prev up next   top/contents search

comp.lang.c FAQ list · Question 15.11

Q: I can't get va_arg to pull in an argument of type pointer-to-function.

A: Try using a typedef for the function pointer type.

The type-rewriting games which the va_arg macro typically plays are stymied by overly-complicated types such as pointer-to-function. To illustrate, a simplified implementation of va_arg is

	#define va_arg(argp, type) \
		(*(type *)(((argp) += sizeof(type)) - sizeof(type)))
where argp's type (va_list) is char *. When you attempt to invoke
	va_arg(argp, int (*)())
the expansion is
	(*(int (*)() *)(((argp) += sizeof(int (*)())) - sizeof(int (*)())))
which is a syntax error (the first cast (int (*)() *) is meaningless). [footnote]

If you use a typedef for the function pointer type, however, all will be well. Given

	typedef int (*funcptr)();
the expansion of
	va_arg(argp, funcptr)
	(*(funcptr *)(((argp) += sizeof(funcptr)) - sizeof(funcptr)))
which works correctly.

See also questions 1.13, 1.17, and 1.21.

References: ISO Sec.
Rationale Sec.

prev up next   contents search
about this FAQ list   about eskimo   search   feedback   copyright

Hosted by Eskimo North