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diff --git a/doc/user/sem.t b/doc/user/sem.t new file mode 100644 index 0000000000..dd51266892 --- /dev/null +++ b/doc/user/sem.t @@ -0,0 +1,832 @@ +@c +@c COPYRIGHT (c) 1988-2002. +@c On-Line Applications Research Corporation (OAR). +@c All rights reserved. +@c +@c $Id$ +@c + +@chapter Semaphore Manager + +@cindex semaphores +@cindex binary semaphores +@cindex counting semaphores +@cindex mutual exclusion + +@section Introduction + +The semaphore manager utilizes standard Dijkstra +counting semaphores to provide synchronization and mutual +exclusion capabilities. The directives provided by the +semaphore manager are: + +@itemize @bullet +@item @code{@value{DIRPREFIX}semaphore_create} - Create a semaphore +@item @code{@value{DIRPREFIX}semaphore_ident} - Get ID of a semaphore +@item @code{@value{DIRPREFIX}semaphore_delete} - Delete a semaphore +@item @code{@value{DIRPREFIX}semaphore_obtain} - Acquire a semaphore +@item @code{@value{DIRPREFIX}semaphore_release} - Release a semaphore +@item @code{@value{DIRPREFIX}semaphore_flush} - Unblock all tasks waiting on a semaphore +@end itemize + +@section Background + +A semaphore can be viewed as a protected variable +whose value can be modified only with the +@code{@value{DIRPREFIX}semaphore_create}, +@code{@value{DIRPREFIX}semaphore_obtain}, and +@code{@value{DIRPREFIX}semaphore_release} directives. RTEMS +supports both binary and counting semaphores. A binary semaphore +is restricted to values of zero or one, while a counting +semaphore can assume any non-negative integer value. + +A binary semaphore can be used to control access to a +single resource. In particular, it can be used to enforce +mutual exclusion for a critical section in user code. In this +instance, the semaphore would be created with an initial count +of one to indicate that no task is executing the critical +section of code. Upon entry to the critical section, a task +must issue the @code{@value{DIRPREFIX}semaphore_obtain} +directive to prevent other tasks from entering the critical section. +Upon exit from the critical section, the task must issue the +@code{@value{DIRPREFIX}semaphore_release} directive to +allow another task to execute the critical section. + +A counting semaphore can be used to control access to +a pool of two or more resources. For example, access to three +printers could be administered by a semaphore created with an +initial count of three. When a task requires access to one of +the printers, it issues the @code{@value{DIRPREFIX}semaphore_obtain} +directive to obtain access to a printer. If a printer is not currently +available, the task can wait for a printer to become available or return +immediately. When the task has completed printing, it should +issue the @code{@value{DIRPREFIX}semaphore_release} +directive to allow other tasks access to the printer. + +Task synchronization may be achieved by creating a +semaphore with an initial count of zero. One task waits for the +arrival of another task by issuing a @code{@value{DIRPREFIX}semaphore_obtain} +directive when it reaches a synchronization point. The other task +performs a corresponding @code{@value{DIRPREFIX}semaphore_release} +operation when it reaches its synchronization point, thus unblocking +the pending task. + +@subsection Nested Resource Access + +Deadlock occurs when a task owning a binary semaphore +attempts to acquire that same semaphore and blocks as result. +Since the semaphore is allocated to a task, it cannot be +deleted. Therefore, the task that currently holds the semaphore +and is also blocked waiting for that semaphore will never +execute again. + +RTEMS addresses this problem by allowing the task +holding the binary semaphore to obtain the same binary semaphore +multiple times in a nested manner. Each +@code{@value{DIRPREFIX}semaphore_obtain} must be accompanied with a +@code{@value{DIRPREFIX}semaphore_release}. The semaphore will +only be made available for acquisition by other tasks when the +outermost @code{@value{DIRPREFIX}semaphore_obtain} is matched with +a @code{@value{DIRPREFIX}semaphore_release}. + +Simple binary semaphores do not allow nested access and so can be used for task