The Extract a Substring of a String routine copies a substring of
    a source string into a destination string.

    Format

      STR$LEN_EXTR  destination-string ,source-string ,start-position

                    ,longword-integer-length

1  –  Corresponding JSB Entry Point

      STR$LEN_EXTR_R8

2  –  Returns

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

3  –  Arguments

 destination-string

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

    Destination string into which STR$LEN_EXTR copies the substring.
    The destination-string argument is the address of a descriptor
    pointing to the destination string.

 source-string

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

    Source string from which STR$LEN_EXTR extracts the substring that
    it copies into the destination string. The source-string argument
    is the address of a descriptor pointing to the source string.

 start-position

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

    Relative position in the source string at which STR$LEN_EXTR
    begins copying the substring. The start-position argument is the
    address of a signed longword containing the starting position.

 longword-integer-length

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

    Number of characters in the substring that STR$LEN_EXTR copies
    to the destination string. The longword-integer-length argument
    is the address of a signed longword containing the length of the
    substring.