January 5, 2009 MQ_SEND 3

### NAME

mq_send,mq_timedsend - send a message to a message queue (REALTIME)

### LIBRARY

POSIX Real-time Library (librt, -lrt)

### SYNOPSIS

```int` mq_send(`` ```mqd_t mqdes` ```const char *msg_ptr` ```size_t msg_len` ```unsigned msg_prio` )
```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 will 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` will be less than or equal to the ```mq_msgsize` attribute of the message queue, or mq_send(``) will fail.

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` will be 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` will 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(``) 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 that has been waiting the longest will be unblocked to send its message. If the specified message queue is full and ```O_NONBLOCK` is set in the message queue description associated with ```mqdes`, the message will not be queued and mq_send(``) will return an error.

The mq_timedsend(``) function will 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 will be terminated when the specified timeout expires. If ```O_NONBLOCK` is set in the message queue description, this function will be equivalent to mq_send(``).

The timeout will expire 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 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 VALUES

Upon successful completion, the mq_send(``) and mq_timedsend(``) functions return a value of zero. Otherwise, no message will be enqueued, the functions will return -1, and the global variable errno will be set to indicate the error.

### ERRORS

The mq_send(``) and mq_timedsend(``) functions 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, or 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.

### SEE ALSO

mq_close(3), mq_getattr(3), mq_notify(3), mq_open(3), mq_receive(3), mq_setattr(3), mq_unlink(3)

### STANDARDS

These functions are expected to conform to the IEEE Std 1003.1-2001 (``POSIX.1'') standard.

### HISTORY

The mq_send(``) and mq_timedsend(``) functions first appeared in NetBSD5.0.

### 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 .