Editing Alias (Mac OS) (section)

== File structure ==

The alias files in macOS start by the [[magic number (programming)|magic number]] <code>62 6F 6F 6B 00 00 00 00 6D 61 72 6B 00 00 00 00</code> which is in [[ASCII]] <big><code>book␀␀␀␀mark␀␀␀␀</code></big> (<big>␀</big> representing the [[Null character]]).

Following the magic number, it has been reported that an alias has a set of records inside it, each record is 150 bytes long and consists of the fields shown below (all integers are big endian).<ref>{{Cite web |title=Some information about MacOS aliases collected from the web. |url=http://sebastien.kirche.free.fr/python_stuff/MacOS-aliases.txt |url-status=live |archive-url=https://web.archive.org/web/20100120201512/http://sebastien.kirche.free.fr/python_stuff/MacOS-aliases.txt |archive-date=2010-01-20}}</ref> However, alias files are far larger than this would explain, and include other information at least including icons.<ref>{{cite web |title=Why are Finder alias files so huge lately? |url=https://discussions.apple.com/thread/3988292?start=0&tstart=0 |url-status=live |archive-url=https://web.archive.org/web/20210507014134/https://discussions.apple.com/thread/3988292 |archive-date=2021-05-07 |website=Apple Support Communities}}</ref><ref>{{Cite web |url=http://forums.macworld.com/index.php?%2Ftopic%2F142842-aliases-vs-symbolic-links%2F |title=Further details, including changes with various Mac OS versions. |access-date=2018-10-11 |archive-url=https://web.archive.org/web/20130430153741/http://forums.macworld.com/index.php?%2Ftopic%2F142842-aliases-vs-symbolic-links%2F |archive-date=2013-04-30 |url-status=dead }}</ref>

# 4 bytes user type name/app creator code = long ASCII text string (none = 0)
# 2 bytes record size = short unsigned total length
# 2 bytes record version = short integer version (current version = 2)
# 2 bytes alias kind = short integer value (file = 0; directory = 1)
# 1 byte volume name string length = byte unsigned length
# 27 bytes volume name string (if volume name string < 27 chars then pad with zeros)
# 4 bytes volume created mac date = long unsigned value in seconds since beginning 1904 to 2040
# 2 bytes volume signature = short unsigned HFS value
# 2 bytes volume type = short integer mac os value (types are Fixed HD = 0; Network Disk = 1; 400kB FD = 2;800kB FD = 3; 1.4MB FD = 4; Other Ejectable Media = 5 )
# 4 bytes parent directory id = long unsigned HFS value
# 1 bytes file name string length = byte unsigned length
# 63 bytes file name string (if file name string < 63 chars then pad with zeros)
# 4 bytes file number = long unsigned HFS value
# 4 bytes file created mac date = long unsigned value in seconds since beginning 1904 to 2040
# 4 bytes file type name = long ASCII text string
# 4 bytes file creator name = long ASCII text string
# 2 bytes nlvl From (directories from alias thru to root) = short integer range
# 2 bytes nlvl To (directories from root thru to source) = short integer range (if alias on different volume then set above to -1)
# 4 bytes volume attributes = long hex flags
# 2 bytes volume file system id = short integer HFS value
# 10 bytes reserved = 80-bit value set to zero
# 4+ bytes optional extra data strings = short integer type + short unsigned string length  (types are Extended Info End = -1; Directory Name = 0; Directory IDs = 1; Absolute Path = 2; AppleShare Zone Name = 3; AppleShare Server Name = 4; AppleShare User Name = 5; Driver Name = 6; Revised AppleShare info = 9; AppleRemoteAccess dialup info = 10)
# string data = hex dump
# odd lengths have a 1 byte odd string length pad = byte value set to zero

=== Alias record structure outside of size length ===
The following is for use with the Apple's Alias Resource Manager.
# 4 bytes resource type name = long ASCII text string
# 2 bytes resource ID = short integer value
# 2 bytes resource end pad = short value set to zero

'''Java code to flag an alias file'''
<syntaxhighlight lang="java">
// This function checks whether a file matches the alias magic number.
public static boolean checkForMacAliasFile(File inputFile) throws FileNotFoundException, IOException {

    // Only files can be aliases.
    // Do not test directories; they will be false.
    if (inputFile.isFile()) {
        byte[] bookmark = new byte[] {
            0x62, 0x6F, 0x6F, 0x6B, 0x00, 0x00, 0x00, 0x00, 0x6D, 0x61, 0x72, 0x6B, 0x00, 0x00, 0x00, 0x00
        };
        long length = inputFile.length();
        if (length > 16) {
            byte[] result = new byte[16];
            FileInputStream fis = new FileInputStream(inputFile);
            fis.read(result);
            fis.close();
            return Arrays.equals(result, bookmark);
        }
    }

    return false;
}
</syntaxhighlight>

There is a [https://github.com/davecotter/cftest/blob/main/source/shared/MacAlias.cpp github repo with working C++ code here].