Used for initializing a variable to the beginning of the argument
    list.

    Format

      #include  <varargs.h> (DEC C Extension)

      void va_start  (va_list ap);

      void va_start_1  (va_list ap, int offset);

1 - Arguments

 ap

    An object pointer. You must declare and use the argument ap as
    shown in the format section.

 offset

    The number of bytes by which ap is to be incremented so that
    it points to a subsequent argument within the list (that is,
    not to the start of the argument list). Using a nonzero offset
    can initialize ap to the address of the first of the optional
    arguments that follow a number of fixed arguments.

2 - Description

    The va_start macro initializes the variable ap to the beginning
    of the argument list.

    The va_start_1 macro initializes ap to the address of an argument
    that is preceded by a known number of defined arguments. The
    printf function is an example of a Compaq C RTL function
    that contains a variable-length argument list offset from the
    beginning of the entire argument list. The variable-length
    argument list is offset by the address of the formatting string.

    When determining the value of the offset argument used in va_
    start_1, the implications of the OpenVMS calling standard must be
    considered.

    On OpenVMS Alpha and Integrity server systems, each argument item
    is a quadword.

                                   NOTE

       When accessing argument lists, especially those passed to a
       subroutine (written in C) by a program written in another
       programming language, consider the implications of the
       OpenVMS calling standard. For more information about the
       OpenVMS calling standard, see the Compaq C User's Guide for
       OpenVMS Systems or the HP OpenVMS Calling Standard.

    The preceding version of va_start and va_start_1 is specific to
    the Compaq C RTL, and is not portable.

    The following syntax describes the va_start macro in the
    <stdarg.h> header file, as defined in the ANSI C standard:

    Format

      #include  <stdarg.h> (ANSI C)

      void va_start  (va_list ap, parmN);

3 - Arguments

 ap

    An object pointer. You must declare and use the argument ap as
    shown in the format section.

 parmN

    The name of the last of the known fixed arguments.

4 - Description

    The pointer ap is initialized to point to the first of the
    optional arguments that follow parmN in the argument list.

    Always use this version of va_start in conjunction with functions
    that are declared and defined with function prototypes. Also use
    this version of va_start to write portable programs.

    For an example of argument-list processing using the <stdarg.h>
    functions and definitions, see the "Character, String, and
    Argument-List Functions" chapter of the HP C RTL Reference
    Manual.