passing va_args to a custom format function - c

Question

I'd like to use a va_args list in a custom formatter function.
I first tried to get vsnprintf() to work to verifiy that my argument pointer list itself is passed correctly => this worked
But when i tried to pass my argument pointer list to another function the pointer does not point to the right element on the stack
#include <stdio.h>
#include <stdarg.h>
int wrapper_snprintf(char *s, size_t n, const char *format, ...)
{
va_list arg;
//Copy Formated string to buffer => adds a terminating null character to the string
va_start(arg, format);
int res = vsnprintf(s, n, format, arg);
va_end(arg);
return res;
}
void Va_Args_Test_C(char *fmt, ...)
{
char test[30] = { 0 };
char test2[30] = { 0 };
{
//vsnprintf
va_list arg;
va_start(arg, fmt);
vsnprintf(test2, 30, fmt, arg);
va_end(arg);
}
{
//custom format function
va_list arg;
va_start(arg, fmt);
wrapper_snprintf(test2, 30, fmt, arg);
va_end(arg);
}
}
the first run worked. The correct argument is copied to string test[].
the second run did not work. test2[] contains the original string + some random number (which looks like a crappy pointer to me). So could you help me? What am I doing wrong?

Answer 1

In the first case you call vsnprintf(test2, 30, fmt, arg);.
This function is defined as
int vsnprintf(char *str, size_t size, const char *format, va_list ap);
In the second case you call wrapper_snprintf(test2, 30, fmt, arg);, but your is defined differently as
int wrapper_snprintf(char *s, size_t n, const char *format, ...)
This is as if you would call snprintf instead of vsnprintf.
If you want a replacement for vsnprintf you have to define your function with an argument of type va_list instead of ...
int wrapper_snprintf(char *s, size_t n, const char *format, va_list ap)
{
//Copy Formated string to buffer => adds a terminating null character to the string
int res = vsnprintf(s, n, format, ap);
return res;
}