synchronization. + + +@subsection Priority Inversion + +Priority inversion is a form of indefinite +postponement which is common in multitasking, preemptive +executives with shared resources. Priority inversion occurs +when a high priority tasks requests access to shared resource +which is currently allocated to low priority task. The high +priority task must block until the low priority task releases +the resource. This problem is exacerbated when the low priority +task is prevented from executing by one or more medium priority +tasks. Because the low priority task is not executing, it +cannot complete its interaction with the resource and release +that resource. The high priority task is effectively prevented +from executing by lower priority tasks. + +@subsection Priority Inheritance + +Priority inheritance is an algorithm that calls for +the lower priority task holding a resource to have its priority +increased to that of the highest priority task blocked waiting +for that resource. Each time a task blocks attempting to obtain +the resource, the task holding the resource may have its +priority increased. + +RTEMS supports priority inheritance for local, binary +semaphores that use the priority task wait queue blocking +discipline. When a task of higher priority than the task +holding the semaphore blocks, the priority of the task holding +the semaphore is increased to that of the blocking task. When +the task holding the task completely releases the binary +semaphore (i.e. not for a nested release), the holder's priority +is restored to the value it had before any higher priority was +inherited. + +The RTEMS implementation of the priority inheritance +algorithm takes into account the scenario in which a task holds +more than one binary semaphore. The holding task will execute +at the priority of the higher of the highest ceiling priority or +at the priority of the highest priority task blocked waiting for +any of the semaphores the task holds. Only when the task +releases ALL of the binary semaphores it holds will its priority +be restored to the normal value. + +@subsection Priority Ceiling + +Priority ceiling is an algorithm that calls for the +lower priority task holding a resource to have its priority +increased to that of the highest priority task which will EVER +block waiting for that resource. This algorithm addresses the +problem of priority inversion although it avoids the possibility +of changing the priority of the task holding the resource +multiple times. The priority ceiling algorithm will only change +the priority of the task holding the resource a maximum of one +time. The ceiling priority is set at creation time and must be +the priority of the highest priority task which will ever +attempt to acquire that semaphore. + +RTEMS supports priority ceiling for local, binary +semaphores that use the priority task wait queue blocking +discipline. When a task of lower priority than the ceiling +priority successfully obtains the semaphore, its priority is +raised to the ceiling priority. When the task holding the task +completely releases the binary semaphore (i.e. not for a nested +release), the holder's priority is restored to the value it had +before any higher priority was put into effect. + +The need to identify the highest priority task which +will attempt to obtain a particular semaphore can be a difficult +task in a large, complicated system. Although the priority +ceiling algorithm is more efficient than the priority +inheritance algorithm with respect to the maximum number of task +priority changes which may occur while a task holds a particular +semaphore, the priority inheritance algorithm is more forgiving +in that it does not require this apriori information. + +The RTEMS implementation of the priority ceiling +algorithm takes into account the scenario in which a task holds +more than one binary semaphore. The holding task will execute +at the priority of the higher of the highest ceiling priority or +at the priority of the highest priority task blocked waiting for +any of the semaphores the task holds. Only when the task +releases ALL of the binary semaphores it holds will its priority +be restored to the normal value. + +@subsection 