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@c

@c  COPYRIGHT (c) 1988-2002.

@c  On-Line Applications Research Corporation (OAR).

@c  All rights reserved.

@c

@c  This is the chapter from the RTEMS ITRON User's Guide that

@c  documents the services provided by the semaphore

@c  manager.

@c

@c  semaphore.t,v 1.12 2002/01/17 21:47:45 joel Exp

@c

@chapter Semaphore Manager

@section Introduction

The semaphore manager provides functions to allocate, delete, and

control counting semaphores.  This manager is based on the

ITRON 3.0 standard.

The services provided by the semaphore manager are:

@itemize @bullet

@item @code{cre_sem} - Create Semaphore

@item @code{del_sem} - Delete Semaphore

@item @code{sig_sem} - Signal Semaphore

@item @code{wai_sem} - Wait on Semaphore

@item @code{preq_sem} - Poll and Request Semaphore

@item @code{twai_sem} - Wait on Semaphore with Timeout

@item @code{ref_sem} - Reference Semaphore Status

@end itemize

@section Background

@subsection Theory

Semaphores are used for synchronization and mutual exclusion by indicating

the availability and number of resources.  The task (the task which is

returning resources) notifying other tasks of an event increases the number

of resources held by the semaphore by one.  The task (the task which

will obtain resources) waiting for the event decreases the number of

resources held by the semaphore by one.  If the number of resources held by a

semaphore is insufficient (namely 0), the task requiring resources will

wait until the next time resources are returned to the semaphore.  If there is

more than one task waiting for a semaphore, the tasks will be placed in the

queue.

@subsection T_CSEM Structure

The T_CSEM structure is used to define the characteristics of a semaphore

and passed an as argument to the @code{cre_sem} service.  The structure

is defined as follows:

@example

@group

/*

 *  Create Semaphore (cre_sem) Structure

 */

typedef struct t_csem @{

  VP    exinf;    /* extended information */

  ATR   sematr;   /* semaphore attributes */

  /* Following is the extended function for [level X]. */

  INT   isemcnt;   /* initial semaphore count */

  INT   maxsem;    /* maximum semaphore count */

  /* additional implementation dependent information may be included */

@} T_CSEM;

/*

 *  sematr - Semaphore Attribute Values

 */

#define TA_TFIFO   0x00   /* waiting tasks are handled by FIFO */

#define TA_TPRI    0x01   /* waiting tasks are handled by priority */

@end group

@end example

where the meaning of each field is:

@table @b

@item exinf

is for any extended information that the implementation may define.

This implementation does not use this field.

@item sematr

is the attributes for this semaphore.  The only attributed

which can be specified is whether tasks wait in FIFO (@code{TA_TFIFO})

or priority (@code{TA_TPRI}) order.

@item isemcnt

is the initial count of the semaphore.

@item maxsem

is the maximum count the semaphore may have.  It places an upper

limit on the value taken by the semaphore.

@end table

@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{TA_TFIFO} - tasks wait by FIFO

@item @code{TA_TPRI} - tasks wait by priority

@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.

@subsection T_RSEM Structure

The T_RSEM structure is filled in by the @code{ref_sem} service with

status and state information on a semaphore.  The structure

is defined as follows:

@example

@group

/*

 *  Reference Semaphore (ref_sem) Structure

 */

typedef struct t_rsem @{

  VP      exinf;    /* extended information */

  BOOL_ID wtsk;     /* indicates whether there is a waiting task */

  INT     semcnt;   /* current semaphore count */

  /* additional implementation dependent information may be included */

@} T_RSEM;

@end group

@end example

@table @b

@item exinf

is for any extended information that the implementation may define.

This implementation does not use this field.

@item wtsk

is TRUE when there is one or more task waiting on the semaphore.

It is FALSE if no tasks are currently waiting.  The meaning of this

field is allowed to vary between ITRON implementations.  It may have

the ID of a waiting task, the number of tasks waiting, or a boolean

indication that one or more tasks are waiting.

@item semcnt

is the current semaphore count.

@end table

The information in this table is very volatile and should be used

with caution in an application.

@section Operations

@subsection Using as a Binary Semaphore

Creating a semaphore with a limit on the count of 1 effectively

restricts the semaphore to being a binary semaphore.  When the

binary semaphore is available, the count is 1.  When the binary

semaphore is unavailable, the count is 0.

Since this does not result in a true binary semaphore, advanced

binary features like the Priority Inheritance and Priority

Ceiling Protocols are not available.

@section System Calls

This section details the semaphore manager's services.

A subsection is dedicated to each of this manager's services

and describes the calling sequence, related constants, usage,

and status codes.

@c

@c  cre_sem

@c

@page

@subsection cre_sem - Create Semaphore

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER cre_sem(

  ID semid,

  T_CSEM *pk_csem

);

@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_NOMEM} - Insufficient memory (Memory for control block cannot be

allocated)

@code{E_OACV} - Object access violation (A semid less than -4 was

specified from a user task.)

