RTEMS 4.10.2 On-Line Library

Semaphore Manager Building a Semaphore Attribute Set

RTEMS Ada User's Guide

9.2.5: Building a Semaphore Attribute Set

In general, an attribute set is built by a bitwise OR of the desired attribute components. The following table lists the set of valid semaphore attributes:

RTEMS.FIFO - tasks wait by FIFO (default)
RTEMS.PRIORITY - tasks wait by priority
RTEMS.BINARY_SEMAPHORE - restrict values to 0 and 1
RTEMS.COUNTING_SEMAPHORE - no restriction on values (default)
RTEMS.SIMPLE_BINARY_SEMAPHORE - restrict values to 0 and 1, do not allow nested access, allow deletion of locked semaphore.
RTEMS.NO_INHERIT_PRIORITY - do not use priority inheritance (default)
RTEMS.INHERIT_PRIORITY - use priority inheritance
RTEMS.PRIORITY_CEILING - use priority ceiling
RTEMS.NO_PRIORITY_CEILING - do not use priority ceiling (default)
RTEMS.LOCAL - local task (default)
RTEMS.GLOBAL - global task

Attribute values are specifically designed to be mutually exclusive, therefore bitwise OR and addition operations are equivalent as long as each attribute appears exactly once in the component list. An attribute listed as a default is not required to appear in the attribute list, although it is a good programming practice to specify default attributes. If all defaults are desired, the attribute RTEMS.DEFAULT_ATTRIBUTES should be specified on this call.

This example demonstrates the attribute_set parameter needed to create a local semaphore with the task priority waiting queue discipline. The attribute_set parameter passed to the rtems.semaphore_create directive could be either RTEMS.PRIORITY or RTEMS.LOCAL or RTEMS.PRIORITY. The attribute_set parameter can be set to RTEMS.PRIORITY because RTEMS.LOCAL is the default for all created tasks. If a similar semaphore were to be known globally, then the attribute_set parameter would be RTEMS.GLOBAL or RTEMS.PRIORITY.

Some combinatinos of these attributes are invalid. For example, priority ordered blocking discipline must be applied to a binary semaphore in order to use either the priority inheritance or priority ceiling functionality. The following tree figure illustrates the valid combinations.

Valid Semaphore Attributes Combinations

RTEMS Ada User's Guide