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: + +@itemize @bullet +@item @code{@value{RPREFIX}FIFO} - tasks wait by FIFO (default) + +@item @code{@value{RPREFIX}PRIORITY} - tasks wait by priority + +@item @code{@value{RPREFIX}BINARY_SEMAPHORE} - restrict values to +0 and 1 + +@item @code{@value{RPREFIX}COUNTING_SEMAPHORE} - no restriction on values +(default) + +@item @code{@value{RPREFIX}SIMPLE_BINARY_SEMAPHORE} - restrict values to +0 and 1, do not allow nested access, allow deletion of locked semaphore. + +@item @code{@value{RPREFIX}NO_INHERIT_PRIORITY} - do not use priority +inheritance (default) + +@item @code{@value{RPREFIX}INHERIT_PRIORITY} - use priority inheritance + +@item @code{@value{RPREFIX}PRIORITY_CEILING} - use priority ceiling + +@item @code{@value{RPREFIX}NO_PRIORITY_CEILING} - do not use priority +ceiling (default) + +@item @code{@value{RPREFIX}LOCAL} - local task (default) + +@item @code{@value{RPREFIX}GLOBAL} - global task +@end itemize + +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 +@code{@value{RPREFIX}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 +@code{@value{DIRPREFIX}semaphore_create} directive could be either +@code{@value{RPREFIX}PRIORITY} or @code{@value{RPREFIX}LOCAL @value{OR} +@value{RPREFIX}PRIORITY}. The attribute_set parameter can be set to +@code{@value{RPREFIX}PRIORITY} because @code{@value{RPREFIX}LOCAL} is the +default for all created tasks. If a similar semaphore were to be known +globally, then the attribute_set parameter would be +@code{@value{RPREFIX}GLOBAL @value{OR} @value{RPREFIX}PRIORITY}. + +@subsection Building a SEMAPHORE_OBTAIN Option Set + +In general, an option is built by a bitwise OR of the +desired option components. The set of valid options for the +@code{@value{DIRPREFIX}semaphore_obtain} directive are listed +in the following table: + +@itemize @bullet +@item @code{@value{RPREFIX}WAIT} - task will wait for semaphore (default) +@item @code{@value{RPREFIX}NO_WAIT} - task should not wait +@end itemize + +Option 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 option +listed as a default is not required to appear in the list, although it is +a good programming practice to specify default options. If all defaults +are desired, the option @code{@value{RPREFIX}DEFAULT_OPTIONS} should be +specified on this call. + +This example demonstrates the option parameter needed +to poll for a semaphore. The option parameter passed to the +@code{@value{DIRPREFIX}semaphore_obtain} +directive should be @code{@value{RPREFIX}NO_WAIT}. + +@section Operations + +@subsection Creating a Semaphore + +The @code{@value{DIRPREFIX}semaphore_create} directive creates a binary or +counting semaphore with a user-specified name as well as an +initial count. If a binary semaphore is created with a count of +zero (0) to indicate that it has been allocated, then the task +creating the semaphore is considered the current holder of the +semaphore. At create time the method for ordering waiting tasks +in the semaphore's task wait queue (by FIFO or task priority) is +specified. Additionally, the priority inheritance or priority +ceiling algorithm may be selected for local, binary semaphores +that use the priority task wait queue blocking discipline. If +the priority ceiling algorithm is selected, then the highest +priority of any task which will attempt to obtain this semaphore +must be specified. RTEMS allocates a Semaphore Control Block +(SMCB) from the SMCB free list. This data structure is used by +RTEMS to manage the newly created semaphore. Also, a unique +semaphore ID is generated and returned to the calling task. + +@subsection Obtaining Semaphore IDs + +When a semaphore is created, RTEMS generates a unique +semaphore ID and assigns it to the created semaphore until it is +deleted. The semaphore ID may be obtained by either of two +methods. First, as the result of an invocation of the +@code{@value{DIRPREFIX}semaphore_create} directive, the +semaphore ID is stored in a user provided location. Second, +the semaphore ID may be obtained later using the +@code{@value{DIRPREFIX}semaphore_ident} directive. The semaphore ID is +used by other semaphore manager directives to access this +semaphore. + +@subsection Acquiring