Memory management services are enabled at compile time by setting the configuration constant OS_CFG_MEM_EN to DEF_ENABLED in os_cfg.h.
There are a number of operations to perform on memory partitions as summarized in the table below.
OSMemCreate() can only be called from task-level code, but OSMemGet() and OSMemPut() can be called by Interrupt Service Routines (ISRs).
The listing show in Returning a Memory Block to a Partition shows an example of how to use the dynamic memory allocation feature of μC/OS-III, as well as message-passing capabilities (see ). In this example, the task on the left reads and checks the value of analog inputs (pressures, temperatures, and voltage) and sends a message to the second task if any of the analog inputs exceed a threshold. The message sent contains information about which channel had the error, an error code, an indication of the severity of the error, and other information.
Error handling in this example is centralized. Other tasks, or even ISRs, can post error messages to the error-handling task. The error-handling task could be responsible for displaying error messages on a monitor (a display), logging errors to a disk, or dispatching other tasks to take corrective action based on the error.
Sometimes it is useful to have a task wait for a memory block in case a partition runs out of blocks. µC/OS-III does not support pending on partitions, but it is possible to support this requirement by adding a counting semaphore (see Resource Management) to guard the memory partition. The initial value of the counting semaphore would be set to the number of blocks in the partition. This is illustrated in the figure below.
The above operations must be performed in order.
Note that the user may call OSMemGet() and OSMemPut() from an ISR since these functions do not block and in fact, execute very quickly. However, you cannot use blocking calls from ISRs.