CHOWN

NAME

SYNOPSIS

#include <unistd.h>

int chown(const char *pathname, uid_t owner, gid_t group);


int fchown(int fd, uid_t owner, gid_t group);


int lchown(const char *pathname, uid_t owner, gid_t group);

#include <fcntl.h>           /* Definition of AT_* constants */
#include <unistd.h>

int fchownat(int dirfd, const char *pathname,
             uid_t owner, gid_t group, int flags);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

fchown(), lchown():

_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L

fchownat():

Since glibc 2.10:

_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L

Before glibc 2.10:

_ATFILE_SOURCE

DESCRIPTION

*

chown() changes the ownership of the file specified by pathname, which is dereferenced if it is a symbolic link.

*

fchown() changes the ownership of the file referred to by the open file descriptor fd.

*

lchown() is like chown(), but does not dereference symbolic links.

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().  

fchownat()

If the pathname given in pathname is relative, then it is interpreted relative to the directory referred to by the file descriptor dirfd (rather than relative to the current working directory of the calling process, as is done by chown() for a relative pathname).

If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like chown()).

If pathname is absolute, then dirfd is ignored.

The flags argument is a bit mask created by ORing together 0 or more of the following values;

AT_EMPTY_PATH (since Linux 2.6.39)

If pathname is an empty string, operate on the file referred to by dirfd (which may have been obtained using the open(2) O_PATH flag). In this case, dirfd can refer to any type of file, not just a directory. If dirfd is AT_FDCWD, the call operates on the current working directory. This flag is Linux-specific; define _GNU_SOURCE to obtain its definition.

AT_SYMLINK_NOFOLLOW

If pathname is a symbolic link, do not dereference it: instead operate on the link itself, like lchown(). (By default, fchownat() dereferences symbolic links, like chown().)

See openat(2) for an explanation of the need for fchownat().  

RETURN VALUE

ERRORS

The more general errors for chown() are listed below.

EACCES

Search permission is denied on a component of the path prefix. (See also path_resolution(7).)

EFAULT

pathname points outside your accessible address space.

ELOOP

Too many symbolic links were encountered in resolving pathname.

ENAMETOOLONG

pathname is too long.

ENOENT

The file does not exist.

ENOMEM

Insufficient kernel memory was available.

ENOTDIR

A component of the path prefix is not a directory.

EPERM

The calling process did not have the required permissions (see above) to change owner and/or group.

EROFS

The named file resides on a read-only filesystem.

The general errors for fchown() are listed below:

EBADF

The descriptor is not valid.

EIO

A low-level I/O error occurred while modifying the inode.

ENOENT

See above.

EPERM

See above.

EROFS

See above.

The same errors that occur for chown() can also occur for fchownat(). The following additional errors can occur for fchownat():

EBADF

dirfd is not a valid file descriptor.

EINVAL

Invalid flag specified in flags.

ENOTDIR

pathname is relative and dirfd is a file descriptor referring to a file other than a directory.

VERSIONS

CONFORMING TO

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

fchownat(): POSIX.1-2008.  

NOTES

Ownership of new files

*

If the filesystem is mounted with -o grpid, then the group of a new file is made the same as that of the parent directory.

*

If the filesystem is mounted with -o nogrpid and the set-group-ID bit is disabled on the parent directory, then the group of a new file is made the same as the process's filesystem GID.

*

If the filesystem is mounted with -o nogrpid and the set-group-ID bit is enabled on the parent directory, then the group of a new file is made the same as that of the parent directory.

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.  

Glibc notes

NFS

Historical details

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).  

Program source

#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

Index

NAME

SYNOPSIS

DESCRIPTION

fchownat()

RETURN VALUE

ERRORS

VERSIONS

CONFORMING TO

NOTES

Ownership of new files

Glibc notes

NFS

Historical details

EXAMPLE

Program source

SEE ALSO