a Semaphore + +The @code{@value{DIRPREFIX}semaphore_obtain} directive is used to acquire the +specified semaphore. A simplified version of the +@code{@value{DIRPREFIX}semaphore_obtain} directive can be described as follows: + +@example +if semaphore's count is greater than zero + then decrement semaphore's count + else wait for release of semaphore + +return SUCCESSFUL +@end example + +When the semaphore cannot be immediately acquired, +one of the following situations applies: + +@itemize @bullet +@item By default, the calling task will wait forever to +acquire the semaphore. + +@item Specifying @code{@value{RPREFIX}NO_WAIT} forces an immediate return +with an error status code. + +@item Specifying a timeout limits the interval the task will +wait before returning with an error status code. +@end itemize + +If the task waits to acquire the semaphore, then it +is placed in the semaphore's task wait queue in either FIFO or +task priority order. If the task blocked waiting for a binary +semaphore using priority inheritance and the task's priority is +greater than that of the task currently holding the semaphore, +then the holding task will inherit the priority of the blocking +task. All tasks waiting on a semaphore are returned an error +code when the semaphore is deleted. + +When a task successfully obtains a semaphore using +priority ceiling and the priority ceiling for this semaphore is +greater than that of the holder, then the holder's priority will +be elevated. + +@subsection Releasing a Semaphore + +The @code{@value{DIRPREFIX}semaphore_release} directive is used to release +the specified semaphore. A simplified version of the +@code{@value{DIRPREFIX}semaphore_release} directive can be described as +follows: + +@example +if no tasks are waiting on this semaphore + then increment semaphore's count + else assign semaphore to a waiting task + +return SUCCESSFUL +@end example + +If this is the outermost release of a binary +semaphore that uses priority inheritance or priority ceiling and +the task does not currently hold any other binary semaphores, +then the task performing the @code{@value{DIRPREFIX}semaphore_release} +will have its priority restored to its normal value. + +@subsection Deleting a Semaphore + +The @code{@value{DIRPREFIX}semaphore_delete} directive removes a semaphore +from the system and frees its control block. A semaphore can be +deleted by any local task that knows the semaphore's ID. As a +result of this directive, all tasks blocked waiting to acquire +the semaphore will be readied and returned a status code which +indicates that the semaphore was deleted. Any subsequent +references to the semaphore's name and ID are invalid. + +@section Directives + +This section details the semaphore manager's +directives. A subsection is dedicated to each of this manager's +directives and describes the calling sequence, related +constants, usage, and status codes. + +@c +@c +@c +@page +@subsection SEMAPHORE_CREATE - Create a semaphore + +@cindex create a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_create +@example +rtems_status_code rtems_semaphore_create( + rtems_name name, + rtems_unsigned32 count, + rtems_attribute attribute_set, + rtems_task_priority priority_ceiling, + rtems_id *id +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Create ( + Name : in RTEMS.Name; + Count : in RTEMS.Unsigned32; + Attribute_Set : in RTEMS.Attribute; + ID : out RTEMS.ID; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore created successfully@* +@code{@value{RPREFIX}INVALID_NAME} - invalid task name@* +@code{@value{RPREFIX}INVALID_ADDRESS} - @code{id} is NULL@* +@code{@value{RPREFIX}TOO_MANY} - too many semaphores created@* +@code{@value{RPREFIX}NOT_DEFINED} - invalid attribute set@* +@code{@value{RPREFIX}INVALID_NUMBER} - invalid starting count for binary semaphore@* +@code{@value{RPREFIX}MP_NOT_CONFIGURED} - multiprocessing not configured@* +@code{@value{RPREFIX}TOO_MANY} - too many global objects + +@subheading DESCRIPTION: + +This directive creates a semaphore which resides on +the local node. The created semaphore has the user-defined name +specified in name and the initial count specified in count. For +control and maintenance of the