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CHMOD(2) Linux Programmer's Manual CHMOD(2)
NAME
chmod, fchmod, fchmodat - change permissions of a file
SYNOPSIS
#include <sys/stat.h>
int chmod(const char *pathname, mode_t mode);
int fchmod(int fd, mode_t mode);
#include <fcntl.h> /* Definition of AT_* constants */
#include <sys/stat.h>
int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchmod():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 || _XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
fchmodat():
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
DESCRIPTION
The chmod() and fchmod() 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‐
name, 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 filesystem, the set-user-ID and set-group-ID exe‐
cution 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 filesystems, 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 filesystems, 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.
fchmodat()
The fchmodat() system call operates in exactly the same way as chmod(), 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 chmod() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then pathname is inter‐
preted relative to the current working directory of the calling process (like chmod()).
If pathname is absolute, then dirfd is ignored.
flags can either be 0, or include the following flag:
AT_SYMLINK_NOFOLLOW
If pathname is a symbolic link, do not dereference it: instead operate on the link
itself. This flag is not currently implemented.
See openat(2) for an explanation of the need for fchmodat().
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
ERRORS
Depending on the filesystem, errors other than those listed below 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_res‐
olution(7).)
EFAULT pathname points outside your accessible address space.
EIO An I/O error occurred.
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 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 filesystem.
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.
The same errors that occur for chmod() can also occur for fchmodat(). The following addi‐
tional errors can occur for fchmodat():
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.
ENOTSUP
flags specified AT_SYMLINK_NOFOLLOW, which is not supported.
VERSIONS
fchmodat() was added to Linux in kernel 2.6.16; library support was added to glibc in ver‐
sion 2.4.
CONFORMING TO
chmod(), fchmod(): 4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
fchmodat(): POSIX.1-2008.
NOTES
C library/kernel ABI differences
The GNU C library fchmodat() wrapper function implements the POSIX-specified interface
described in this page. This interface differs from the underlying Linux system call,
which does not have a flags argument.
Glibc notes
On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to
the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname
based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
SEE ALSO
chown(2), execve(2), open(2), stat(2), path_resolution(7), symlink(7)
COLOPHON
This page is part of release 3.74 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2014-08-19 CHMOD(2)
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