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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<head>
    <title>TCF Agent Porting Guide</title>
</head>

<body lang='EN-US'>

    <h1>TCF Agent Porting Guide</h1>

    <p>
        Copyright (c) 2009 Wind River Systems, Inc. Made available under the EPL v1.0
    </p>
    <p>
        Direct comments, questions to the <a href="mailto:dsdp-tcf-dev@eclipse.org">dsdp-tcf-dev@eclipse.org</a> mailing list
    </p>

    <h2>Table of Contents</h2>
    <ul>
        <li>
            <a href='#Introduction'>Introduction</a>
        </li>
        <li>
            <a href='#Customizing'>Customizing and Porting TCF Agent</a>
        </li>
        <li>
            <a href='#NewOS'>Porting TCF Agent to a New OS Platform</a>
        </li>
        <li>
            <a href='#NewCPU'>Porting TCF Agent to a New CPU Type</a>
        </li>
        <li>
            <a href='#NewExeFile'>Adding Support For a New Executable File Format</a>
        </li>
        <li>
            <a href='#NewDebugData'>Adding Support For a New Debug Data Format</a>
        </li>
    </ul>

    <h2>
        <a name='Introduction'>Introduction</a>
    </h2>

    <p>
        TCF Agent is a lightweight reference implementation of TCF protocol that supports basic debugging and other TCF services.
        It is written in C and can be used for remote debugging of software written for Linux, Windows XP or VxWorks.
        See <a href='TCF Getting Started.html'>TCF Getting Started</a> for instructions on how to get the source code and build the agent.
    </p>


    <h2>
        <a name='Customizing'>Customizing and Porting TCF Agent</a>
    </h2>

    <p>
        Most of TCF agent configuration is done at compile time.
        Conditional compilation statements in the source code assume that both the agent and inferior code will run on same OS platform and
        on same CPU type that were used to build the agent.
        Building an agent that can run on one machine while controlling execution of code on another machine might be possible, but not fully supported at this time.
    </p>

    <p>
        It is important to know concurrency model used by the agent code before making any changes.
        Most of the agent code is event driven: it has a main loop that retrieves events from an event queue and executes them sequentially by calling event handlers by a single thread.
        Single threaded event driven design provides good level of concurrency (equivalent to cooperative multithreading), while greatly reduces need for synchronization -
        each event dispatch cycle can be viewed as fully synchronized atomic operation.
    </p>
    
    <p>
        Event driven code should avoid long running or potentially blocking operations in event handlers since they can stop all event processing for indefinite time.
        Such operations should use asynchronous APIs (like POSIX Asynchronous I/O), or should be performed by background threads.
        Treat background threads with extreme caution - agent data structures are not intended for multithreaded access.
        Background thread scope should be limited to a single module and it should not call agent public APIs.
        Instead they should communicate with the rest of the code by posting events.
    </p>
    
    <p>
        An event is essentially a function pointer (a call-back) that points to event handler, plus a data pointer.
        Call-backs are also used throughout the agent code to subscribe listeners for various state change notifications.
        Using events and call-backs, as a design principle, is also known as inversion of control.
        Note that, in general, inversion of control is not compatible with traditional multithreaded programming model that used mutexes to protect shared data from racing conditions.
    </p>
    
    <p>
        It should be much easier to port the agent if you don't need all TCF services.
        For example, for RSE integration you only need File System, System Monitor and Processes services,
        so you can disable all other services by editing <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/config.h'>config.h</a>.
    </p>

    <p>
        Source file context.c implements low level debugger functionality, however it is still pulled in even if you disable all debugger related services.
        If you don't need debugger support in the agent, you can replace context.c with a dummy implementation that does nothing and always returns an error.
    </p>

