#include <stdarg.h>
#include <stdio.h>
int vsnprintf(char *__restrict__ s, size_t n,
const char *__restrict__ format, va_list arg);The vsnprintf() function is equivalent to snprintf(), except that instead of being called with a variable number of arguments, it is called with an argument list as defined by stdarg.h. For a specification of the format string, see sprintf() — Format and Write Data.
Initialize the argument list by using the va_start macro before each call. These functions do not invoke the va_end macro, but instead invoke the va_arg macro causing the value of arg after the return to be unspecified.
The vsnprintf() function returns the number of characters that would have been written had n been sufficiently large, not counting the terminating null character, or a negative value if an encoding error occurred. Thus, the null-terminated output has been completely written if and only if the returned value is nonnegative and less than n.