How to use va_start()?

Question

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.

See the C11 standard. 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
– pmg, Commented Jul 15, 2019 at 9:24
Some light reading (examples are at the bottom). Bookmark that site, btw, for all your future C and C++ needs. — WhozCraig
– WhozCraig, Commented Jul 15, 2019 at 9:27
I've checked it already. Couldnt understand.. "macros have no concept of type;" that must be the point. — Kewal Takhellambam
– Kewal Takhellambam, Commented Jul 15, 2019 at 9:27
Indeed. They're about straight up substitution more than anything else. — WhozCraig
– WhozCraig, Commented Jul 15, 2019 at 9:28

Jonathan Leffler · Accepted Answer · 2019-07-15 11:46:51Z

7

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.

edited Jul 15, 2019 at 11:46

Jonathan Leffler

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answered Jul 15, 2019 at 9:27

Ingo Leonhardt

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2 Comments

Kewal Takhellambam Over a year ago

now i understand. I have been missing that va_start() is a macro ,not a function.

Jonathan Leffler Over a year ago

@KewalTakhellambam — the va_start() and va_arg() macros have to be macros because the second argument is the name of a type, and you can't pass type names to functions.

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