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diff --git a/c-user/region/operations.rst b/c-user/region/operations.rst new file mode 100644 index 0000000..ef41d39 --- /dev/null +++ b/c-user/region/operations.rst @@ -0,0 +1,101 @@ +.. SPDX-License-Identifier: CC-BY-SA-4.0 + +.. Copyright (C) 1988, 2008 On-Line Applications Research Corporation (OAR) + +Operations +========== + +Creating a Region +----------------- + +The ``rtems_region_create`` directive creates a region with the user-defined +name. The user may select FIFO or task priority as the method for placing +waiting tasks in the task wait queue. RTEMS allocates a Region Control Block +(RNCB) from the RNCB free list to maintain the newly created region. RTEMS +also generates a unique region ID which is returned to the calling task. + +It is not possible to calculate the exact number of bytes available to the user +since RTEMS requires overhead for each segment allocated. For example, a +region with one segment that is the size of the entire region has more +available bytes than a region with two segments that collectively are the size +of the entire region. This is because the region with one segment requires +only the overhead for one segment, while the other region requires the overhead +for two segments. + +Due to automatic coalescing, the number of segments in the region dynamically +changes. Therefore, the total overhead required by RTEMS dynamically changes. + +Obtaining Region IDs +-------------------- + +When a region is created, RTEMS generates a unique region ID and assigns it to +the created region until it is deleted. The region ID may be obtained by +either of two methods. First, as the result of an invocation of the +``rtems_region_create`` directive, the region ID is stored in a user provided +location. Second, the region ID may be obtained later using the +``rtems_region_ident`` directive. The region ID is used by other region +manager directives to access this region. + +Adding Memory to a Region +------------------------- + +The ``rtems_region_extend`` directive may be used to add memory to an existing +region. The caller specifies the size in bytes and starting address of the +memory being added. + +Acquiring a Segment +------------------- + +The ``rtems_region_get_segment`` directive attempts to acquire a segment from a +specified region. If the region has enough available free memory, then a +segment is returned successfully to the caller. When the segment cannot be +allocated, one of the following situations applies: + +- By default, the calling task will wait forever to acquire the segment. + +- Specifying the ``RTEMS_NO_WAIT`` option forces an immediate return with an + error status code. + +- Specifying a timeout limits the interval the task will wait before returning + with an error status code. + +If the task waits for the segment, then it is placed in the region's task wait +queue in either FIFO or task priority order. All tasks waiting on a region are +returned an error when the message queue is deleted. + +Releasing a Segment +------------------- + +When a segment is returned to a region by the ``rtems_region_return_segment`` +directive, it is merged with its unallocated neighbors to form the largest +possible segment. The first task on the wait queue is examined to determine if +its segment request can now be satisfied. If so, it is given a segment and +unblocked. This process is repeated until the first task's segment request +cannot be satisfied. + +Obtaining the Size of a Segment +------------------------------- + +The ``rtems_region_get_segment_size`` directive returns the size in bytes of +the specified segment. The size returned includes any "extra" memory included +in the segment because of rounding up to a page size boundary. + +Changing the Size of a Segment +------------------------------ + +The ``rtems_region_resize_segment`` directive is used to change the size in +bytes of the specified segment. The size may be increased or decreased. When +increasing the size of a segment, it is possible that the request cannot be +satisfied. This directive provides functionality similar to the ``realloc()`` +function in the Standard C Library. + +Deleting a Region +----------------- + +A region can be removed from the system and returned to RTEMS with the +``rtems_region_delete`` directive. When a region is deleted, its control block +is returned to the RNCB free list. A region with segments still allocated is +not allowed to be deleted. Any task attempting to do so will be returned an +error. As a result of this directive, all tasks blocked waiting to obtain a +segment from the region will be readied and returned a status code which +indicates that the region was deleted. |