Technotes

Don't Look at ioPosOffset

Technote FL 24 April 1988

Written by: Darin Adler April 1988

The Device Manager chapter of Inside Macintosh Volume II says that ioPosOffset is passed to and returned by Read and Write calls. It also says that "After the read [or write] is completed, the position is returned in ioPosOffset..." Actually, ioPosOffset is not changed by either call.

Also note that device drivers should only look at the dCtlPosition field of the DCE, and should not look directly at the ioPosOffset field of the parameter block. The Device Manager sets up dCtlPosition for the driver, taking into account both the ioPosMode and the ioPosOffset.

Further Reference:

* The Device Manager

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