# Apple Macintosh "icon libraries"

## Introduction

The equivalent of Windows icon libraries on Mac systems is probably the so-called resource fork which is a container format for resources such as icons and cursors.

- Resource forks are an integral part of the Mac file system (HFS). When extracted and ported to a Windows or Linux system, they usually have .RSRC extension.
  - On a Mac, each file can have a data fork and a resource fork, and possibly other kinds of "forks".
  - The data fork is the actual data of the file.
  - The resource fork is akin to a miniature file system, because it is a structured collection of icons, cursors and other resources pertaining to the file.
- When copied to Windows/Linux, Mac files are sometimes converted to a compound format called AppleSingle or AppleDouble. This file (or two files in case of AppleDouble) is a bundle of the data fork, the resource fork and all other forks that may exist in connection with the original Mac file. It may have any extension, for example, .AS or .ADF.
- The resource fork is a somewhat obsolete concept.
  - It is still used by the system, but no API for developers is provided on modern Macs any more.
- As a consequence, Greenfish Icon Editor Pro supports loading but not saving Mac resource forks and AppleSingle/AppleDouble files.
- When loaded into Greenfish Icon Editor Pro, Mac resource collections are displayed exactly like Windows icon libraries.
  - Technical detail: The following types of resources are extracted by GFIE Pro (each type is identified by a four-character code, and each resource has a numeric ID and, optionally, a human-readable name):
  - acur - 16x16-pixel animated cursor
  - cicn - Color icon. Can only have at most two pages, a color and a monochrome one.
  - crsr - 16x16-pixel color cursor. Similarly to color icons, color cursors may only have up to 2 pages.
  - curs - 16x16-pixel black and white cursor
  - icns - Apple Icon Image Format. Sometimes the individual icon pages are stored in separate resource chunks, GFIE combines these into a single ICNS resource.
- After modifying them, you cannot save them in their original format, but you can save a copy in a Windows icon library format such as .ICL or .RES. You can also extract the individual resources as .ICO or .CUR files.

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