source-string

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    Source string within which STR$POSITION searches for the
    substring. The source-string argument is the address of a
    descriptor pointing to the source string.

 substring

    OpenVMS usage:char_string
    type:         character string
    access:       read only
    mechanism:    by descriptor

    Substring for which STR$POSITION searches. The substring argument
    is the address of a descriptor pointing to the substring.

 start-position

    OpenVMS usage:longword_signed
    type:         longword (signed)
    access:       read only
    mechanism:    by reference

    Relative position in the source string at which STR$POSITION
    begins the search. The start-position argument is the address of
    a signed longword containing the starting position. Although this
    is an optional argument, it is required if you are using the JSB
    entry point.

    If start-position is not supplied, STR$POSITION starts the search
    at the first character position of source-string.