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.Dd October 31, 2024
.Dt CHMOD 2
.Os
.Sh NAME
.Nm chmod ,
.Nm fchmod ,
.Nm lchmod ,
.Nm fchmodat
.Nd change mode of file
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/stat.h
.Ft int
.Fn chmod "const char *path" "mode_t mode"
.Ft int
.Fn fchmod "int fd" "mode_t mode"
.Ft int
.Fn lchmod "const char *path" "mode_t mode"
.Ft int
.Fn fchmodat "int fd" "const char *path" "mode_t mode" "int flag"
.Sh DESCRIPTION
The file permission bits of the file named specified by
.Fa path
or referenced by the file descriptor
.Fa fd
are changed to
.Fa mode .
The
.Fn chmod
system call verifies that the process owner (user) either owns
the file specified by
.Fa path
(or
.Fa fd ) ,
or
is the super-user.
The
.Fn chmod
system call follows symbolic links to operate on the target of the link
rather than the link itself.
.Pp
The
.Fn lchmod
system call is similar to
.Fn chmod
but does not follow symbolic links.
.Pp
The
.Fn fchmodat
is equivalent to either
.Fn chmod
or
.Fn lchmod
depending on the
.Fa flag
except in the case where
.Fa path
specifies a relative path.
In this case the file to be changed is determined relative to the directory
associated with the file descriptor
.Fa fd
instead of the current working directory.
The values for the
.Fa flag
are constructed by a bitwise-inclusive OR of flags from the following list, defined
in
.In fcntl.h :
.Bl -tag -width indent
.It Dv AT_SYMLINK_NOFOLLOW
If
.Fa path
names a symbolic link, then the mode of the symbolic link is changed.
.It Dv AT_RESOLVE_BENEATH
Only walk paths below the directory specified by the
.Ar fd
descriptor.
See the description of the
.Dv O_RESOLVE_BENEATH
flag in the
.Xr open 2
manual page.
.It Dv AT_EMPTY_PATH
If the
.Fa path
argument is an empty string, operate on the file or directory
referenced by the descriptor
.Fa fd .
If
.Fa fd
is equal to
.Dv AT_FDCWD ,
operate on the current working directory.
.El
.Pp
If
.Fn fchmodat
is passed the special value
.Dv AT_FDCWD
in the
.Fa fd
parameter, the current working directory is used.
If also
.Fa flag
is zero, the behavior is identical to a call to
.Fn chmod .
.Pp
A mode is created from
.Em or'd
permission bit masks
defined in
.In sys/stat.h :
.Pp
.Bd -literal -offset indent -compact
#define S_IRWXU 0000700    /* RWX mask for owner */
#define S_IRUSR 0000400    /* R for owner */
#define S_IWUSR 0000200    /* W for owner */
#define S_IXUSR 0000100    /* X for owner */

#define S_IRWXG 0000070    /* RWX mask for group */
#define S_IRGRP 0000040    /* R for group */
#define S_IWGRP 0000020    /* W for group */
#define S_IXGRP 0000010    /* X for group */

#define S_IRWXO 0000007    /* RWX mask for other */
#define S_IROTH 0000004    /* R for other */
#define S_IWOTH 0000002    /* W for other */
#define S_IXOTH 0000001    /* X for other */

