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In a function with variable arguments, we initialize an object of type va_list ,'ap' with the function va_start() as:

void va_start(va_list ap, parmN);

I don't understand

1.what type of objects can be passed as parMN(last known parameter). I've done with examples of passing integers, strings with format specifiers, structures etc.

2. How the parMN describes the following optional parameters.

Kewal Takhellambam
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    See the [C11 standard](http://port70.net/~nsz/c/c11/n1570.html#7.16.1.4). In short: 1. `va_start` is a macro; macros have no concept of *type*; they work their magic with source text; 2. the following optional parameters are described with `va_arg` (also a macro) – pmg Jul 15 '19 at 09:24
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    [Some light reading](https://en.cppreference.com/w/cpp/utility/variadic/va_start) (examples are at the bottom). Bookmark that site, btw, for all your future C and C++ needs. – WhozCraig Jul 15 '19 at 09:27
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    I've checked it already. Couldnt understand.. "macros have no concept of type;" that must be the point. – Kewal Takhellambam Jul 15 '19 at 09:27
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    Indeed. They're about straight up substitution more than anything else. – WhozCraig Jul 15 '19 at 09:28

1 Answers1

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The C standard says that va_start() is actually a macro, not a function, so it can do things a function couldn't. The details are highly implementation dependent but you could imagine that it takes the address of parmN to determine the stack address of the next parameter. There's no need for va_start() to know about the types of the following parameters because that information is passed to va_arg(), which is another macro.

Jonathan Leffler
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Ingo Leonhardt
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