Prints formatted output based on an argument list.

    This function is the same as the printf 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 the
    va_start macro (and possibly with subsequent va_arg calls) from
    <stdarg.h>.

    Format

      #include  <stdio.h>

      int vprintf  (const char *format, va_list ap);

1 - Arguments

 format

    A pointer to the string containing the format specification.

 ap

    A variable list of the items needed for output.

2 - Description

    See the vfprintf and vsprintf functions.

3 - Return Values

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