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643 lines
17 KiB
Perl
643 lines
17 KiB
Perl
@c
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@c COPYRIGHT (c) 1988-2002.
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@c On-Line Applications Research Corporation (OAR).
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@c All rights reserved.
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@c
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@c This is the chapter from the RTEMS ITRON User's Guide that
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@c documents the services provided by the semaphore
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@c manager.
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@c
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@c $Id$
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@c
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@chapter Semaphore Manager
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@section Introduction
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The semaphore manager provides functions to allocate, delete, and
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control counting semaphores. This manager is based on the
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ITRON 3.0 standard.
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The services provided by the semaphore manager are:
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@itemize @bullet
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@item @code{cre_sem} - Create Semaphore
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@item @code{del_sem} - Delete Semaphore
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@item @code{sig_sem} - Signal Semaphore
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@item @code{wai_sem} - Wait on Semaphore
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@item @code{preq_sem} - Poll and Request Semaphore
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@item @code{twai_sem} - Wait on Semaphore with Timeout
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@item @code{ref_sem} - Reference Semaphore Status
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@end itemize
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@section Background
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@subsection Theory
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Semaphores are used for synchronization and mutual exclusion by indicating
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the availability and number of resources. The task (the task which is
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returning resources) notifying other tasks of an event increases the number
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of resources held by the semaphore by one. The task (the task which
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will obtain resources) waiting for the event decreases the number of
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resources held by the semaphore by one. If the number of resources held by a
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semaphore is insufficient (namely 0), the task requiring resources will
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wait until the next time resources are returned to the semaphore. If there is
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more than one task waiting for a semaphore, the tasks will be placed in the
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queue.
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@subsection T_CSEM Structure
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The T_CSEM structure is used to define the characteristics of a semaphore
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and passed an as argument to the @code{cre_sem} service. The structure
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is defined as follows:
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@example
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@group
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/*
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* Create Semaphore (cre_sem) Structure
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*/
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typedef struct t_csem @{
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VP exinf; /* extended information */
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ATR sematr; /* semaphore attributes */
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/* Following is the extended function for [level X]. */
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INT isemcnt; /* initial semaphore count */
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INT maxsem; /* maximum semaphore count */
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/* additional implementation dependent information may be included */
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@} T_CSEM;
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/*
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* sematr - Semaphore Attribute Values
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*/
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#define TA_TFIFO 0x00 /* waiting tasks are handled by FIFO */
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#define TA_TPRI 0x01 /* waiting tasks are handled by priority */
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@end group
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@end example
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where the meaning of each field is:
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@table @b
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@item exinf
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is for any extended information that the implementation may define.
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This implementation does not use this field.
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@item sematr
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is the attributes for this semaphore. The only attributed
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which can be specified is whether tasks wait in FIFO (@code{TA_TFIFO})
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or priority (@code{TA_TPRI}) order.
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@item isemcnt
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is the initial count of the semaphore.
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@item maxsem
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is the maximum count the semaphore may have. It places an upper
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limit on the value taken by the semaphore.
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@end table
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@subsection Building a Semaphore Attribute Set
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In general, an attribute set is built by a bitwise OR
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of the desired attribute components. The following table lists
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the set of valid semaphore attributes:
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@itemize @bullet
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@item @code{TA_TFIFO} - tasks wait by FIFO
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@item @code{TA_TPRI} - tasks wait by priority
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@end itemize
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Attribute values are specifically designed to be
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mutually exclusive, therefore bitwise OR and addition operations
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are equivalent as long as each attribute appears exactly once in
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the component list.
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@subsection T_RSEM Structure
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The T_RSEM structure is filled in by the @code{ref_sem} service with
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status and state information on a semaphore. The structure
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is defined as follows:
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@example
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@group
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/*
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* Reference Semaphore (ref_sem) Structure
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*/
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typedef struct t_rsem @{
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VP exinf; /* extended information */
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BOOL_ID wtsk; /* indicates whether there is a waiting task */
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INT semcnt; /* current semaphore count */
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/* additional implementation dependent information may be included */
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@} T_RSEM;
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@end group
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@end example
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@table @b
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@item exinf
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is for any extended information that the implementation may define.
