va_start
From cppreference.com
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.
Only the first argument passed to va_start is evaluated. Any additional arguments are neither expanded nor used in any way.
[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
Run this code
#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:
150 150
[edit] References
- C17 standard (ISO/IEC 9899:2018):
- 7.16.1.4 The va_start macro (p: 198-199)
- C11 standard (ISO/IEC 9899:2011):
- 7.16.1.4 The va_start macro (p: 271-272)
- C99 standard (ISO/IEC 9899:1999):
- 7.15.1.4 The va_start macro (p: 251-252)
- C89/C90 standard (ISO/IEC 9899:1990):
- 4.8.1.1 The va_start macro
[edit] See also
C++ documentation for va_start