**`vprintf()`** is a built-in function defined in the `<stdio.h>` header.

The following is the function prototype:
```C
int vprintf (const char * format, va_list arg);
```

# Functionality
`vprintf()` writes the input string to `stdout`.

The function works in a similar way to `printf()`. However, `vprintf()` uses elements in the `variable argument list` to replace format specifiers rather than using additional arguments.

# Parameters and return
`vprintf()` takes *two* input arguments.

- `format`: the string to be written to `stdout`. Contains format specifiers as in `printf()`.

- `arg`: a variable argument list that contains the placeholders for the format specifiers.

The `vprintf()` function returns the total number of characters written.

In case of an error, the function returns a negative number.

# Code

#include <stdio.h>
#include <stdarg.h>

void VariadicFunction ( const char * format, ... )
{
  va_list args;
  va_start (args, format);
  vprintf (format, args);
  va_end (args);
}

int main ()
{
   VariadicFunction("My format contains a %s\n", "string");
   VariadicFunction("My format contains %d and a %s\n", 7, "string");
   
   return 0;
}

In the above example, we begin by declaring a `variadic` function. 

This function takes the format string and variable arguments as input parameters.

When the format string and variable arguments pass to the function, `va_list` is created to store the variable arguments.

>`va_list` is defined in `<stdarg.h>`.

`vprintf()` can write the string onto `stdout`, replacing the format specifiers with the relevant argument from `va_list`. 

What is vprintf() in C?

vprintf() in C, from <stdio.h>, writes a formatted string to stdout using a variable argument list and returns the character count written or a negative error value.