newlib-cygwin/winsup/cygwin/how-autoload-works.txt

Copyright 2002 Red Hat Inc., Egor Duda

How does function autoloading work?

Cygwin has the ability to handle win32 functions which are present on
some platforms and not present on others via autoload mechanism.  It's
essentially a lazy binding of symbols.  It works as following.  For
(almost) every function from OS API which cygwin uses, a stub is created
in file autoload.cc.  Each reference to the such function from win32 API
in cygwin dll source code is actually pointing to this stub.

When the function, say GetConsoleWindow(), is called for the first time,
the control is passed to its stub.  The stub tries to load the
appropriate system dll via LoadModule() and get the actual function
address via GetProcAddress().  If this operation succeeds, the stub is
"patched" to pass control to actual address of GetConsoleWindow() in
appropriate system dll, so that next time we won't have to load dll and
perform address lookup in it again.  From this point on, the call to the
function is performed as if the dll/function were linked statically.

If LoadModule() or GetProcAddress() fail, (and on nt4 the latter indeed
fails because GetConsoleWindow() is not available in kernel32.dll), then
the application, depending on what kind of stub is created in
autoload.cc, will either:

1) Exit with fatal error.

2) Or return a predefined value indicating an error; and set the windows
error code to 127 (ERROR_PROC_NOT_FOUND).

Almost all w32api functions are linked into the cygwin dll in this
manner, dynamically, at runtime.

The costs:
1) A tiny overhead in the initial call to a function call as each call
is performed, indirectly, via a stub.  For the first lookup of a symbol
of an unloaded dll, there is also some overhead in loading the dll for
the first time.  The dll is only loaded by the first call to a symbol
in the dll.  After the first call to a function, subsequent calls are
as fast as a normal, statically loaded function.

The benefits:
1) Speedup at startup time.  Applications only load those dlls which are
actually needed.  For example, if application never uses socket
functions, winsock dlls are never loaded.

2) Greatly simplify wincap system -- we don't need to have a separate
capability for every win32 function which may or may not be present on
particular win32 platform.

3) Allows a single cygwin1.dll for all win32 platforms.

If you're changing in cygwin1.dll source code and if you use some
function that was not used there before, you should add a stub so it
will be autoloaded.  To do so, add one of the LoadDllfunc* macros to
autoload.cc.  All macros eventually resolve to the following form:

LoadDLLfuncEx2 (function name, parameter block length, dll name,
                non-fatality flag , value to return if function not available)

Parameter block length is a sum of sizes (in bytes) of parameters which are
being passed to the function. If non-fatality flag is set to 0, then failure
to load dll and find a function will cause fatal error. If non fatality flag
is set to 1, then call to the function will return default value.
You can also use shorter versions -- LoadDLLfuncEx and LoadDLLfunc, if the
defaults they provide suit your needs.