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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
.Nd change mode of file
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/stat.h>
.Ft int
.Fn chmod "const char *path" "mode_t mode"
.Ft int
.Fn fchmod "int fd" "mode_t mode"
.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 fd .
.Fn Chmod
verifies that the process owner (user) either owns
the file specified by
.Fa path
(or
.Fa fd ) ,
or
is the super-user.
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 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.
.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
.Fn Chmod
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 
.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 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 EROFS
The named file resides on a read-only file system.
.It Bq Er EFAULT
.Fa Path
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.
.El
.Pp
.Fn Fchmod
will fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The descriptor is not valid.
.It Bq Er EINVAL
.Fa fd
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.
.El
.Sh SEE ALSO
.Xr chmod 1 ,
.Xr open 2 ,
.Xr chown 2 ,
.Xr stat 2 ,
.Xr sticky 8
.Sh STANDARDS
The
.Fn chmod
function is expected to conform to 
.St -p1003.1-88 .
.Sh HISTORY
The
.Fn fchmod
function call
appeared in
.Bx 4.2 .