Begins message processing. You must call this routine before
    calling any other message routines.

    Format

      MAIL$MESSAGE_BEGIN  context ,in_item_list ,out_item_list

1  –  Returns

    OpenVMS usage:cond_value
    type:         longword (unsigned)
    access:       write only
    mechanism:    by value

    Longword condition value. All utility routines return a condition
    value in R0. Condition values that can be returned by this
    routine are listed under Condition Values Returned.

2  –  Arguments

 context

    OpenVMS usage:context
    type:         longword (unsigned)
    access:       modify
    mechanism:    by reference
    Message context information to be passed to various message
    routines. The context argument is the address of a longword that
    contains message context information.

    You should specify the value of this argument as 0 in the first
    of a sequence of calls to message routines. In the following
    calls, you should specify the message context value returned by
    this routine.

 in_item_list

    OpenVMS usage:itmlst_3
    type:         longword (unsigned)
    access:       read only
    mechanism:    by reference
    Item list specifying options for the routine. The in_item_list
    argument is the address of a list of item descriptors, each of
    which specifies an option and provides the information needed to
    perform the operation.

    The item list is terminated by longword value of 0.

3  –  Input Item Codes

 MAIL$_MESSAGE_FILE_CTX

    MAIL$_MESSAGE_FILE_CTX specifies the mail file context received
    from MAIL$MAILFILE_BEGIN to be passed to the message routines.
    The buffer address field of the item descriptor points to a
    longword that contains mail file context information.

    The item code MAIL$_MESSAGE_FILE_CTX is required.

 out_item_list

    OpenVMS usage:itmlst_3
    type:         longword
    access:       write only
    mechanism:    by reference
    Item list specifying the information you want the routine to
    return. The out_item_list argument is the address of a list of
    item descriptors, each of which describes an item of information.
    The list of item descriptors is terminated by longword value of
    0.

4  –  Output Item Code

 MAIL$_MESSAGE_SELECTED

    When you specify MAIL$_MESSAGE_SELECTED, MAIL$MESSAGE_BEGIN
    returns the number of messages selected as a longword value.

5  –  Description

    MAIL$MESSAGE_BEGIN creates and initializes a message context for
    subsequent calls to message routines.

6  –  Condition Values Returned

    MAIL$_ILLCTXADR    The context block address is illegal.
    MAIL$_INVITMCOD    The specified item code is invalid.
    MAIL$_INVITMLEN    The specified item length is invalid.
    MAIL$_MISREQITEM   The required item is missing.
    MAIL$_NOFILEOPEN   The mail file is not open.
    MAIL$_WRONGCTX     The context block is incorrect.
    MAIL$_WRONGFILE    The specified file is incorrect in this
                       context.
    SS$_ACCVIO         Access violation.
    Any condition value returned by $GET and LIB$GET_VM.