What exactly is va_end for? Is it always necessary to call it?

Question

va_end - Macro to reset arg_ptr.

After accessing a variable argument list, the arg_ptr pointer is usually reset with va_end(). I understand that it is required if you want to re-iterate the list, but is it really needed if you aren't going to? Is it just good practice, like the rule "always have a default: in your switch"?

Answer 1

va_end is used to do cleanup. You don't want to smash the stack, do you?

From man va_start:

va_end()

Each invocation of va_start() must be matched by a corresponding invocation of va_end() in the same function. After the call va_end(ap) the variable ap is undefined. Multiple traversals of the list, each bracketed by va_start() and va_end() are possible. va_end() may be a macro or a function.

Note the presence of the word must.

The stack could become corrupted because you don't know what va_start() is doing. The va_* macros are meant to be treated as black boxes. Every compiler on every platform can do whatever it wants there. It may do nothing, or it may do a lot.

Some ABIs pass the first few args in registers, and the remainder on the stack. A va_arg() there may be more complicated. You can look up how a given implementation does varargs, which may be interesting, but in writing portable code you should treat them as opaque operations.