Section: Linux Programmer's Manual (2)
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chmod, fchmod, fchmodat - change permissions of a file
int chmod(const char *pathname, mode_t mode);
int fchmod(int fd, mode_t mode);
#include <fcntl.h> /* Definition of AT_* constants */
int fchmodat(int dirfd, const char *pathname, mode_t mode, int flags);
Feature Test Macro Requirements for glibc (see
_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
- Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
system calls change the permissions of a file.
They differ only in how the file is specified:
changes the permissions of the file specified whose pathname is given in
which is dereferenced if it is a symbolic link.
changes the permissions of the file referred to by the open file descriptor
The new file permissions are specified in
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
- S_ISGID (02000)
set-group-ID (set process effective group ID on
mandatory locking, as described in
take a new file's group from parent directory, as described in
- S_ISVTX (01000)
sticky bit (restricted deletion flag, as described in
- 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
If the calling process is not privileged (Linux: does not have the
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
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 execution bits
may be turned off if a file is written.
(On Linux this occurs if the writing process does not have the
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
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.
system call operates in exactly the same way as
except for the differences described here.
If the pathname given in
is relative, then it is interpreted relative to the directory
referred to by the file descriptor
(rather than relative to the current working directory of
the calling process, as is done by
for a relative pathname).
is relative and
is the special value
is interpreted relative to the current working
directory of the calling process (like
is absolute, then
can either be 0, or include the following flag:
is a symbolic link, do not dereference it:
instead operate on the link itself.
This flag is not currently implemented.
for an explanation of the need for
On success, zero is returned.
On error, -1 is returned, and
is set appropriately.
Depending on the filesystem,
errors other than those listed below can be returned.
The more general errors for
are listed below:
Search permission is denied on a component of the path prefix.
points outside your accessible address space.
An I/O error occurred.
Too many symbolic links were encountered in resolving
is too long.
The file does not exist.
Insufficient kernel memory was available.
A component of the path prefix is not a directory.
The effective UID does not match the owner of the file,
and the process is not privileged (Linux: it does not have the
The named file resides on a read-only filesystem.
The general errors for
are listed below:
The file descriptor
is not valid.
The same errors that occur for
can also occur for
The following additional errors can occur for
is not a valid file descriptor.
Invalid flag specified in
is relative and
is a file descriptor referring to a file other than a directory.
which is not supported.
was added to Linux in kernel 2.6.16;
library support was added to glibc in version 2.4.
4.4BSD, SVr4, POSIX.1-2001i, POSIX.1-2008.
C library/kernel ABI differences
The GNU C library
wrapper function implements the POSIX-specified
interface described in this page.
This interface differs from the underlying Linux system call, which does
On older kernels where
is unavailable, the glibc wrapper function falls back to the use of
is a relative pathname,
glibc constructs a pathname based on the symbolic link in
that corresponds to the
This page is part of release 3.74 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at
- RETURN VALUE
- CONFORMING TO
- C library/kernel ABI differences
- Glibc notes
- SEE ALSO