4.11. chown, fchown, and lchown Functions

The chown functions allow us to change the user ID of a file and the group ID of a file.

[View full width] #include <unistd.h> int chown(const char *pathname, uid_t owner, gid_t group); int fchown(int filedes, uid_t owner, gid_t group); int lchown(const char *pathname, uid_t owner, gid_t group);
All three return: 0 if OK, 1 on error

These three functions operate similarly unless the referenced file is a symbolic link. In that case, lchown changes the owners of the symbolic link itself, not the file pointed to by the symbolic link.

The lchown function is an XSI extension to the POSIX.1 functionality defined in the Single UNIX Specification. As such, all UNIX System implementations are expected to provide it.

If either of the arguments owner or group is -1, the corresponding ID is left unchanged.

Historically, BSD-based systems have enforced the restriction that only the superuser can change the ownership of a file. This is to prevent users from giving away their files to others, thereby defeating any disk space quota restrictions. System V, however, has allowed any user to change the ownership of any files they own.

POSIX.1 allows either form of operation, depending on the value of _POSIX_CHOWN_RESTRICTED.

With Solaris 9, this functionality is a configuration option, whose default value is to enforce the restriction. FreeBSD 5.2.1, Linux 2.4.22, and Mac OS X 10.3 always enforce the chown restriction.

Recall from Section 2.6 that the _POSIX_CHOWN_RESTRICTED constant can optionally be defined in the header <unistd.h>, and can always be queried using either the pathconf function or the fpathconf function. Also recall that this option can depend on the referenced file; it can be enabled or disabled on a per file system basis. We'll use the phrase, if _POSIX_CHOWN_RESTRICTED is in effect, to mean if it applies to the particular file that we're talking about, regardless of whether this actual constant is defined in the header.

If _POSIX_CHOWN_RESTRICTED is in effect for the specified file, then

This means that when _POSIX_CHOWN_RESTRICTED is in effect, you can't change the user ID of other users' files. You can change the group ID of files that you own, but only to groups that you belong to.

If these functions are called by a process other than a superuser process, on successful return, both the set-user-ID and the set-group-ID bits are cleared.