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.
(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:
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