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Swapping Bytes

Two primary byte-ordering methods (or endian formats) exist in the world of computing. An endian format specifies how to store the individual bytes of multibyte numerical data in memory. Big-endian byte ordering specifies to store multibyte data with its most significant byte first. Little-endian byte ordering specifies to store multibyte data with its least significant byte first. The PowerPC processor uses big-endian byte ordering. The x86 processor family uses little-endian byte ordering. By convention, multibyte data sent over the network uses big-endian byte ordering.

If your application assumes that data is in one endian format, but the data is actually in another, then it will interpret the data incorrectly. You will want to analyze your code for routines that read multibyte data (16 bits, 32 bits, or 64 bits) from, or write multibyte data to, disk or to the network, as these routines are sensitive to byte-ordering format. There are two general approaches for handling byte ordering differences: swap bytes when necessary or use XML or another byte-order-independent data format such as those offered by Core Foundation (CFPreferences, CFPropertyList, CFXMLParser).

Whether you should swap bytes or use a byte-order-independent data format depends on how you use the data in your application. If you have an existing file format to support, the binary-compatible solution is to accept the big-endian file format you have been using in your application, and write code that swaps bytes when the file is read or written on an Intel-based Macintosh. If you don’t have legacy files to support, you could consider redesigning your file format to use XML (extensible markup language), XDR (external data representation), or NSCoding (Objective C) to represent data.

This chapter describes why byte ordering matters, gives guidelines for swapping bytes, describes the byte-swapping APIs available in Mac OS X, and provides solutions for most of the situations where byte ordering matters.

Contents:

Why Byte Ordering Matters
Guidelines for Swapping Bytes
Byte-Swapping Routines
Byte-Swapping Strategies
Writing a Callback to Swap Data Bytes
See Also

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