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Advising a primitive function (also called a “subr”) is risky. Some primitive functions are used by the advice mechanism; advising them could cause an infinite recursion. Also, many primitive functions are called directly from C code. Calls to the primitive from Lisp code will take note of the advice, but calls from C code will ignore the advice.
When the advice facility constructs the combined definition, it needs
to know the argument list of the original function. This is not
always possible for primitive functions. When advice cannot determine
the argument list, it uses (&rest ad-subr-args), which always
works but is inefficient because it constructs a list of the argument
values. You can use ad-define-subr-args to declare the proper
argument names for a primitive function:
This function specifies that arglist should be used as the argument list for function function.
For example,
(ad-define-subr-args 'fset '(sym newdef))
specifies the argument list for the function fset.