56 lines
2.6 KiB
Plaintext
56 lines
2.6 KiB
Plaintext
Copyright 2002 Red Hat Inc., Egor Duda
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How does function autoloading work?
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Cygwin has an ability to handle win32 functions which are present on some
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platforms and not present on others via autoload mechanism. It's essentially a
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lazy binding of symbols. It works as following. For (almost) every function
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from OS API which cygwin uses, a stub is created in file autoload.cc. Each
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reference to the such function from win32 API in cygwin dll source code is
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actually pointing to this stub.
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When the function, say GetConsoleWindow(), is called first time, the
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control is passed to its stub. Stub tries to load appropriate system dll
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via LoadModule() and get actual function address via GetProcAddress().
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If this operation succeeds, the stub is "patched" to pass control to actual
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address of GetConsoleWindow() in appropriate system dll, so that next
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time we won't have to load dll and perform address lookup in it again.
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If LoadModule() or GetProcAddress() fail, (and on nt4 the latter indeed
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fails because GetConsoleWindow() is not available in kernel32.dll), then
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application, depending on what kind of stub is created in autoload.cc, is
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either:
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1) Exits with fatal error.
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2) Or returns a predefined value indicating an error; and sets error code to
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127 (ERROR_PROC_NOT_FOUND).
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That is, almost all w32api functions are linked into cygwin dll
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dynamically, at runtime.
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The costs:
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1) A tiny overhead in function call as each call is performed,
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indirectly, via stub.
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The benefits:
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1) Speedup at startup time. Application only loads those dlls which are
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actually needed. For example, if application never uses socket functions,
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winsock dlls are never loaded.
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2) Greatly simplify wincap system -- we don't need to have a separate
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capability for every win32 function which may or may not be present on
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particular win32 platform.
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3) Allow single cygwin1.dll for all win32 platforms.
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If you're changing in cygwin1.dll source code and if you use some function that
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was not used there before, you should add a stub so it will be autoloaded. To
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do so, add
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LoadDLLfuncEx2 (function name, parameter block length, dll name,
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non-fatality flag , value to return if function not available)
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to autoload.cc file.
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Parameter block length is a sum of sizes (in bytes) of parameters which are
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being passed to the function. If non-fatality flag is set to 0, then failure
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to load dll and find a function will cause fatal error. If non fatality flag
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is set to 1, then call to the function will return default value.
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You can also use shorter versions -- LoadDLLfuncEx and LoadDLLfunc, if the
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defaults they provide suit your needs.
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