mq_setattr()

Set a queue's attributes

Synopsis:

#include <mqueue.h>

int mq_setattr( mqd_t mqdes, 
                const struct mq_attr* mqstat,
                struct mq_attr* omqstat );

Arguments:

mqdes

The message-queue descriptor, returned by mq_open(), of the message queue that you want to set the attributes of.

mqstat

A pointer to a mq_attr structure that specifies the attributes that you want to use for the message queue. For more information about this structure, see mq_getattr(); for information about which attributes you can set, see below.

omqstat

NULL, or a pointer to a mq_attr structure where the function can store the old attributes of the message queue.

Library:

• For the traditional implementation, libc

  Use the -l c option to qcc to link against this library. This library is usually included automatically.

• For the alternate implementation that uses the mq server and a queue in kernel space, libmq

  Use the -l mq option to qcc to link against this library.

Description:

The mq_setattr() function sets the mq_flags field for the specified queue (passed as the mq_flags field in mqstat). If omqstat isn't NULL, then the old attribute structure is stored in the location that it points to.

---

The message queue manager needs to be running. Neutrino supports two implementations of message queues: a traditional implementation, and an alternate one that uses the mq server and a queue in kernel space. For more information, see the entries for mq and mqueue in the Utilities Reference, as well as the POSIX Message Queues: Two Implementations technote.

---

This function ignores the mq_maxmsg, mq_msgsize, and mq_curmsgs fields of mqstat. The mq_flags field is the bitwise OR of zero or more of the following constants:

O_NONBLOCK

No mq_receive() or mq_send() will ever block on this queue. If the queue is in such a condition that the given operation can't be performed without blocking, then an error is returned, and errno is set to EAGAIN.

---

The settings that you make for mq_flags apply only to the given message-queue description (i.e., locally), not to the queue itself.

---

Returns:

-1 if the function couldn't change the attributes (errno is set). Any other value indicates success.

Errors:

EBADF

Invalid message queue mqdes.

Classification:

POSIX 1003.1 MSG

See also:

mq_getattr(), mq_open(), mq_receive(), mq_send()

mq, mqueue in the Utilities Reference

“POSIX message queues” in the “Interprocess Communication (IPC)” chapter of System Architecture

POSIX Message Queues: Two Implementations technote.