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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 the the figure below. 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 the figure below.

.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.

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The figure below 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 DEF_ENABLED in os_cfg.h. The figure below 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.