An FSRef is an opaque data structure that refers to an existing file system object (i.e. file, directory, volume). It is part of the CoreServices File Manager (aka Carbon File Manager).

An FSRef can only refer to existing objects, and it is not a permanent reference. As such:

• don’t write it out flatly as part of a file
• don’t pass it to another process (it is meaningless in a different address space)

One should not assume an FSRef is still valid after:

• file moves/renames
• if the underlying FileSystem goes away (i.e. unmounting a removable disk, disconnecting a share)

To refer to a file you wish to create, use an FSRef to the parent folder and a unicode string for the new filename (see FSCreateFileUnicode()).

Since FSRef is opaque:

• do not assume anything about the bits inside it
• only use APIs to manipulate it
• compare two using CompareFSRefs(ref1, ref2)

Although opaque, it’s documented as 80 bytes in Files.h, and memcpy() can be used to copy one.

An Alias provides several features that FSRef does not:

• remains a valid file reference across file moves/renames (on the same volume)
• can be flattened and stored on disk or passed between processes

An alias is also opaque.

Interfacing with Cocoa:

• Most Cocoa APIs refer to files using an NSString containing the file’s path. This has the disadvantage of not being opaque. (Which can also be an advantage of course.)
• NSDocument in Mac OS X 10.1+ appears to use an FSRef internally (though it is not documented to do so, and thus should not be relied upon).
• NathanDay’s NDAlias provides a Cocoa way to interface with FSRef. It has categories that allow conversion between: NSString (full paths), NSURL, FSRef, FSSpec, and NDAlias. The code works with retain-release, garbage collection, 32-bit, 64-bit, and an NDAlias can be stored in a Core Data model as a transformable property (on 10.5). This provides a modern way for a Cocoa app to save file references more robustly than a path.

Other Notes:

• #import <CoreServices/CoreServices.h> for FSRef (specifically, it’s in the very well documented Files.h).
• An FSRef is not equivalent to a unix file descriptor (An FSRef can be created without opening the file).
• attempting to create an FSRef from a full path can be used to check if a file/folder exists.
• FSRef is not deprecated (on the contrary). It works in 32 and 64 bit apps.
• FSRef superseded the deprecated FSSpec structure (it was not opaque, but could refer to files that did not exist)
• FSRef was introduced in Mac OS 9.
• FSRef likely stands for File System Reference.
• The modern Carbon File Manager supports many high-level file system operations that Cocoa does not.
• See also: Alias, FSSpec, NSFileManager, NSWorkspace, NDAlias, BDAlias.

The following NSString category methods, from the NDAlias project, convert NSString (full path) <-> FSRef:

• (NSString )stringWithFSRef:(const FSRef *)aFSRef { CFURLRef theURL = CFURLCreateFromFSRef( kCFAllocatorDefault, aFSRef ); NSString thePath = [(NSURL *)theURL path]; CFRelease ( theURL ); return thePath; }

• (BOOL)getFSRef:(FSRef *)aFSRef { return FSPathMakeRef( (const UInt8 *)[self fileSystemRepresentation], aFSRef, NULL ) == noErr; }

• I took the good tidbits from the old page and made this a more encyclopedic article. Hope that’s ok. – smcbride 2008-01-27

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