Tribblix: manual page: mq

MQ_SEND(3C) Standard C Library Functions MQ_SEND(3C)

NAME

mq_send, mq_timedsend, mq_reltimedsend_np - send a message to a
message queue

SYNOPSIS

#include <mqueue.h>

int mq_send(mqd_t mqdes, const char *msg_ptr, size_t msg_len,
unsigned msg_prio);

#include <mqueue.h>
#include <time.h>

int mq_timedsend(mqd_t mqdes, const char *msg_ptr,
size_t msg_len, unsigned msg_prio,
const struct timespec *restrict abs_timeout);

int mq_reltimedsend_np(mqd_t mqdes, const char *msg_ptr,
size_t msg_len, unsigned msg_prio,
const struct timespec *restrict rel_timeout);

DESCRIPTION

The mq_send() function adds the message pointed to by the argument
msg_ptr to the message queue specified by mqdes. The msg_len argument
specifies the length of the message in bytes pointed to by msg_ptr.
The value of msg_len is less than or equal to the mq_msgsize
attribute of the message queue, or mq_send() fails.

If the specified message queue is not full, mq_send() behaves as if
the message is inserted into the message queue at the position
indicated by the msg_prio argument. A message with a larger numeric
value of msg_prio is inserted before messages with lower values of
msg_prio. A message will be inserted after other messages in the
queue, if any, with equal msg_prio. The value of msg_prio must range
from zero to MQ_PRIO_MAX - 1.

If the specified message queue is full and O_NONBLOCK is not set in
the message queue description associated with mqdes (see mq_open(3C)
and mq_setattr(3C)), mq_send() blocks until space becomes available
to enqueue the message, or until mq_send() is interrupted by a
signal. If more than one thread is waiting to send when space becomes
available in the message queue, then the thread of the highest
priority which has been waiting the longest is unblocked to send its
message. Otherwise, it is unspecified which waiting thread is
unblocked. If the specified message queue is full and O_NONBLOCK is
set in the message queue description associated with mqdes, the
message is not queued and mq_send() returns an error.

The mq_timedsend() function adds a message to the message queue
specified by mqdes in the manner defined for the mq_send() function.
However, if the specified message queue is full and O_NONBLOCK is not
set in the message queue description associated with mqdes, the wait
for sufficient room in the queue is terminated when the specified
timeout expires. If O_NONBLOCK is set in the message queue
description, this function is equivalent to mq_send().

The mq_reltimedsend_np() function is identical to the mq_timedsend()
function, except that the timeout is specified as a relative time
interval.

For mq_timedsend(), 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_reltimedsend_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 there
is sufficient room in the queue to add the message immediately. The
validity of the timeout parameter need not be checked when there is
sufficient room in the queue.

RETURN VALUES

Upon successful completion, mq_send(), mq_timedsend(), and
mq_reltimedsend_np() return 0. Otherwise, no message is enqueued, the
functions return -1, and errno is set to indicate the error.

ERRORS

The mq_send(), mq_timedsend(), and mq_reltimedsend_np() functions
will fail if:

EAGAIN
The O_NONBLOCK flag is set in the message queue
description associated with mqdes, and the specified
message queue is full.

EBADF
The mqdes argument is not a valid message queue
descriptor open for writing.

EINTR
A signal interrupted the function call.

EINVAL
The value of msg_prio was outside the valid range.

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 length, msg_len, exceeds the
message size attribute of the message queue.

ETIMEDOUT
The O_NONBLOCK flag was not set when the message queue
was opened, but the timeout expired before the message
could be added to the queue.