semaphore, RTEMS allocates and +initializes a SMCB. The RTEMS-assigned semaphore id is returned +in id. This semaphore id is used with other semaphore related +directives to access the semaphore. + +Specifying PRIORITY in attribute_set causes tasks +waiting for a semaphore to be serviced according to task +priority. When FIFO is selected, tasks are serviced in First +In-First Out order. + +@subheading NOTES: + +This directive will not cause the calling task to be +preempted. + +The priority inheritance and priority ceiling +algorithms are only supported for local, binary semaphores that +use the priority task wait queue blocking discipline. + +The following semaphore attribute constants are +defined by RTEMS: + +@itemize @bullet +@item @code{@value{RPREFIX}FIFO} - tasks wait by FIFO (default) + +@item @code{@value{RPREFIX}PRIORITY} - tasks wait by priority + +@item @code{@value{RPREFIX}BINARY_SEMAPHORE} - restrict values to +0 and 1 + +@item @code{@value{RPREFIX}COUNTING_SEMAPHORE} - no restriction on values +(default) + +@item @code{@value{RPREFIX}SIMPLE_BINARY_SEMAPHORE} - restrict values to +0 and 1, block on nested access, allow deletion of locked semaphore. + +@item @code{@value{RPREFIX}NO_INHERIT_PRIORITY} - do not use priority +inheritance (default) + +@item @code{@value{RPREFIX}INHERIT_PRIORITY} - use priority inheritance + +@item @code{@value{RPREFIX}PRIORITY_CEILING} - use priority ceiling + +@item @code{@value{RPREFIX}NO_PRIORITY_CEILING} - do not use priority +ceiling (default) + +@item @code{@value{RPREFIX}LOCAL} - local task (default) + +@item @code{@value{RPREFIX}GLOBAL} - global task +@end itemize + +Semaphores should not be made global unless remote +tasks must interact with the created semaphore. This is to +avoid the system overhead incurred by the creation of a global +semaphore. When a global semaphore is created, the semaphore's +name and id must be transmitted to every node in the system for +insertion in the local copy of the global object table. + +The total number of global objects, including +semaphores, is limited by the maximum_global_objects field in +the Configuration Table. + +@c +@c +@c +@page +@subsection SEMAPHORE_IDENT - Get ID of a semaphore + +@cindex get ID of a semaphore +@cindex obtain ID of a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_ident +@example +rtems_status_code rtems_semaphore_ident( + rtems_name name, + rtems_unsigned32 node, + rtems_id *id +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Ident ( + Name : in RTEMS.Name; + Node : in RTEMS.Unsigned32; + ID : out RTEMS.ID; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore identified successfully@* +@code{@value{RPREFIX}INVALID_NAME} - semaphore name not found@* +@code{@value{RPREFIX}INVALID_NODE} - invalid node id + +@subheading DESCRIPTION: + +This directive obtains the semaphore id associated +with the semaphore name. If the semaphore name is not unique, +then the semaphore id will match one of the semaphores with that +name. However, this semaphore id is not guaranteed to +correspond to the desired semaphore. The semaphore id is used +by other semaphore related directives to access the semaphore. + +@subheading NOTES: + +This directive will not cause the running task to be +preempted. + +If node is @code{@value{RPREFIX}SEARCH_ALL_NODES}, all nodes are searched +with the local node being searched first. All other nodes are +searched with the lowest numbered node searched first. + +If node is a valid node number which does not +represent the local node, then only the semaphores exported by +the designated node are searched. + +This directive does not generate activity on remote +nodes. It accesses only the local copy of the global object +table. + +@c +@c +@c +@page +@subsection SEMAPHORE_DELETE - Delete a semaphore + +@cindex delete a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_delete +@example +rtems_status_code rtems_semaphore_delete( + rtems_id id +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Delete ( + ID : in RTEMS.ID; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore deleted successfully@* +@code{@value{RPREFIX}INVALID_ID} - invalid semaphore id@* +@code{@value{RPREFIX}ILLEGAL_ON_REMOTE_OBJECT} - cannot delete remote