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4.2.6 Limitations--original function signatures

As shown in the above example, to call the original you must use a macro of the form ‘CALL_FN_*’. For technical reasons it is impossible to create a single macro to deal with all argument types and numbers, so a family of macros covering the most common cases is supplied. In what follows, 'W' denotes a machine-word-typed value (a pointer or a C ‘long’), and 'v' denotes C's ‘void’ type. The currently available macros are:

Macro: CALL_FN_v_v

Call an original function of type void fn (void).

Macro: CALL_FN_W_v

Call an original function of type long fn (void).

Macro: CALL_FN_v_W

Call an original function of type void fn (long).

Macro: CALL_FN_W_W

Call an original function of type long fn (long).

Macro: CALL_FN_v_WW

Call an original function of type void fn (long, long).

Macro: CALL_FN_W_WW

Call an original function of type long fn (long, long).

Macro: CALL_FN_v_WWW

Call an original function of type void fn (long, long, long).

Macro: CALL_FN_W_WWW

Call an original function of type long fn (long, long, long).

Macro: CALL_FN_W_WWWW

Call an original function of type long fn (long, long, long, long).

Macro: CALL_FN_W_5W

Call an original function of type long fn (long, long, long, long, long).

Macro: CALL_FN_W_6W

Call an original function of type long fn (long, long, long, long, long, long).

The set of supported types can be expanded as needed. It is regrettable that this limitation exists. Function wrapping has proven difficult to implement, with a certain apparently unavoidable level of ickyness. After several implementation attempts, the present arrangement appears to be the least-worst tradeoff. At least it works reliably in the presence of dynamic linking and dynamic code loading/unloading.

You should not attempt to wrap a function of one type signature with a wrapper of a different type signature. Such trickery will surely lead to crashes or strange behaviour. This is not of course a limitation of the function wrapping implementation, merely a reflection of the fact that it gives you sweeping powers to shoot yourself in the foot if you are not careful. Imagine the instant havoc you could wreak by writing a wrapper which matched any function name in any soname--in effect, one which claimed to be a wrapper for all functions in the process.