Tribblix: manual page: mq

MQ_RECEIVE(3C) Standard C Library Functions 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
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 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_timedreceive() function, except that the timeout is specified as a
relative time interval.

For mq_timedreceive(), the timeout expires when the absolute time
specified 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
message 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()
functions 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()
functions may fail if:

EBADMSG
A data corruption problem with the message has been
detected.