RTEMS 4.8.0 On-Line Library

Message Passing Manager Building a Message Queue Attribute Set

RTEMS POSIX API User's Guide

16.2.4: Building a Message Queue Attribute Set

The mq_attr structure is used to define the characteristics of the message queue.

typedef struct mq_attr{
  long mq_flags;
  long mq_maxmsg;
  long mq_msgsize;
  long mq_curmsgs;
};

All of these attributes are set when the message queue is created using mq_open. The mq_flags field is not used in the creation of a message queue, it is only used by mq_setattr and mq_getattr. The structure mq_attr is passed as an argument to mq_setattr and mq_getattr.

The mq_flags contain information affecting the behavior of the message queue. The O_NONBLOCK mq_flag is the only flag that is defined. In mq_setattr, the mq_flag can be set to dynamically change the blocking and non-blocking behavior of the message queue. If the non-block flag is set then the message queue is non-blocking, and requests to send and receive messages do not block waiting for resources. For a blocking message queue, a request to send might have to wait for an empty message queue, and a request to receive might have to wait for a message to arrive on the queue. Both mq_maxmsg and mq_msgsize affect the sizing of the message queue. mq_maxmsg specifies how many messages the queue can hold at any one time. mq_msgsize specifies the size of any one message on the queue. If either of these limits is exceeded, an error message results.

Upon return from mq_getattr, the mq_curmsgs is set according to the current state of the message queue. This specifies the number of messages currently on the queue.

RTEMS POSIX API User's Guide