semaphore@* +@code{@value{RPREFIX}RESOURCE_IN_USE} - binary semaphore is in use + +@subheading DESCRIPTION: + +This directive deletes the semaphore specified by @code{id}. +All tasks blocked waiting to acquire the semaphore will be +readied and returned a status code which indicates that the +semaphore was deleted. The SMCB for this semaphore is reclaimed +by RTEMS. + +@subheading NOTES: + +The calling task will be preempted if it is enabled +by the task's execution mode and a higher priority local task is +waiting on the deleted semaphore. The calling task will NOT be +preempted if all of the tasks that are waiting on the semaphore +are remote tasks. + +The calling task does not have to be the task that +created the semaphore. Any local task that knows the semaphore +id can delete the semaphore. + +When a global semaphore is deleted, the semaphore id +must be transmitted to every node in the system for deletion +from the local copy of the global object table. + +The semaphore must reside on the local node, even if +the semaphore was created with the @code{@value{RPREFIX}GLOBAL} option. + +Proxies, used to represent remote tasks, are +reclaimed when the semaphore is deleted. + +@c +@c +@c +@page +@subsection SEMAPHORE_OBTAIN - Acquire a semaphore + +@cindex obtain a semaphore +@cindex lock a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_obtain +@example +rtems_status_code rtems_semaphore_obtain( + rtems_id id, + rtems_unsigned32 option_set, + rtems_interval timeout +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Obtain ( + ID : in RTEMS.ID; + Option_Set : in RTEMS.Option; + Timeout : in RTEMS.Interval; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore obtained successfully@* +@code{@value{RPREFIX}UNSATISFIED} - semaphore not available@* +@code{@value{RPREFIX}TIMEOUT} - timed out waiting for semaphore@* +@code{@value{RPREFIX}OBJECT_WAS_DELETED} - semaphore deleted while waiting@* +@code{@value{RPREFIX}INVALID_ID} - invalid semaphore id + +@subheading DESCRIPTION: + +This directive acquires the semaphore specified by +id. The @code{@value{RPREFIX}WAIT} and @code{@value{RPREFIX}NO_WAIT} components of the options parameter +indicate whether the calling task wants to wait for the +semaphore to become available or return immediately if the +semaphore is not currently available. With either @code{@value{RPREFIX}WAIT} or +@code{@value{RPREFIX}NO_WAIT}, if the current semaphore count is positive, then it is +decremented by one and the semaphore is successfully acquired by +returning immediately with a successful return code. + +If the calling task chooses to return immediately and the current +semaphore count is zero or negative, then a status code is returned +indicating that the semaphore is not available. If the calling task +chooses to wait for a semaphore and the current semaphore count is zero or +negative, then it is decremented by one and the calling task is placed on +the semaphore's wait queue and blocked. If the semaphore was created with +the @code{@value{RPREFIX}PRIORITY} attribute, then the calling task is +inserted into the queue according to its priority. However, if the +semaphore was created with the @code{@value{RPREFIX}FIFO} attribute, then +the calling task is placed at the rear of the wait queue. If the binary +semaphore was created with the @code{@value{RPREFIX}INHERIT_PRIORITY} +attribute, then the priority of the task currently holding the binary +semaphore is guaranteed to be greater than or equal to that of the +blocking task. If the binary semaphore was created with the +@code{@value{RPREFIX}PRIORITY_CEILING} attribute, a task successfully +obtains the semaphore, and the priority of that task is greater than the +ceiling priority for this semaphore, then the priority of the task +obtaining the semaphore is elevated to that of the ceiling. + +The timeout parameter specifies the maximum interval the calling task is +willing to be blocked waiting for the semaphore. If it is set to +@code{@value{RPREFIX}NO_TIMEOUT}, then the calling task will wait forever. +If the semaphore is available or the @code{@value{RPREFIX}NO_WAIT} option +component is set, then timeout is ignored. + +@subheading NOTES: +The following semaphore acquisition