va_start - cppreference.com (original) (raw)

| Defined in header <stdarg.h> | | | | ----------------------------------------------------------------------- | | ----------- | | void va_start( va_list ap, parmN ); | | (until C23) | | void va_start( va_list ap, ... ); | | (since C23) |

The va_start macro enables access to the variable arguments following the named argument parmN(until C23).

va_start shall be invoked with an instance to a valid va_list object ap before any calls to va_arg.

If parmN is declared with register storage class specifier, with an array type, with a function type, or with a type not compatible with the type that results from default argument promotions, the behavior is undefined. (until C23)
Only the first argument passed to va_start is evaluated. Any additional arguments are neither expanded nor used in any way. (since C23)

[edit] Parameters

ap - an instance of the va_list type
parmN - the named parameter preceding the first variable parameter

[edit] Expanded value

(none)

[edit] Example

#include <stdio.h> #include <stdarg.h>   int add_nums_C99(int count, ...) { int result = 0; va_list args; va_start(args, count); // count can be omitted since C23   for (int i = 0; i < count; ++i) { result += va_arg(args, int); }   va_end(args); return result; }   #if __STDC_VERSION__ > 201710L // Same as above, valid since C23 int add_nums_C23(...) { int result = 0; va_list args; va_start(args);   int count = va_arg(args, int); for (int i = 0; i < count; ++i) { result += va_arg(args, int); }   va_end(args); return result; } #endif   int main(void) { printf("%d\n", add_nums_C99(4, 25, 25, 50, 50)); #if __STDC_VERSION__ > 201710L printf("%d\n", add_nums_C23(4, 25, 25, 50, 50)); #endif }

Possible output:

[edit] References

[edit] See also

| | accesses the next variadic function argument (function macro) [edit] | | ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | | | makes a copy of the variadic function arguments (function macro) [edit] | | | ends traversal of the variadic function arguments (function macro) [edit] | | | holds the information needed by va_start, va_arg, va_end, and va_copy (typedef) [edit] | | |