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This implementation does not use this field.
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@item wtsk
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is TRUE when there is one or more task waiting on the semaphore.
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It is FALSE if no tasks are currently waiting. The meaning of this
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field is allowed to vary between ITRON implementations. It may have
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the ID of a waiting task, the number of tasks waiting, or a boolean
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indication that one or more tasks are waiting.
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@item semcnt
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is the current semaphore count.
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@end table
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The information in this table is very volatile and should be used
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with caution in an application.
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@section Operations
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@subsection Using as a Binary Semaphore
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Creating a semaphore with a limit on the count of 1 effectively
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restricts the semaphore to being a binary semaphore. When the
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binary semaphore is available, the count is 1. When the binary
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semaphore is unavailable, the count is 0.
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Since this does not result in a true binary semaphore, advanced
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binary features like the Priority Inheritance and Priority
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Ceiling Protocols are not available.
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@section System Calls
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This section details the semaphore manager's services.
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A subsection is dedicated to each of this manager's services
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and describes the calling sequence, related constants, usage,
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and status codes.
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@c
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@c cre_sem
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@c
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@page
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@subsection cre_sem - Create Semaphore
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER cre_sem(
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ID semid,
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T_CSEM *pk_csem
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOMEM} - Insufficient memory (Memory for control block cannot be
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allocated)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task.)
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@code{E_RSATR} - Reserved attribute (sematr was invalid or could not be
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used)
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@code{E_OBJ} - Invalid object state (a semaphore of the same ID already
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exists)
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@code{E_PAR} - Parameter error (pk_csem is invalid and/or isemcnt or
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maxsem is negative or invalid)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_CTXID} - Specified an object on another node when the system call
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was issued from a task in dispatch disabled state or from a task-
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independent portion
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@code{EN_PAR} - A value outside the range supported by the target node
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and/or transmission packet format was specified as a parameter (a value
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outside supported range was specified for exinf, sematr, isemcnt and/or
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maxsem)
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@subheading DESCRIPTION:
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This routine creates a semaphore that resides on the local node. The
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semaphore is initialized based on the attributes specified in the
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@code{pk_csem} structure. The initial and maximum counts of the
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semaphore are set based on the @code{isemcnt} and @code{maxsem} fields
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in this structure.
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Specifying @code{TA_TPRI} in the @code{sematr} field of the
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semaphore attributes structure causes tasks
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waiting for a semaphore to be serviced according to task
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priority. When @code{TA_TFIFO} is selected, tasks are serviced in First
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In-First Out order.
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@subheading NOTES:
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Multiprocessing is not supported. Thus none of the "EN_" status codes
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will be returned.
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All memory is preallocated for RTEMS ITRON objects. Thus, no dynamic
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memory allocation is performed by @code{cre_sem} and the @code{E_NOMEM}
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error can not be returned.
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This directive will not cause the running task to be preempted.
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The following semaphore attribute constants are
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defined by RTEMS:
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@itemize @bullet
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@item @code{TA_TFIFO} - tasks wait by FIFO
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@item @code{TA_TPRI} - tasks wait by priority
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@end itemize
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@c
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@c del_sem
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@c
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@page
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@subsection del_sem - Delete Semaphore
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER del_sem(
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ID semid
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
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does not exist)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task. This is implementation dependent.)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_CTXID} - Specified an object on another node when the system call
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was issued from a task in dispatch disabled state or from a
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task-independent portion
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@subheading DESCRIPTION:
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This routine deletes the semaphore specified by @code{semid}.