@code{E_RSATR} - Reserved attribute (sematr was invalid or could not be

used)

@code{E_OBJ} - Invalid object state (a semaphore of the same ID already

exists)

@code{E_PAR} - Parameter error (pk_csem is invalid and/or isemcnt or

maxsem is negative or invalid)

@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_PAR} - A value outside the range supported by the target node

and/or transmission packet format was specified as a parameter (a value

outside supported range was specified for exinf, sematr, isemcnt and/or

maxsem)

@subheading DESCRIPTION:

This routine creates a semaphore that resides on the local node.  The

semaphore is initialized based on the attributes specified in the

@code{pk_csem} structure.  The initial and maximum counts of the

semaphore are set based on the @code{isemcnt} and @code{maxsem} fields

in this structure.

Specifying @code{TA_TPRI} in the @code{sematr} field of the

semaphore attributes structure causes tasks

waiting for a semaphore to be serviced according to task

priority.  When @code{TA_TFIFO} is selected, tasks are serviced in First

In-First Out order.

@subheading NOTES:

Multiprocessing is not supported.  Thus none of the "EN_" status codes

will be returned.

All memory is preallocated for RTEMS ITRON objects.  Thus, no dynamic

memory allocation is performed by @code{cre_sem} and the @code{E_NOMEM}

error can not be returned.

This directive will not cause the running task to be preempted.

The following semaphore attribute constants are

defined by RTEMS:

@itemize @bullet

@item @code{TA_TFIFO} - tasks wait by FIFO

@item @code{TA_TPRI} - tasks wait by priority

@end itemize

@c

@c  del_sem

@c

@page

@subsection del_sem - Delete Semaphore

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER del_sem(

  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{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

@subheading DESCRIPTION:

This routine deletes the semaphore specified by @code{semid}.

All tasks blocked waiting to acquire the semaphore will be

readied and returned a status code which indicates that the

semaphore was deleted.  The control block for this semaphore

is reclaimed by RTEMS.

@subheading NOTES:

Multiprocessing is not supported.  Thus none of the "EN_" status codes

will be returned.

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.

@c

@c  sig_sem

@c

@page

@subsection sig_sem - Signal Semaphore

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER sig_sem(

  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_QOVR} - Queuing or nesting overflow (the queuing count given by

semcnt went over the maximum allowed)

@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

@subheading DESCRIPTION:

@subheading NOTES:

Multiprocessing is not supported.  Thus none of the "EN_" status codes

will be returned.

@c

@c  wai_sem

@c

@page

@subsection wai_sem - Wait on Semaphore

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER wai_sem(

  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_DLT} - The object being waited for was deleted (the specified

semaphore was deleted while waiting)

@code{E_RLWAI} - Wait state was forcibly released (rel_wai was received

while waiting)

@code{E_CTX} - Context error (issued from task-independent portions or a

task in dispatch disabled state)

@code{EN_OBJNO} - An object number which could not be accessed on the

target node is specified.

@code{EN_PAR} - A value outside the range supported by the target node

and/or transmission packet format was specified as a parameter (a value

outside supported range was specified for tmout)

@subheading DESCRIPTION:

This routine attempts to acquire the semaphore specified by @code{semid}.

If the semaphore is available (i.e. positive semaphore count), then the

semaphore count is decremented and the calling task returns immediately.

Otherwise the calling tasking is blocked until the semaphore is released

by a subsequent invocation of @code{sig_sem}.

@subheading NOTES:

Multiprocessing is not supported.  Thus none of the "EN_" status codes

will be returned.

If the semaphore is not available, then the calling task will be blocked.

@c

@c  preq_sem

@c

@page

@subsection preq_sem - Poll and Request Semaphore

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER preq_sem(

  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_TMOUT} - Polling failure or timeout exceeded

@code{E_CTX} - Context error (issued from task-independent portions or a

task in dispatch disabled state)

@code{EN_OBJNO} - An object number which could not be accessed on the

target node is specified.

@code{EN_PAR} - A value outside the range supported by the target node

and/or transmission packet format was specified as a parameter (a value

outside supported range was specified for tmout)

@subheading DESCRIPTION:

This routine attempts to acquire the semaphore specified by @code{semid}.

If the semaphore is available (i.e. positive semaphore count), then the

semaphore count is decremented and the calling task returns immediately.

Otherwise, the @code{E_TMOUT} error is returned to the calling task to

indicate the semaphore is unavailable.

@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.

@c

@c  twai_sem

@c

@page

@subsection twai_sem - Wait on Semaphore with Timeout

@subheading CALLING SEQUENCE:

@ifset is-C

@example

ER twai_sem(

  ID semid,

  TMO tmout

);

@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 (tmout is -2 or less)

@code{E_DLT} - The object being waited for was deleted (the specified

semaphore was deleted while waiting)

@code{E_RLWAI} - Wait state was forcibly released (rel_wai was received

while waiting)

@code{E_TMOUT} - Polling failure or timeout exceeded

@code{E_CTX} - Context error (issued from task-independent portions or a

task in dispatch disabled state)

@code{EN_OBJNO} - An object number which could not be accessed on the

target node is specified.

@code{EN_PAR} - A value outside the range supported by the target node

and/or transmission packet format was specified as a parameter (a value

outside supported range was specified for tmout)

@subheading DESCRIPTION:

This routine attempts to acquire the semaphore specified by @code{semid}.

If the semaphore is available (i.e. positive semaphore count), then the

semaphore count is decremented and the calling task returns immediately.

Otherwise the calling tasking is blocked until the semaphore is released

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.