RTEMS POSIX API User's Guide
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
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