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All tasks blocked waiting to acquire the semaphore will be
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readied and returned a status code which indicates that the
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semaphore was deleted. The control block for this semaphore
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is reclaimed by RTEMS.
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@subheading NOTES:
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Multiprocessing is not supported. Thus none of the "EN_" status codes
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will be returned.
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The calling task will be preempted if it is enabled
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by the task's execution mode and a higher priority local task is
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waiting on the deleted semaphore. The calling task will NOT be
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preempted if all of the tasks that are waiting on the semaphore
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are remote tasks.
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The calling task does not have to be the task that
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created the semaphore. Any local task that knows the semaphore
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id can delete the semaphore.
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@c
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@c sig_sem
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@c
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@page
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@subsection sig_sem - Signal Semaphore
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER sig_sem(
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ID semid
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
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does not exist)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task. This is implementation dependent.)
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@code{E_QOVR} - Queuing or nesting overflow (the queuing count given by
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semcnt went over the maximum allowed)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_CTXID} - Specified an object on another node when the system call
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was issued from a task in dispatch disabled state or from a
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task-independent portion
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@subheading DESCRIPTION:
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@subheading NOTES:
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Multiprocessing is not supported. Thus none of the "EN_" status codes
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will be returned.
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@c
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@c wai_sem
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@c
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@page
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@subsection wai_sem - Wait on Semaphore
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER wai_sem(
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ID semid
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
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does not exist)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task. This is implementation dependent.)
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@code{E_DLT} - The object being waited for was deleted (the specified
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semaphore was deleted while waiting)
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@code{E_RLWAI} - Wait state was forcibly released (rel_wai was received
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while waiting)
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@code{E_CTX} - Context error (issued from task-independent portions or a
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task in dispatch disabled state)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_PAR} - A value outside the range supported by the target node
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and/or transmission packet format was specified as a parameter (a value
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outside supported range was specified for tmout)
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@subheading DESCRIPTION:
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This routine attempts to acquire the semaphore specified by @code{semid}.
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If the semaphore is available (i.e. positive semaphore count), then the
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semaphore count is decremented and the calling task returns immediately.
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Otherwise the calling tasking is blocked until the semaphore is released
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by a subsequent invocation of @code{sig_sem}.
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@subheading NOTES:
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Multiprocessing is not supported. Thus none of the "EN_" status codes
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will be returned.
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If the semaphore is not available, then the calling task will be blocked.
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@c
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@c preq_sem
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@c
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@page
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@subsection preq_sem - Poll and Request Semaphore
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER preq_sem(
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ID semid
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
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does not exist)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task. This is implementation dependent.)
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@code{E_TMOUT} - Polling failure or timeout exceeded
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@code{E_CTX} - Context error (issued from task-independent portions or a
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task in dispatch disabled state)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_PAR} - A value outside the range supported by the target node
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and/or transmission packet format was specified as a parameter (a value
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outside supported range was specified for tmout)
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@subheading DESCRIPTION:
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This routine attempts to acquire the semaphore specified by @code{semid}.
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If the semaphore is available (i.e. positive semaphore count), then the
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semaphore count is decremented and the calling task returns immediately.
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Otherwise, the @code{E_TMOUT} error is returned to the calling task to
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indicate the semaphore is unavailable.
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@subheading NOTES:
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Multiprocessing is not supported. Thus none of the "EN_" status codes
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will be returned.
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This routine will not cause the running task to be preempted.
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@c
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@c twai_sem
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@c
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@page
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@subsection twai_sem - Wait on Semaphore with Timeout
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@subheading CALLING SEQUENCE:
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@ifset is-C
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@example
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ER twai_sem(
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ID semid,
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TMO tmout
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);
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@end example
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@end ifset
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@ifset is-Ada
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@end ifset
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@subheading STATUS CODES:
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@code{E_OK} - Normal Completion
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@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
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@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
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does not exist)
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@code{E_OACV} - Object access violation (A semid less than -4 was
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specified from a user task. This is implementation dependent.)
