chmod(2) — Linux manual pages

Linux manual pages	Section 2

Name

chmod, fchmod — change permissions of a file

Synopsis

#include <sys/stat.h>

`int chmod(`	const char *`path`,
	mode_t `mode)`;

`int fchmod(`	int `fd`,
	mode_t `mode)`;

Note

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
`fchmod`():
`_BSD_SOURCE` \|\| `_XOPEN_SOURCE` >= 500

DESCRIPTION

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

chmod() changes the permissions of the file specified whose pathname is given in path, which is dereferenced if it is a symbolic link.
fchmod() changes the permissions of the file referred to by the open file descriptor fd.

The new file permissions are specified in mode, which is a bit mask created by ORing together zero or more of the following:

S_ISUID (04000): set-user-ID (set process effective user ID on execve(2))
S_ISGID (02000): set-group-ID (set process effective group ID on execve(2); mandatory locking, as described in fcntl(2); take a new file's group from parent directory, as described in chown(2) and mkdir(2))
S_ISVTX (01000): sticky bit (restricted deletion flag, as described in unlink(2))
S_IRUSR (00400): read by owner
S_IWUSR (00200): write by owner
S_IXUSR (00100): execute/search by owner ("search" applies for directories, and means that entries within the directory can be accessed)
S_IRGRP (00040): read by group
S_IWGRP (00020): write by group
S_IXGRP (00010): execute/search by group
S_IROTH (00004): read by others
S_IWOTH (00002): write by others
S_IXOTH (00001): execute/search by others

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability).

If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned.

As a security measure, depending on the file system, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux this occurs if the writing process does not have the CAP_FSETID capability.) On some file systems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2).

On NFS file systems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.

RETURN VALUE

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

ERRORS

Depending on the file system, other errors can be returned. The more general errors for chmod() are listed below:

EACCES: Search permission is denied on a component of the path prefix. (See also path_resolution(7).)
EFAULT: path points outside your accessible address space.
EIO: An I/O error occurred.
ELOOP: Too many symbolic links were encountered in resolving path.
ENAMETOOLONG: path 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 effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability).
EROFS: The named file resides on a read-only file system.

The general errors for fchmod() are listed below:

EBADF: The file descriptor fd is not valid.
EIO: See above.
EPERM: See above.
EROFS: See above.

CONFORMING TO

4.4BSD, SVr4, POSIX.1-2001.

COLOPHON

This page is part of release 3.24 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.

Copyright (c) 1992 Drew Eckhardt (drewcs.colorado.edu), March 28, 1992

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

Permission is granted to copy and distribute modified versions of this
manual under the conditions for verbatim copying, provided that the
entire resulting derived work is distributed under the terms of a
permission notice identical to this one.

Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
have taken the same level of care in the production of this manual,
which is licensed free of charge, as they might when working
professionally.

Formatted or processed versions of this manual, if unaccompanied by
the source, must acknowledge the copyright and authors of this work.

Modified by Michael Haardt <michaelmoria.de>
Modified 1993-07-21 by Rik Faith <faithcs.unc.edu>
Modified 1997-01-12 by Michael Haardt
  <michaelcantor.informatik.rwth-aachen.de>: NFS details
Modified 2004-06-23 by Michael Kerrisk <mtk.manpagesgmail.com>