    <p>
        It is better to create a separate directory with alternative versions of
        <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/config.h'>config.h</a>,
        <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/context.h'>context.h,</a>
        <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/context.c'>context.c,</a>
        <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/Makefile'>Makefile,</a>
        etc., instead of editing original files.
        The idea is that Makefile will search that directory first, and if a file not found there, it will search original agent sources.
        See <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/examples/org.eclipse.tm.tcf.examples.daytime.agent'>examples/org.eclipse.tm.tcf.examples.daytime.agent</a>
        for an example of a custom TCF agent.
        Of course, if changes are generic enough to be useful for other ports, then it is better to change code in the main directory.
    </p>

    <p>
        Please, consider contributing your changes of the source code back to eclipse.org.
    </p>

    <h2>
        <a name='NewOS'>Porting TCF Agent to a New OS Platform</a>
    </h2>

    <p>
        TBD
    </p>

    <h2>
        <a name='NewCPU'>Porting TCF Agent to a New CPU Type</a>
    </h2>

    <p>Searching TCF agent source code for __i386__ is a good way to find all places where the source code depends on CPU type.</p>
    <p>There are several files in the code that might need changes in order to support a new CPU type:</p>

    <dl>
        <dt>
            <b>
                <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/context.c'>context.c</a>
            </b>
        </dt>
        <dd>
            The module provides low level debugger functionality: attach/detach, suspend/resume, single step, memory read/write.
            It uses OS debug APIs to do its job. Most of the code does not depend on CPU type, however, single stepping is not always directly
            supported by OS, and its implementation needs to be reviewed and changed to support new CPU type.
        </dd>
        <dt>
            <b>
                <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/dwarfexpr.c'>dwarfexpr.c</a>
            </b>
        </dt>
        <dd>
            The module implements evaluation of <a href='http://en.wikipedia.org/wiki/DWARF'>DWARF</a> expressions.
            The module is used only if the agent is built to support
            <a href='http://en.wikipedia.org/wiki/Executable_and_Linkable_Format'>ELF</a> executable file format and
            <a href='http://en.wikipedia.org/wiki/DWARF'>DWARF</a> debugging data format.
            No need to change the module if <a href='http://en.wikipedia.org/wiki/Executable_and_Linkable_Format'>ELF</a> or
            <a href='http://en.wikipedia.org/wiki/DWARF'>DWARF</a> support is not required.
            <a href='http://en.wikipedia.org/wiki/DWARF'>DWARF</a> expressions can have references to CPU registers.
            Register access code needs to be changed to support new CPU type.
            Note that different compilers can use different numbers to identify same registers of same CPU.
        </dd>
        <dt>
            <b>
                <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/registers.c'>registers.c</a>
            </b>
        </dt>
        <dd>
            The module implements <a href='TCF Service - Registers.html'>Registers service</a>. The code has static variable "regs_index" that contains a table of CPU registers.
            The table holds names, offsets and sizes of CPU registers. Offset and size define location of register data in REG_SET structure,
            which represents snapshot of register values of an execution context. Definition of the variable needs to be changed to support new CPU type.
        </dd>
        <dt>
            <b>
                <a href='http://dev.eclipse.org/svnroot/dsdp/org.eclipse.tm.tcf/trunk/agent/stacktrace.c'>stacktrace.c</a>
            </b>
        </dt>
        <dd>
            The module implements <a href='TCF Service - Stack Trace.html'>Stack Trace service</a>.
            The module contains "trace_stack" function that creates stack trace by walking a stack of an executable context.
            Stack trace data format is defined by two struct declarations: StackTrace and StackFrame.
            The data structure is generic, however the code that created the data structure is CPU dependand.
            Alternative version of "trace_stack" function needs to be provided to support new CPU type.
        </dd>
    </dl>

    <h2>
        <a name='NewExeFile'>Adding Support For a New Executable File Format</a>
    </h2>

    <p>
        TBD
    </p>

    <h2>
        <a name='NewDebugData'>Adding Support For a New Debug Data Format</a>
    </h2>

    <p>
        TBD
    </p>

</body>
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