chown, fchown, lchown, fchownat - change ownership of a file
#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)):
These system calls change the owner and group of a file. The chown(), fchown(), and lchown() system calls differ only in how the file is specified:
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 is 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, and since Linux 2.2.13, root is treated like other users. 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().
When the owner or group of an executable file is changed (by any user), all capability sets for the file are cleared.
The fchownat() system call operates in exactly the same way as chown(), except for the differences described here.
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;
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.
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().
On success, zero is returned. On error, -1 is returned, and
errno is set to indicate the error.
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[])
{
char *endptr;
uid_t uid;
struct passwd *pwd;
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);
}