chmod, fchmod — change permissions of a file


#include <sys/stat.h>
int chmod( const char *path,
  mode_t mode);
int fchmod( int fd,
  mode_t mode);
[Note] Note
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
|| /* Since glibc 2.12: */
_POSIX_C_SOURCE >= 200809L


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 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 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.


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


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


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


path points outside your accessible address space.


An I/O error occurred.


Too many symbolic links were encountered in resolving path.


path 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 CAP_FOWNER capability).


The named file resides on a read-only filesystem.

The general errors for fchmod() are listed below:


The file descriptor fd is not valid.


See above.


See above.


See above.


4.4BSD, SVr4, POSIX.1-2001.


chown(2), execve(2), fchmodat(2), open(2), stat(2), path_resolution(7)


This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at−pages/.

  Copyright (c) 1992 Drew Eckhardt (, March 28, 1992

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Modified by Michael Haardt <>
Modified 1993-07-21 by Rik Faith <>
Modified 1997-01-12 by Michael Haardt
  <>: NFS details
Modified 2004-06-23 by Michael Kerrisk <>