#define S_ISUID 0004000    /* set user id on execution */
#define S_ISGID 0002000    /* set group id on execution */
#define S_ISVTX 0001000    /* sticky bit */
.Ed
.Pp
The non-standard
.Dv S_ISTXT
is a synonym for
.Dv S_ISVTX .
.Pp
The
.Fx
VM system totally ignores the sticky bit
.Pq Dv S_ISVTX
for executables.
On UFS-based file systems (FFS, LFS) the sticky
bit may only be set upon directories.
.Pp
If mode
.Dv S_ISVTX
(the `sticky bit') is set on a directory,
an unprivileged user may not delete or rename
files of other users in that directory.
The sticky bit may be
set by any user on a directory which the user owns or has appropriate
permissions.
For more details of the properties of the sticky bit, see
.Xr sticky 7 .
.Pp
If mode ISUID (set UID) is set on a directory,
and the MNT_SUIDDIR option was used in the mount of the file system,
then the owner of any new files and sub-directories
created within this directory are set
to be the same as the owner of that directory.
If this function is enabled, new directories will inherit
the bit from their parents.
Execute bits are removed from
the file, and it will not be given to root.
This behavior does not change the
requirements for the user to be allowed to write the file, but only the eventual
owner after it has been created.
Group inheritance is not affected.
.Pp
This feature is designed for use on fileservers serving PC users via
ftp, SAMBA, or netatalk.
It provides security holes for shell users and as
such should not be used on shell machines, especially on home directories.
This option requires the SUIDDIR
option in the kernel to work.
Only UFS file systems support this option.
For more details of the suiddir mount option, see
.Xr mount 8 .
.Pp
Writing or changing the owner of a file
turns off the set-user-id and set-group-id bits
unless the user is the super-user.
This makes the system somewhat more secure
by protecting set-user-id (set-group-id) files
from remaining set-user-id (set-group-id) if they are modified,
at the expense of a degree of compatibility.
.Pp
While it is normally an error to invoke
.Fn fchmod
on a socket, it is possible to do so on
.Dv AF_LOCAL
sockets before they are bound to a file name; see
.Xr unix 4 .
.Sh RETURN VALUES
.Rv -std
.Sh ERRORS
The
.Fn chmod
system call
will fail and the file mode will be unchanged if:
.Bl -tag -width Er
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 255 characters,
or an entire path name exceeded 1023 characters.
.It Bq Er ENOENT
The named file does not exist.
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
.It Bq Er EPERM
The effective user ID does not match the owner of the file and
the effective user ID is not the super-user.
.It Bq Er EPERM
The effective user ID is not the super-user, the effective user ID do match the
owner of the file, but the group ID of the file does not match the effective
group ID nor one of the supplementary group IDs.
.It Bq Er EPERM
The named file has its immutable or append-only flag set, see the
.Xr chflags 2
manual page for more information.
.It Bq Er EROFS
The named file resides on a read-only file system.
.It Bq Er EFAULT
The
.Fa path
argument
points outside the process's allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.It Bq Er EFTYPE
The effective user ID is not the super-user, the mode includes the sticky bit
.Dv ( S_ISVTX ) ,
and path does not refer to a directory.
.El
.Pp
The
.Fn fchmod
system call will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The descriptor is not valid.
.It Bq Er EINVAL
The
.Fa fd
argument
refers to a socket, not to a file.
.It Bq Er EROFS
The file resides on a read-only file system.
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.It Bq Er EINTEGRITY
Corrupted data was detected while reading from the file system.
.El
.Pp
In addition to the
.Fn chmod
errors,
.Fn fchmodat
fails if:
.Bl -tag -width Er
.It Bq Er EBADF
The
.Fa path
argument does not specify an absolute path and the
.Fa fd
argument is neither
.Fa AT_FDCWD
nor a valid file descriptor open for searching.
.It Bq Er EINVAL
The value of the
.Fa flag
argument is not valid.
.It Bq Er ENOTDIR
The
.Fa path
argument is not an absolute path and
.Fa fd
is neither
.Dv AT_FDCWD
nor a file descriptor associated with a directory.
.It Bq Er ENOTCAPABLE
.Fa path
is an absolute path,
or contained a ".." component leading to a
directory outside of the directory hierarchy specified by
.Fa fd ,
and the process is in capability mode or the
.Dv AT_RESOLVE_BENEATH
flag was specified.
.El
.Sh SEE ALSO
.Xr chmod 1 ,
.Xr chflags 2 ,
.Xr chown 2 ,
.Xr open 2 ,
.Xr stat 2 ,
.Xr sticky 7
.Sh STANDARDS
The
.Fn chmod
system call is expected to conform to
.St -p1003.1-90 ,
except for the return of
.Er EFTYPE .
The
.Dv S_ISVTX
bit on directories is expected to conform to
.St -susv3 .
The
.Fn fchmodat
system call is expected to conform to
.St -p1003.1-2008 .
.Sh HISTORY
The
.Fn chmod
function appeared in
.At v1 .
The
.Fn fchmod
system call appeared in
.Bx 4.2 .
The
.Fn lchmod
system call appeared in
.Fx 3.0 .
The
.Fn fchmodat
system call appeared in
.Fx 8.0 .