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.TH CHMOD 2 2008-05-26 "Linux" "Linux Programmer's Manual"
.SH NAME
chmod, fchmod \- change permissions of a file
.SH SYNOPSIS
.B #include <sys/stat.h>
.sp
.BI "int chmod(const char *" path ", mode_t " mode );
.br
.BI "int fchmod(int " fd ", mode_t " mode );
.sp
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.sp
.BR fchmod ():
_BSD_SOURCE || _XOPEN_SOURCE\ >=\ 500
.SH DESCRIPTION
These system calls change the permissions of a file.
They differ only in how the file is specified:
.IP * 2
.BR chmod ()
changes the permissions of the file specified whose pathname is given in
.IR path ,
which is dereferenced if it is a symbolic link.
.IP *
.BR fchmod ()
changes the permissions of the file referred to by the open file descriptor
.IR fd .
.PP
The new file permissions are specified in
.IR mode ,
which is a bit mask created by ORing together zero or
more of the following:
.TP 18
.BR S_ISUID "  (04000)"
set-user-ID (set process effective user ID on
.BR execve (2))
.TP
.BR S_ISGID "  (02000)"
set-group-ID (set process effective group ID on
.BR execve (2);
mandatory locking, as described in
.BR fcntl (2);
take a new file's group from parent directory, as described in
.BR chown (2)
and
.BR mkdir (2))
.TP
.BR S_ISVTX "  (01000)"
sticky bit (restricted deletion flag, as described in
.BR unlink (2))
.TP
.BR S_IRUSR "  (00400)"
read by owner
.TP
.BR S_IWUSR "  (00200)"
write by owner
.TP
.BR S_IXUSR "  (00100)"
execute/search by owner ("search" applies for directories,
and means that entries within the directory can be accessed)
.TP
.BR S_IRGRP "  (00040)"
read by group
.TP
.BR S_IWGRP "  (00020)"
write by group
.TP
.BR S_IXGRP "  (00010)"
execute/search by group
.TP
.BR S_IROTH "  (00004)"
read by others
.TP
.BR S_IWOTH "  (00002)"
write by others
.TP
.BR S_IXOTH "  (00001)"
execute/search by others
.PP
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
.B CAP_FOWNER
capability).

If the calling process is not privileged (Linux: does not have the
.B 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
.B 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
.B 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
.BR 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.
.SH "RETURN VALUE"
On success, zero is returned.
On error, \-1 is returned, and
.I errno
is set appropriately.
.SH ERRORS
Depending on the file system, other errors can be returned.
The more general errors for
.BR chmod ()
are listed below:
.TP
.B EACCES
Search permission is denied on a component of the path prefix.
(See also
.BR path_resolution (7).)
.TP
.B EFAULT
.I path
points outside your accessible address space.
.TP
.B EIO
An I/O error occurred.
.TP
.B ELOOP
Too many symbolic links were encountered in resolving
.IR path .
.TP
.B ENAMETOOLONG
.I path
is too long.
.TP
.B ENOENT
The file does not exist.
.TP
.B ENOMEM
Insufficient kernel memory was available.
.TP
.B ENOTDIR
A component of the path prefix is not a directory.
.TP
.B EPERM
The effective UID does not match the owner of the file,
and the process is not privileged (Linux: it does not have the
.B CAP_FOWNER
capability).
.TP
.B EROFS
The named file resides on a read-only file system.
.PP
The general errors for
.BR fchmod ()
are listed below:
.TP
.B EBADF
The file descriptor
.I fd
is not valid.
.TP
.B EIO
See above.
.TP
.B EPERM
See above.
.TP
.B EROFS
See above.
.SH "CONFORMING TO"
4.4BSD, SVr4, POSIX.1-2001.
.SH "SEE ALSO"
.BR chown (2),
.BR execve (2),
.BR fchmodat (2),
.BR open (2),
.BR stat (2),
.BR path_resolution (7)
.SH COLOPHON
This page is part of release 3.22 of the Linux
.I man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
