Questions tagged [xattr]

Extended attributes are descriptions of the file stored separately from the file data. They are read by special system calls. People often first encounter them as a result of seeing an @ after the permissions in the output from ls -l. (The related symbol, a +, indicates that there are ACLs — access control lists — associated with the file.)

On macOS (or Mac OS X), you find out about the extended attributes with the @ option to ls. For example, a file might have the extra attribute:

$ ls -l@d Cscope
drwxr-xr-x@ 3 someuser  staff  102 Mar  8 21:49 Cscope
    com.apple.quarantine     30 
$

indicating that the directory had been copied from the internet and had not yet been marked as clean/safe. The Time Machine software makes extensive use of extended attributes too.

For macOS, the system calls are listxattr(), flistxattr() getxattr(), fgetxattr(), removexattr(), setxattr(). Linux adds lgetxattr() to this set.