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).

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)

is

(*(funcptr *)(((argp) += sizeof(funcptr)) - sizeof(funcptr)))

which works correctly.