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213 lines
8.9 KiB
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<title>xptcall Porting Guide</title>
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<body bgcolor = "white">
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<h2><center>xptcall Porting Guide</center></h2>
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<h3>Overview</h3>
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<blockquote>
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<a href="http://www.mozilla.org/scriptable/xptcall-faq.html"> xptcall</a> is a
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library that supports both invoking methods on arbitrary xpcom objects and
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implementing classes whose objects can impersonate any xpcom interface. It does
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this using platform specific assembly language code. This code needs to be
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ported to all platforms that want to support xptcall (and thus mozilla).
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</blockquote>
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<h3>The tree</h3>
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<blockquote>
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<pre>
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<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall">mozilla/xpcom/libxpt/xptcall</a>
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+--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public">public</a> // exported headers
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+--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src">src</a> // core source
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| \--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md">md</a> // platform specific parts
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| +--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/mac">mac</a> // mac ppc
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| +--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/unix">unix</a> // all unix
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| \--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/win32">win32</a> // win32
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| +--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/test">test</a> // simple tests to get started
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\--<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/tests">tests</a> // full tests via api
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</pre>
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Porters are free to create subdirectories under the base <code>md</code>
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directory for their given platforms and to integrate into the build system as
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appropriate for their platform.
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</blockquote>
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<h3>Theory of operation</h3>
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<blockquote>
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There are really two pieces of functionality: <i>invoke</i> and <i>stubs</i>...
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<p>
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The <b><i>invoke</i></b> functionality requires the implementation of the
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following on each platform (from <a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public/xptcall.h#131">xptcall/public/xptcall.h</a>):
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<pre>
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XPTC_PUBLIC_API(nsresult)
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XPTC_InvokeByIndex(nsISupports* that, PRUint32 methodIndex,
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PRUint32 paramCount, nsXPTCVariant* params);
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</pre>
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Calling code is expected to supply an array of <code>nsXPTCVariant</code>
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structs. These are discriminated unions describing the type and value of each
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parameter of the target function. The platform specific code then builds a call
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frame and invokes the method indicated by the index <code>methodIndex</code> on
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the xpcom interface <code>that</code>.
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<p>
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Here are examples of this implementation for
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<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/win32/xptcinvoke.cpp">Win32</a>
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and
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<a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/unix/xptcinvoke_unixish_x86.cpp">Linux x86, NetBSD x86, and FreeBSD</a>.
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Both of these implementations use the basic strategy of: figure out how much
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stack space is needed for the params, make the space in a new frame, copy the
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params to that space, invoke the method, cleanup and return. C++ is used where
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appropriate, Assembly language is used where necessary. Inline assembly language is used here,
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but it is equally valid to use separate assembly language source files. Porters
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can decide how best to do this for their platforms.
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<p>
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The <b><i>stubs</i></b> functionality is more complex. The goal here is a class
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whose vtbl can look like the vtbl of any arbitrary xpcom interface. Objects of
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this class can then be built to impersonate any xpcom object. The base interface
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for this is (from <a href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public/xptcall.h#109">xptcall/public/xptcall.h</a>):
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<pre>
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class nsXPTCStubBase : public nsISupports
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{
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public:
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// Include generated vtbl stub declarations.
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// These are virtual and *also* implemented by this class..
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#include "xptcstubsdecl.inc"
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// The following methods must be provided by inheritor of this class.
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// return a refcounted pointer to the InterfaceInfo for this object
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// NOTE: on some platforms this MUST not fail or we crash!
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NS_IMETHOD GetInterfaceInfo(nsIInterfaceInfo** info) = 0;
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// call this method and return result
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NS_IMETHOD CallMethod(PRUint16 methodIndex,
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const nsXPTMethodInfo* info,
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nsXPTCMiniVariant* params) = 0;
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};
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</pre>
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Code that wishes to make use of this <i>stubs</i> functionality (such as
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<a href="http://www.mozilla.org/scriptable/">XPConnect</a>) implement a class
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which inherits from <code>nsXPTCStubBase</code> and implements the
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<code>GetInterfaceInfo</code> and <code>CallMethod</code> to let the
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platform specific code know how to get interface information and how to dispatch methods
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once their parameters have been pulled out of the platform specific calling
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frame.
