path: root/c/src/lib/librtems++/README

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RTEMS C++ Library
=================

The RTEMS C++ Library or librtems++ is a wrapper for the RTEMS API.
The classes provide as close a match to the RTEMS C API, for
performance, to share the existing C documentation as much as
possible, and to allow easy tracking of any changes to the RTEMS C
API.

The C++ interface only uses RTEMS API calls.  No external references
or internal interfaces are used.  This allows the classes to be used
in separately compiled modules or applications which link to the RTEMS
trap interface.

(This is the goal, which has not quite been reached. The TOD macro for
micro-seconds to ticks is used, and this uses an internal global RTEMS
variable)

The C++ interface does not deal with RTEMS initialisation or the
device driver interface.  The current view is these parts of a system
are best handled in the current manner.  This means BSP for
initialisation and the C API for drivers.

RTEMS C++ Classes
=================

The classes map to the managers of RTEMS.

The methods have default values selected which try to fit most cases
or follow the documented RTEMS default values.  Moving from left to
right the parameters become less used, allowing the defaults to be
selected. An example is the scope parameter for most classes.  This
can be local or global.  I assume that most RTEMS objects are local,
therefore it has been made the last parameter.

Inline methods have been used for methods which are commonly used in
applications.  This tries to add the minimum of overhead.  For
example, the methods to send or receive events are inline, while all
methods for control of a task are not.

The RTEMS types, enumerations, and defines are used.  If a new type,
enumeration or define is made it will map directly to the RTEMS
equivalent.  For example the enumeration Scope is defined for various
classes which can be local or global. The elements of the enumeration
are forced to the same value as the RTEMS values.  An enumeration is
used in this case to allow the compiler to type check a little
better. It saves having to check only RTEMS_LOCAL or RTEMS_GLOBAL is
passed as a parameter (I am not convinced this is really needed as the
goal was to not define anything and to only use what RTEMS provided).

Where possible the various parts of an option bit set, or mode can be
controlled separately or controlled as a group.  An example is the
task mode.  The RTEMS C API allows a set of modes to be modified at
once.  The TaskMode class allows this to occur, while also providing
methods to control a single mode item.

The name of an object is always passed as a string.  The classes turn
the string into a rtems_name variable.  The string does not have to be
nul character terminated.

The RTEMS C API uses 'delete' to remove or kill an RTEMS object.  This
is a reserved word in C++, so the word 'destroy' is used instead.

Calling the classes from interrupts follows the rules of RTEMS.  An
exception introduced by the class library is the last status code.
There is only one last status code for each instance of the library's
classes and it is not protected.  This needs to be watched for.  Maybe
a better solution needs to be found, such as interrupt calls do not set
the last status code.

RTEMS objects created by the C++ library can be operated on by C code
just as any other RTEMS object. If limitations exist they should be
documented in under the class.

RTEMS Object Ownership
======================

The concept of ownership of an object is not defined as part of the
RTEMS C API.  A piece of code executing as part a task can create a
message queue.  Another piece of code running as part of a different
task can destroy the message queue.  Correct behavior between the code
that creates the message queue and the code which destroy's the
message queue must be provided by the programmer.

The librtems++ supports the concept of ownership of an RTEMS object.
Only the C++ object that creates the RTEMS object can destroy it.  A
C++ object can connect to an existing RTEMS object and control it,
how-ever it can not destroy it.

Copy constructors and assignment operators are provided to in-force
this rule.

Ownership only applies to classes that create RTEMS objects.  These
classes contain a flag which signals ownership of the id.

Timeouts
========

The timeout value is specified in micro-seconds.  The classes turn the
micro-second timeout value into ticks required by the RTEMS C API.

This causes a problem for timeout values which are less than one tick.
This case is tested for and the timeout value is set to one tick.  All
other cases round down to the nearest tick.

Status Codes
============

All classes which form the C++ API are derived from the StatusCode
class.  This class provides a common method for handling the status
code returned by RTEMS.

The last returned status code is held in the StatusCode object.  It
can be queried directly, or as a boolean.  You can also obtain an
error string for the status code.

