67 lines
3.1 KiB
Plaintext
67 lines
3.1 KiB
Plaintext
Contributed by Egor Duda
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How does function autoloading work?
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Cygwin has the ability to handle win32 functions which are present on
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some platforms and not present on others via autoload mechanism. It's
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essentially a lazy binding of symbols. It works as following. For
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(almost) every function from OS API which cygwin uses, a stub is created
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in file autoload.cc. Each reference to the such function from win32 API
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in cygwin dll source code is actually pointing to this stub.
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When the function, say GetConsoleWindow(), is called for the first time,
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the control is passed to its stub. The stub tries to load the
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appropriate system dll via LoadModule() and get the actual function
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address via GetProcAddress(). If this operation succeeds, the stub is
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"patched" to pass control to actual address of GetConsoleWindow() in
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appropriate system dll, so that next time we won't have to load dll and
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perform address lookup in it again. From this point on, the call to the
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function is performed as if the dll/function were linked statically.
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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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the application, depending on what kind of stub is created in
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autoload.cc, will either:
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1) Exit with fatal error.
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2) Or return a predefined value indicating an error; and set the windows
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error code to 127 (ERROR_PROC_NOT_FOUND).
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Almost all w32api functions are linked into the cygwin dll in this
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manner, dynamically, at runtime.
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The costs:
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1) A tiny overhead in the initial call to a function call as each call
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is performed, indirectly, via a stub. For the first lookup of a symbol
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of an unloaded dll, there is also some overhead in loading the dll for
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the first time. The dll is only loaded by the first call to a symbol
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in the dll. After the first call to a function, subsequent calls are
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as fast as a normal, statically loaded function.
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The benefits:
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1) Speedup at startup time. Applications only load those dlls which are
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actually needed. For example, if application never uses socket
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functions, 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) Allows a 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
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function that was not used there before, you should add a stub so it
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will be autoloaded. To do so, add one of the LoadDllfunc* macros to
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autoload.cc. All macros eventually resolve to the following form:
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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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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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