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<p>
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Porters of this functionality implement the platform specific code for the
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<i>stub</i> methods that fill the vtbl for this class. The idea here is that the
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class has a vtbl full of a large number of generic stubs. All instances of this
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class share that vtbl and the same stubs. The stubs forward calls to a platform
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specific method that uses the interface information supplied by
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the overridden <code>GetInterfaceInfo</code> to extract the parameters and build
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an array of platform independent <code>nsXPTCMiniVariant</code> structs which
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are in turn passed on to the overridden <code>CallMethod</code>. The
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platform dependent code is responsible for doing any cleanup and returning.
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<p>
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The stub methods are declared in <a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public/xptcstubsdecl.inc">xptcall/public/xptcstubsdecl.inc</a>.
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These are '#included' into the declaration of <code>nsXPTCStubBase</code>. A
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similar include file (<a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public/xptcstubsdef.inc">xptcall/public/xptcstubsdef.inc</a>)
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is expanded using platform specific macros to define the stub functions. These
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'.inc' files are checked into cvs. However, they can be regenerated as necessary
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(i.e. to change the number of stubs or to change their specific declaration)
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using the Perl script <a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/public/genstubs.pl">xptcall/public/genstubs.pl</a>.
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<p>
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Here are examples of this implementation for <a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/win32/xptcstubs.cpp">Win32</a>
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and <a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/unix/xptcstubs_unixish_x86.cpp">Linux x86, NetBSD x86, and FreeBSD</a>.
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Both of these examples use inline assembly language. That is just how I
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decided to do it. You can do it as you choose.
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<p>
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The Win32 version is somewhat tighter because the __declspec(naked) feature
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allows for very small stubs. However, the __stdcall requires the callee to clean
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up the stack, so it is imperative that the interface information scheme allow
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the code to determine the correct stack pointer fixup for return without fail,
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else the process will crash.
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<p>
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I opted to use inline assembler for the gcc Linux x86 port. I ended up with
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larger stubs than I would have preferred rather than battle the compiler over
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what would happen to the stack before my asm code began running.
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<p>
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I believe that the non-assembly parts of these files can be copied and reused
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with minimal (but not zero) platform specific tweaks. Feel free to copy and
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paste as necessary. Please remember that safety and reliability are more
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important than speed optimizations. This code is primarily used to connect XPCOM
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components with JavaScript; function call overhead is a <b>tiny</b> part of the
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time involved.
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<p>
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I put together
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<a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/src/md/test">xptcall/src/md/test
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</a> as a place to evolve the basic functionality as a port is coming together.
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Not all of the functionality is exercised, but it is a place to get started.
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<a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/tests">xptcall/tests
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</a> has an api level test for <code>XPTC_InvokeByIndex</code>, but no tests for
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the <i>stubs</i> functionality. Such a test ought to be written, but this has not
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yet been done.
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<p>
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A full 'test' at this point requires building the client and running the
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XPConnect test called <i>TestXPC</i> in
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<a
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href="http://lxr.mozilla.org/mozilla/source/js/src/xpconnect/tests">mozilla/js/src/xpconnect/tests
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</a>.
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<p>
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Getting these ports done is very important. Please let <a
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href="mailto:jband@netscape.com">me</a> know if you are interested in doing one.
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I'll answer any questions as I get them.
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<p>
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<a
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href="http://lxr.mozilla.org/mozilla/source/xpcom/libxpt/xptcall/status.html">
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Porting Status
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</a>
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</blockquote>
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<hr>
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<b>Author:</b> <a href="mailto:jband@netscape.com">John Bandhauer <jband@netscape.com></a><br>
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<b>Last modified:</b> 30 April 1999
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