The setting of a status code is restricted to derived classes.

The last status code attribute of the class is only ever set to an
RTEMS defined status code.

Event Class
===========

The event class allows users to send and receive events to and from
tasks.

Events objects are by default connected the RTEMS_SELF task.  A send
or receive will operate on the task currently executing.

An Event object can be connected to a task using the connect method.
The name is the name of the task.  Connection can also be achieved by
using the copy constructor or assignment operator.

Events can be sent to a task by specifying an RTEMS task id, or by
passing a reference to a Task object.

Interrupt Class
===============

The interrupt class allows a protected virtual method of a derived
class to be an interrupt handler.

You derive from this class and provide the handler method.  The next
interrupt after the vector is caught will cause the handler method to
be entered.

You can chain the interrupt by calling the chain method.  If the old
handler is not an instance of this class the chain is passed as "void
(*)(void)".  If it is an instance of this class, the handler method is
directly called. (Chaining has not been tested)

This class implements a table of pointers to the last instance to
catch the interrupt.  A static method of the class catches the
interrupt and re-directs the interrupt to the instance in the table.
The re-direct adds a additional virtual function call and return to
the overhead of the interrupt.  For a i386 type processor this is
about 12 instructions including the function call entry.

Message Queue Class
===================

The MessageQueue class allows message queue's to be created, or
connected too.  Only the creator can destroy a message queue.

The class implements, sending, urgent sending, broadcast, flushing,
and receiving.

Semaphore Class
===============

The Semaphore class allows semaphores to be created, or connected
too.  Only the creator can destroy a semaphore.

All types of semaphores can be created.

(Not tested in the test code)

Task Class
==========

The Task class allows tasks to be created, or connected too.  Only the
creator can destroy a task.

If creating a task, derive from the Task class and provide the body
method.  The body method is the entry point for a task.  When
connecting to an existing task, no body method is required to be
provided.  It is how-ever required if you create a task.  This is not
enforced by the compiler, how-ever the default body will be entered,
and it contains no code.  The RTEMS default behaviour for a task that
returns occurs.

The mode of a task is controlled using the TaskMode class.

The Task class allows you to start, restart, suspend, and resume a
task.  You can control the priority, and access the note-pad
registers.  The task can also be slept using the wake_after and
wake_when methods.

Currently the task argument is used to pass the 'this' pointer to the
libraries default task body. The actual argument is held in the class
instance and passed to the virtual body method. This means of passing
the 'this' pointer through RTEMS to the default task body requires the
actual task object to perform a restart call. This is not really the
best solution to the problem. Another solution is to remove a notpad
register, say 31 from the task and use it. This would mean any Task
object could stop and restart a task how-ever a notpad register is
lost. Any other ideas are welcome.

Task Mode Class
===============

The TaskMode class allows you to query or change the mode of a task.
The object only operates on the currently executing task.

The standard flags defined in RTEMS are used.

Methods are provided to operate on a group of modes which are required
to be changed in a single operation.  The mode and mask is specified
by ORing the required flags as documented in the RTEMS manual.

Methods are provided for accessing and controlling a specific mode.
The returned value will only contain the requested mode's flags, and
only the that mode will be changed when setting a mode.

Timer Class
===========

The Timer class allows timers to be created.  You cannot connect to an
existing timer.

You derive from the Timer class and provide the trigger method.  This
method is called when the timer triggers or times out.

You can request a single shot timer using the fire_after or fire_when
methods, or a periodic timer by calling the repeat_file_at method.

You cannot copy timer objects.

Contact
=======
Send any question to me Chris Johns at cjohns@plessey.com.au, or the RTEMS
mailing list.

To Do
=====

1) Develop a complete test suite (under way, cjohns@plessey.com.au).

2) Complete wrapping the remaining RTEMS C API.

3) Provide light weight cout/cerr/clog classes based on printf for
embedded systems.

4) Provide a memory serial class which maps the <</>> operators onto
raw memory in network byte order independent of CPU byte order.

5) Fix the Task class so any Task object can restart a task.

6) Provide some frame work classes which allow actor type objects that
start in an ordered manner.