RTEMS 4.9.4 On-Line Library

Message Passing Manager mq_setattr - Set Message Queue Attributes

RTEMS POSIX API User's Guide

16.4.7: mq_setattr - Set Message Queue Attributes

CALLING SEQUENCE:

#include <mqueue.h>

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

STATUS CODES:

EBADF - The message queue descriptor does not refer to a valid, open queue. EINVAL - The mq_flag value is invalid.

DESCRIPTION:

The mq_setattr function is used to set attributes associated with the open message queue description referenced by the message queue descriptor specified by mqdes. The *omqstat represents the old or previous attributes. If omqstat is non-NULL, the function mq_setattr() stores, in the location referenced by omqstat, the previous message queue attributes and the current queue status. These values are the same as would be returned by a call to mq_getattr() at that point.

There is only one mq_attr.mq_flag which can be altered by this call. This is the flag that deals with the blocking and non-blocking behavior of the message queue. If the flag is set then the message queue is non-blocking, and requests to send or receive do not block while waiting for resources. If the flag is not set, then message send and receive may involve waiting for an empty queue or waiting for a message to arrive.

Upon successful completion, the function returns a value of zero and the attributes of the message queue have been changed as specified. Otherwise, the message queue attributes is unchanged, and the function returns a value of -1 and sets errno to indicate the error.

NOTES:

All other fields in the mq_attr are ignored by this call.

RTEMS POSIX API User's Guide