MQ_SEND(P) POSIX Programmer's Manual MQ_SEND(P)NAME
mq_send, mq_timedsend - send a message to a message queue (REALTIME)
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 *abs_timeout);
DESCRIPTION
The mq_send() function shall add 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 shall be less than or equal to the mq_msgsize
attribute of the message queue, or mq_send() shall fail.
If the specified message queue is not full, mq_send() shall behave as
if the message is inserted into the message queue at the position indi‐
cated by the msg_prio argument. A message with a larger numeric value
of msg_prio shall be inserted before messages with lower values of
msg_prio. A message shall be inserted after other messages in the
queue, if any, with equal msg_prio. The value of msg_prio shall be less
than {MQ_PRIO_MAX}.
If the specified message queue is full and O_NONBLOCK is not set in the
message queue description associated with mqdes, mq_send() shall block
until space becomes available to enqueue the message, or until
mq_send() is interrupted by a signal. If more than one thread is wait‐
ing to send when space becomes available in the message queue and the
Priority Scheduling option is supported, then the thread of the highest
priority that has been waiting the longest shall be 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
shall not be queued and mq_send() shall return an error.
The mq_timedsend() function shall add 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 shall be terminated when the specified
timeout expires. If O_NONBLOCK is set in the message queue description,
this function shall be equivalent to mq_send().
The timeout shall expire when the absolute time specified by abs_time‐
out 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.
If the Timers option is supported, the timeout shall be based on the
CLOCK_REALTIME clock; if the Timers option is not supported, the time‐
out shall be based on the system clock as returned by the time() func‐
tion.
The resolution of the timeout shall be the resolution of the clock on
which it is based. The timespec argument is defined in the <time.h>
header.
Under no circumstance shall the operation fail with a timeout if there
is sufficient room in the queue to add the message immediately. The
validity of the abs_timeout parameter need not be checked when there is
sufficient room in the queue.
RETURN VALUE
Upon successful completion, the mq_send() and mq_timedsend() func‐
tions shall return a value of zero. Otherwise, no message shall be
enqueued, the functions shall return -1, and errno shall be set to
indicate the error.
ERRORS
The mq_send() and mq_timedsend() functions shall 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 call to mq_send() or mq_timedsend().
EINVAL The value of msg_prio was outside the valid range.
EINVAL The process or thread would have blocked, and the abs_timeout
parameter specified a nanoseconds field value less than zero or
greater than or equal to 1000 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.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
The value of the symbol {MQ_PRIO_MAX} limits the number of priority
levels supported by the application. Message priorities range from 0 to
{MQ_PRIO_MAX}-1.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSOmq_open() , mq_receive() , mq_setattr() , mq_timedreceive() , time() ,
the Base Definitions volume of IEEE Std 1003.1-2001, <mqueue.h>,
<time.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 MQ_SEND(P)
