System-Dependent Address In A File Stream
     QP_stream:      union QP_cookie magic;

The system-dependent address of the current position in a stream is stored in the magic field of the stream structure. It is only used when there is any kind of seek to be performed on the stream. This field is a C type union cookie, which is defined in <quintus/quintus.h> as follows:

                     union QP_cookie {
                           struct RFA {
                                  int  BlockNumber;
                                  short Offset;
                           } vms_rfa;
                           int mvs_rrn;
                           int  cms_recno;
                           off_t byteno;
                           int user_defined[2];
                     } magic;

Depending on the host operating system, different field names of union cookie are used to store the position address of the stream depending on the host operating system. Under UNIX, magic.byteno is used to record the current location of the file pointer as an absolute byte offset from the beginning of the stream; magic.vms_rfa is used on VMS; magic.mvs_rrn is used on MVS; magic.cms_recno is used on CMS. magic.user_defined is used for a user's specific method of recording the current location of a stream. These values must be maintained in the bottom layer functions of a stream with seek permission.