queue and copied to
the buffer pointed to by msg_ptr.
If the value of msg_len is greater than {SSIZE_MAX}, the result is
implementation-defined.
If msg_prio is not NULL, the priority of the selected message is 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, (see mq_open(3C)
and mq_setattr(3C)), mq_receive() blocks, waiting until a message is
enqueued on the message queue, or until mq_receive() is interrupted by
a signal. If more than one process (or thread) is waiting to receive a
message when a message arrives at an empty queue, then the process of
highest priority that has been waiting the longest is 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 is removed from the queue, and mq_receive() returns
an error.
The mq_timedreceive() function receives the oldest of the highest pri‐
ority 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 with the mq_open(3C) function, and no
message exists on the queue to satisfy the receive, the wait for such a
message is terminated when the specified timeout expires. If O_NONBLOCK
is set, this function is equivalent to mq_receive().
The mq_reltimedreceive_np() function is identical to the mq_time‐
dreceive() function, except that the timeout is specified as a relative
time interval.
For mq_timedreceive(), the timeout expires when the absolute time spec‐
ified by abs_timeout passes, as measured by the CLOCK_REALTIME clock
(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.
For mq_reltimedreceive_np(), the timeout expires when the time interval
specified by rel_timeout passes, as measured by the CLOCK_REALTIME
clock, or if the time interval specified by rel_timeout is negative at
the time of the call.
The resolution of the timeout is the resolution of the CLOCK_REALTIME
clock. The timespec argument is defined in the <time.h> header.
Under no circumstance does the operation fail with a timeout if a mes‐
sage can be removed from the message queue immediately. The validity of
the timeout parameter need not be checked if a message can be removed
from the message queue immediately.
RETURN VALUES
Upon successful completion, mq_receive(), mq_timedreceive(), and
mq_reltimedreceive_np() return the length of the selected message in
bytes and the message is removed from the queue. Otherwise, no message
is removed from the queue, the functions return a value of −1, and sets
errno to indicate the error condition.
ERRORS
The mq_receive(), mq_timedreceive(), and mq_reltimedreceive_np() func‐
tions will 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.
EINTR
The function was interrupted by a signal.
EINVAL
The process or thread would have blocked, and the timeout
parameter specified a nanoseconds field value less than
zero or greater than or equal to 1,000 million.
EMSGSIZE
The specified message buffer size, msg_len, is less than
the message size member of the message queue.
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(), mq_timedreceive(), and mq_reltimedreceive_np() func‐
tions may fail if:
EBADMSG
A data corruption problem with the message has been
detected.
ATTRIBUTES
See attributes(5) for descriptions of the following attributes:
┌────────────────────┬─────────────────┐
│ATTRIBUTE TYPE │ ATTRIBUTE VALUE │
├────────────────────┼─────────────────┤
│Interface Stability │ Committed │
├────────────────────┼─────────────────┤
│MT-Level │ MT-Safe │
├────────────────────┼─────────────────┤
│Standard │ See below. │
└────────────────────┴─────────────────┘
For mq_receive() and mq_timedreceive(). see standards(5).
SEE ALSOmqueue.h(3HEAD), mq_open(3C), mq_send(3C), mq_setattr(3C),
attributes(5), standards(5)
Feb 5, 2008 MQ_RECEIVE(3C)
