CrossWorks for ARM User Guide

21Message queuesA message queue is a structure that enables tasks to post and receive messages. Amessage is a generic (void) pointer and as such can be used to send data that will fitinto a pointer type (2 or 4 bytes depending upon processor word size) or can be usedto pass a pointer to a block of memory. The message queue has a buffer that enablesa number of posts to be completed without receives occuring. The buffer keeps theposted messages in a fifo order so the oldest message is received first. When the bufferisnt full a post will put the message at the back of the queue and the calling taskcontinues execution. When the buffer is full a post will block the calling task untilthere is room for the message. When the buffer isnt empty a receive will return themessage from the front of the queue and continue execution of the calling task. Whenthe buffer is empty a receive will block the calling task until a message is posted.You allocate a message queue by declaring it as a C variableCTL_MESSAGE_QUEUE_t m1;A message queue is initialised using the ctl_message_queue_init function.void *queue[20];…ctl_message_queue_init(&m1, queue, 20);This example uses an 20 element array for the message queue. Note that the array isa void * which enables pointers to memory or (cast) integers to be communicatedvia a message queue.You can post a message to a message queue with an optional timeout using thectl_message_queue_post function.ctl_message_queue_post(&m1, (void *)45, 0, 0);