The Return Relative Position of Substring routine searches for
    the first occurrence of a single substring within a source
    string. If STR$POSITION finds the substring, it returns the
    relative position of that substring. If the substring is not
    found, STR$POSITION returns a zero.

    Format

      STR$POSITION  source-string ,substring [,start-position]

1  –  Corresponding JSB Entry Point

      STR$POSITION_R6

2  –  Returns

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

    Relative position of the first character of the substring. Zero
    is the value returned if STR$POSITION did not find the substring.

    On Alpha systems, if the relative position of the substring can
    exceed 2**32-1, assign the return value to a quadword to ensure
    that you retrieve the correct relative position.

3  –  Arguments

 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.