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.\"     @(#)chmod.2	8.1 (Berkeley) 6/4/93
.\"
.Dd June 4, 1993
.Dt CHMOD 2
.Os BSD 4
.Sh NAME
.Nm chmod ,
.Nm fchmod ,
.Nm fchmodat
.Nd change mode of file
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/stat.h>
.Ft int
.Fo chmod
.Fa "const char *path"
.Fa "mode_t mode"
.Fc
.Ft int
.Fo fchmod
.Fa "int fildes"
.Fa "mode_t mode"
.Fc
.Ft int
.Fn fchmodat "int fd" "const char *path" "mode_t mode" "int flag"
.Sh DESCRIPTION
The function
.Fn chmod
sets the file permission bits
of the file
specified by the pathname
.Fa path
to
.Fa mode .
.Fn fchmod
sets the permission bits of the specified
file descriptor
.Fa fildes .
.Fn chmod
verifies that the process owner (user) either owns
the file specified by
.Fa path
(or
.Fa fildes ) ,
or
is the super-user.
.Pp
The
.Fn fchmodat
is equivalent to
.Fn chmod
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.
.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 .
A mode is created from
.Em or'd
permission bit masks
defined in
.Aq Pa sys/stat.h :
.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    /* save swapped text even after use */
.Ed
.Pp
The
.Dv ISVTX
(the
.Em sticky bit )
indicates to the system which executable files are shareable (the
default) and the system maintains the program text of the files
in the swap area. The sticky bit may only be set by the super user
on shareable executable files.
.Pp
If mode
.Dv 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
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.
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn chmod
system call will fail and the file mode will be unchanged if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EACCES
Search permission is denied for a component of the path prefix.
.\" ==========
.It Bq Er EFAULT
.Fa Path
points outside the process's allocated address space.
.\" ==========
.It Bq Er EINTR
Its execution was interrupted by a signal.
.\" ==========
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.\" ==========
.It Bq Er ELOOP
Too many symbolic links were encountered in translating the pathname.
This is taken to be indicative of a looping symbolic link.
.\" ==========
.It Bq Er ENAMETOOLONG
A component of a pathname exceeded 
.Dv {NAME_MAX}
characters, or an entire path name exceeded 
.Dv {PATH_MAX}
characters.
.\" ==========
.It Bq Er ENOENT
The named file does not exist.
.\" ==========
.It Bq Er ENOTDIR
A component of the path prefix is not a directory.
.\" ==========
.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 EROFS
The named file resides on a read-only file system.
.El
.Pp
.Fn fchmod
will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EBADF
.Fa fildes
is not a valid file descriptor.
.\" ==========
.It Bq Er EINVAL
.Fa fildes
refers to a socket, not to a file.
.\" ==========
.It Bq Er EINVAL
.Fa mode
is not a valid file mode.
.\" ==========
.It Bq Er EINTR
Its execution was interrupted by a signal.
.\" ==========
.It Bq Er EIO
An I/O error occurred while reading from or writing to the file system.
.\" ==========
.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 EROFS
The file resides on a read-only 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.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/stat.h>
.Pp
The include file
.In sys/types.h
is necessary.
.Sh SEE ALSO
.Xr chmod 1 ,
.Xr chown 2 ,
.Xr open 2 ,
.Xr stat 2 ,
.Xr compat 5 ,
.Xr sticky 7
.Sh STANDARDS
The
.Fn chmod
function is expected to conform to 
.St -p1003.1-88 .
The
.Fn fchmodat
function is expected to conform to POSIX.1-2008 . 
.Sh HISTORY
The
.Fn fchmod
function call
appeared in
.Bx 4.2 .
The
.Fn fchmodat
system call appeared in OS X 10.10