As previously described, a message consists of a pointer to actual data, a variable indicating the size of the data being pointed to and a timestamp indicating when the message was actually sent. When sent, a message is placed in a data structure of type OS_MSG, shown in Figure 15-11.
The sender and receiver are unaware of this data structure since everything is hidden through the APIs provided by µC/OS-III.
µC/OS-III maintains a pool of free OS_MSGs. The total number of available messages in the pool is determined by the value of OS_CFG_MSG_POOL_SIZE found in os_cfg_app.h. When µC/OS-III is initialized, OS_MSGs are linked in a single linked list as shown in Figure 15-12. Notice that the free list is maintained by a data structure of type OS_MSG_POOL, which contains four (4) fields: .NextPtr, which points to the free list; .NbrFree, which contains the number of free OS_MSGs in the pool, .NbrUsed, which contains the number of OS_MSGs allocated to the application and, .NbrUsedMax which detects the maximum number of messages allocated to the application.
Messages are queued using a data structure of type OS_MSG_Q, as shown in Figure 15-13.
.InPtr
This field contains a pointer to where the next OS_MSG will be inserted in the queue. In fact, the OS_MSG will be inserted “after” the OS_MSG pointed to.
.OutPtr
This field contains a pointer to where the next OS_MSG will be extracted.
.NbrEntriesSize
This field contains the maximum number of OS_MSGs that the queue will hold. If an application attempts to send a message and the .NbrEntries matches this value, the queue is considered to be full and the OS_MSG will not be inserted.
.NbrEntries
This field contains the current number of OS_MSGs in the queue.
.NbrEntriesMax
This field contains the highest number of OS_MSGs existing in the queue at any given time.
A number of internal functions are used by µC/OS-III to manipulate the free list and messages. Specifically, OS_MsgQPut() inserts an OS_MSG in an OS_MSG_Q, OS_MsgQGet() extracts an OS_MSG from an OS_MSG_Q, and OS_MsgQFreeAll() returns all OS_MSGs in an OS_MSG_Q to the pool of free OS_MSGs. There are other OS_MsgQ??() functions in os_msg.c that are used during initialization.
Figure 15-14 shows an example of an OS_MSG_Q when four OS_MSGs are inserted.
OS_MSG_Qs are used inside two additional data structures: OS_Q and OS_TCB. Recall that an OS_Q is declared when creating a message queue object. An OS_TCB is a task control block and, as previously mentioned, each OS_TCB can have its own message queue when the configuration constant OS_CFG_TASK_Q_EN is set to 1 in os_cfg.h. Figure 15-15 shows the contents of an OS_Q and partial contents of an OS_TCB containing an OS_MSG_Q. The OS_MSG_Q data structure is shown as an “exploded view” to emphasize the structure within the structure.