option constants +are defined by RTEMS: + +@itemize @bullet +@item @code{@value{RPREFIX}WAIT} - task will wait for semaphore (default) +@item @code{@value{RPREFIX}NO_WAIT} - task should not wait +@end itemize + +Attempting to obtain a global semaphore which does not reside on the local +node will generate a request to the remote node to access the semaphore. +If the semaphore is not available and @code{@value{RPREFIX}NO_WAIT} was +not specified, then the task must be blocked until the semaphore is +released. A proxy is allocated on the remote node to represent the task +until the semaphore is released. + +A clock tick is required to support the timeout functionality of +this directive. + +@c +@c +@c +@page +@subsection SEMAPHORE_RELEASE - Release a semaphore + +@cindex release a semaphore +@cindex unlock a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_release +@example +rtems_status_code rtems_semaphore_release( + rtems_id id +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Release ( + ID : in RTEMS.ID; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore released successfully@* +@code{@value{RPREFIX}INVALID_ID} - invalid semaphore id@* +@code{@value{RPREFIX}NOT_OWNER_OF_RESOURCE} - calling task does not own semaphore + +@subheading DESCRIPTION: + +This directive releases the semaphore specified by +id. The semaphore count is incremented by one. If the count is +zero or negative, then the first task on this semaphore's wait +queue is removed and unblocked. The unblocked task may preempt +the running task if the running task's preemption mode is +enabled and the unblocked task has a higher priority than the +running task. + +@subheading NOTES: + +The calling task may be preempted if it causes a +higher priority task to be made ready for execution. + +Releasing a global semaphore which does not reside on +the local node will generate a request telling the remote node +to release the semaphore. + +If the task to be unblocked resides on a different +node from the semaphore, then the semaphore allocation is +forwarded to the appropriate node, the waiting task is +unblocked, and the proxy used to represent the task is reclaimed. + +The outermost release of a local, binary, priority +inheritance or priority ceiling semaphore may result in the +calling task having its priority lowered. This will occur if +the calling task holds no other binary semaphores and it has +inherited a higher priority. + +@c +@c +@c +@page +@subsection SEMAPHORE_FLUSH - Unblock all tasks waiting on a semaphore + +@cindex flush a semaphore +@cindex unblock all tasks waiting on a semaphore + +@subheading CALLING SEQUENCE: + +@ifset is-C +@findex rtems_semaphore_flush +@example +rtems_status_code rtems_semaphore_flush( + rtems_id id +); +@end example +@end ifset + +@ifset is-Ada +@example +procedure Semaphore_Flush ( + ID : in RTEMS.ID; + Result : out RTEMS.Status_Codes +); +@end example +@end ifset + +@subheading DIRECTIVE STATUS CODES: +@code{@value{RPREFIX}SUCCESSFUL} - semaphore released successfully@* +@code{@value{RPREFIX}INVALID_ID} - invalid semaphore id@* +@code{@value{RPREFIX}ILLEGAL_ON_REMOTE_OBJECT} - not supported for remote semaphores + +@subheading DESCRIPTION: + +This directive unblocks all tasks waiting on the semaphore specified by +id. Since there are tasks blocked on the semaphore, the semaphore's +count is not changed by this directive and thus is zero before and +after this directive is executed. Tasks which are unblocked as the +result of this directive will return from the +@code{@value{DIRPREFIX}semaphore_release} directive with a +status code of @code{@value{RPREFIX}UNSATISFIED} to indicate +that the semaphore was not obtained. + +This directive may unblock any number of tasks. Any of the unblocked +tasks may preempt the running task if the running task's preemption mode is +enabled and an unblocked task has a higher priority than the +running task. + +@subheading NOTES: + +The calling task may be preempted if it causes a +higher priority task to be made ready for execution. + +If the task to be unblocked resides on a different +node from the semaphore, then the waiting task is +unblocked, and the proxy used to represent the task is reclaimed. + + |