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@code{E_PAR} - Parameter error (tmout is -2 or less)
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@code{E_DLT} - The object being waited for was deleted (the specified
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semaphore was deleted while waiting)
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@code{E_RLWAI} - Wait state was forcibly released (rel_wai was received
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while waiting)
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@code{E_TMOUT} - Polling failure or timeout exceeded
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@code{E_CTX} - Context error (issued from task-independent portions or a
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task in dispatch disabled state)
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@code{EN_OBJNO} - An object number which could not be accessed on the
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target node is specified.
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@code{EN_PAR} - A value outside the range supported by the target node
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and/or transmission packet format was specified as a parameter (a value
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outside supported range was specified for tmout)
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@subheading DESCRIPTION:
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This routine attempts to acquire the semaphore specified by @code{semid}.
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If the semaphore is available (i.e. positive semaphore count), then the
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semaphore count is decremented and the calling task returns immediately.
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Otherwise the calling tasking is blocked until the semaphore is released
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by a subsequent invocation of @code{sig_sem} or the timeout period specified
|
|
by @code{tmout} milliseconds is exceeded. If the timeout period is exceeded,
|
|
then the @code{E_TMOUT} error is returned.
|
|
|
|
By specifiying @code{tmout} as @code{TMO_FEVR}, this routine has the same
|
|
behavior as @code{wai_sem}. Similarly, by specifiying @code{tmout} as
|
|
@code{TMO_POL}, this routine has the same behavior as @code{preq_sem}.
|
|
|
|
@subheading NOTES:
|
|
|
|
Multiprocessing is not supported. Thus none of the "EN_" status codes
|
|
will be returned.
|
|
|
|
This routine may cause the calling task to block.
|
|
|
|
A clock tick is required to support the timeout functionality of
|
|
this routine.
|
|
|
|
@c
|
|
@c ref_sem
|
|
@c
|
|
|
|
@page
|
|
@subsection ref_sem - Reference Semaphore Status
|
|
|
|
@subheading CALLING SEQUENCE:
|
|
|
|
@ifset is-C
|
|
@example
|
|
ER ref_sem(
|
|
T_RSEM *pk_rsem,
|
|
ID semid
|
|
);
|
|
@end example
|
|
@end ifset
|
|
|
|
@ifset is-Ada
|
|
@end ifset
|
|
|
|
@subheading STATUS CODES:
|
|
|
|
@code{E_OK} - Normal Completion
|
|
|
|
@code{E_ID} - Invalid ID number (semid was invalid or could not be used)
|
|
|
|
@code{E_NOEXS} - Object does not exist (the semaphore specified by semid
|
|
does not exist)
|
|
|
|
@code{E_OACV} - Object access violation (A semid less than -4 was
|
|
specified from a user task. This is implementation dependent.)
|
|
|
|
@code{E_PAR} - Parameter error (the packet address for the return
|
|
parameters could not be used)
|
|
|
|
@code{EN_OBJNO} - An object number which could not be accessed on the
|
|
target node is specified.
|
|
|
|
@code{EN_CTXID} - Specified an object on another node when the system call
|
|
was issued from a task in dispatch disabled state or from a
|
|
task-independent portion
|
|
|
|
@code{EN_RPAR} - A value outside the range supported by the requesting
|
|
node and/or transmission packet format was returned as a parameter (a
|
|
value outside supported range was specified for exinf, wtsk or semcnt)
|
|
|
|
@subheading DESCRIPTION:
|
|
|
|
This routine returns status information on the semaphore specified
|
|
by @code{semid}. The @code{pk_rsem} structure is filled in by
|
|
this service call.
|
|
|
|
@subheading NOTES:
|
|
|
|
Multiprocessing is not supported. Thus none of the "EN_" status codes
|
|
will be returned.
|
|
|
|
This routine will not cause the running task to be preempted.
|
|
|