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6.25 Pointer Arguments in Variadic Functions

Standard C requires that pointer types used with va_arg in functions with variable argument lists either must be compatible with that of the actual argument, or that one type must be a pointer to void and the other a pointer to a character type. GNU C implements the POSIX XSI extension that additionally permits the use of va_arg with a pointer type to receive arguments of any other pointer type.

In particular, in GNU C va_arg (ap, void *) can safely be used to consume an argument of any pointer type.