DESCRIPTION

These system calls change the owner and group of a file. They differ only in how the file is specified:

Only a privileged process (Linux: one with the CAP_CHOWN capability) may change the owner of a file. The owner of a file may change the group of the file to any group of which that owner is a member. A privileged process (Linux: with CAP_CHOWN) may change the group arbitrarily.

If the owner or group is specified as −1, then that ID is not changed.

When the owner or group of an executable file are changed by an unprivileged user the S_ISUID and S_ISGID mode bits are cleared. POSIX does not specify whether this also should happen when root does the chown(); the Linux behavior depends on the kernel version. In case of a non-group-executable file (i.e., one for which the S_IXGRP bit is not set) the S_ISGID bit indicates mandatory locking, and is not cleared by a chown().

RETURN VALUE

On success, zero is returned. On error, −1 is returned, and errno is set appropriately.

ERRORS

Depending on the filesystem, other errors can be returned. The more general errors for chown() are listed below.

The general errors for fchown() are listed below:

CONFORMING TO

4.4BSD, SVr4, POSIX.1-2001.

The 4.4BSD version can be used only by the superuser (that is, ordinary users cannot give away files).

NOTES

The original Linux chown(), fchown(), and lchown() system calls supported only 16-bit user and group IDs. Subsequently, Linux 2.4 added chown32(), fchown32(), and lchown32(), supporting 32-bit IDs. The glibc chown(), fchown(), and lchown() wrapper functions transparently deal with the variations across kernel versions.

When a new file is created (by, for example, open(2) or mkdir(2)), its owner is made the same as the filesystem user ID of the creating process. The group of the file depends on a range of factors, including the type of filesystem, the options used to mount the filesystem, and whether or not the set-group-ID permission bit is enabled on the parent directory. If the filesystem supports the −o grpid (or, synonymously −o bsdgroups) and −o nogrpid (or, synonymously −o sysvgroups) mount(8) options, then the rules are as follows:

As at Linux 2.6.25, the −o grpid and −o nogrpid mount options are supported by ext2, ext3, ext4, and XFS. Filesystems that don't support these mount options follow the −o nogrpid rules.

The chown() semantics are deliberately violated on NFS filesystems which have UID mapping enabled. Additionally, the semantics of all system calls which access the file contents are violated, because chown() may cause immediate access revocation on already open files. Client side caching may lead to a delay between the time where ownership have been changed to allow access for a user and the time where the file can actually be accessed by the user on other clients.

In versions of Linux prior to 2.1.81 (and distinct from 2.1.46), chown() did not follow symbolic links. Since Linux 2.1.81, chown() does follow symbolic links, and there is a new system call lchown() that does not follow symbolic links. Since Linux 2.1.86, this new call (that has the same semantics as the old chown()) has got the same syscall number, and chown() got the newly introduced number.

EXAMPLE

The following program changes the ownership of the file named in its second command-line argument to the value specified in its first command-line argument. The new owner can be specified either as a numeric user ID, or as a username (which is converted to a user ID by using getpwnam(3) to perform a lookup in the system password file).

#include <pwd.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int
main(int argc, char *argv[])
{
    uid_t uid;
    struct passwd *pwd;
    char *endptr;

    if (argc != 3 || argv[1][0] == '\0') {
        fprintf(stderr, "%s <owner> <file>\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    uid = strtol(argv[1], &endptr, 10);  /* Allow a numeric string */

    if (*endptr != '\0') {         /* Was not pure numeric string */
        pwd = getpwnam(argv[1]);   /* Try getting UID for username */
        if (pwd == NULL) {
            perror("getpwnam");
            exit(EXIT_FAILURE);
        }

        uid = pwd−>pw_uid;
    }

    if (chown(argv[2], uid, −1) == −1) {
        perror("chown");
        exit(EXIT_FAILURE);
    }

    exit(EXIT_SUCCESS);
}

SEE ALSO

chmod(2), fchownat(2), flock(2), path_resolution(7), symlink(7)