TIP 129: New Format Codes for the [binary] Command

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by Aug 21.
Author:         Arjen Markus <arjen.markus@wldelft.nl>
Author:         Torsten Reincke <reincke@typoscriptics.de>
State:          Final
Type:           Project
Vote:           Done
Created:        14-Mar-2003
Post-History:   
Keywords:       IEEE,binary data,Tcl
Tcl-Version:    8.5

Abstract

This TIP proposes to add a set of new format codes to the binary command to enhance its ability to deal with especially non-native floating-point data. The assumption is that current limitations are due to the distinction between little-endian and big-endian storage of such data.

Introduction

The current binary command can manipulate little-endian and big-endian integer data, but only native floating-point data. This means that binary data from other computer systems that use a different representation of floating-point data can not be directly handled.

The lack of format codes to handle "native" integer data means that one has to distinguish the current platform's byte ordering to be platform-independent, whenever the binary command is used.

Most current computer systems use either little-endian or big-endian byte order and the so-called IEEE representation for the exponent and mantissa. So, the main variation to deal with is the endian-ness.

Some popular file formats, like ESRI's ArcView shape files, use both types of byte order. It is difficult (though not impossible) to handle these files with the current set of format codes.

It should be noted that there is more variety among floating-point representation than just the byte order. This TIP will not solve this more general problem.

Proposed Format Codes

Format codes should be available to catch the two main varieties of byte ordering. There should, both for reasons of symmetry and for practical purposes, also be a full set to deal with "native" data.

For integer types there are no codes to deal with native ordering. So:

The floating-point types will be handled via:

where the lower-case is associated with little-endian order and the upper-case with big-endian order.

Implementation Notes

The implementation for the integer types is simple:

The implementation for the floating-point types is somewhat more complicated, this involves adding byte swapping, if the ordering of the platform does not correspond to that of the format code.

Copyright

This document is placed in the public domain.

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