Prints formatted output based on an argument list.

    This function is the same as the sprintf function except that
    instead of being called with a variable number of arguments, it
    is called with an argument list that has been initialized by va_
    start (and possibly with subsequent va_arg calls).

    Format

      #include  <stdio.h>

      int vsprintf  (char *str, const char *format, va_list ap);

1 - Arguments

 str

    A pointer to a string that will receive the formatted output.
    This string is assumed to be large enough to hold the output.

 format

    A pointer to a character string that contains the format
    specification.

 ap

    A list of expressions whose resultant types correspond to the
    conversion specifications given in the format specifications.

2 - Return Value

    x                  The number of bytes written.
    Negative value     Indicates an output error occurred. The
                       function sets errno. For a list of possible
                       errno values set, see fprintf.