va_start initializes a va_list so a variadic function can access arguments after its final named parameter. Pass the va_list object itself—not its address—and identify that last named parameter. section 16.5.2
Initializing a variable-argument list
double sumIt(size_t n, ...) { double ret = 0.0; va_list va; va_start(va, n); for (size_t i = 0; i < n; ++i) ret += va_arg(va, double); va_end(va); return ret; } Here, n is the last named parameter, so va_start(va, n) prepares va for va_arg. The function must track the number of values itself; va_start and va_arg provide no list length. section 16.5.2
Passing the initialized list onward
va_start can prepare arguments for an existing library function instead of manually processing them: va_list va; va_start(va, format); ret = vfprintf(iodebug, format, va); va_end(va); This pattern uses va_start and va_end to delimit the va_list's use. section 16.5.2
1 void va_start(va_list ap, parmN);
2 void va_end(va_list ap);
3 type va_arg(va_list ap, type);
4 void va_copy(va_list dest, va_list src);
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Compare the pseudo-interfaces for va_start, va_end, va_arg, and va_copy. section 16.5.2
