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)is
(*(funcptr *)(((argp) += sizeof(funcptr)) - sizeof(funcptr)))which works correctly.
See also questions 1.13, 1.17, and 1.21.
References:
ISO Sec. 7.8.1.2
Rationale Sec. 4.8.1.2
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