Inside Macintosh: Files /

Chapter 2 - File Manager / File Manager Reference

High-Level File Access Routines

Allocating File Blocks

The File Manager provides two functions, Allocate and AllocContig, that allow you to allocate additional blocks to a file. The File Manager automatically allocates file blocks if you move the logical end-of-file past the physical end-of-file, and it automatically deallocates unneeded blocks from a file if you move the logical end-of-file to a position more than one allocation block before the current physical end-of-file. Consequently,
you do not in general need to be concerned with allocating or deallocating file
blocks. However, you can improve file block contiguity if you use the Allocate
or AllocContig function to preallocate file blocks. This is most useful if you know
in advance how big a file is likely to become.

Note

When the File Manager allocates (or deallocates) file blocks automatically, it always adds (or removes) blocks in clumps. The Allocate and AllocContig functions allow you to add blocks
in allocation blocks, which may be smaller than clumps.

The Allocate and AllocContig functions are not supported by AppleShare volumes. Instead, use SetEOF or PBSetEOF to extend a file by setting the end-of-file.

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Subtopics

Allocate

AllocContig