CHMOD(2) —

NAME

chmod, fchmod − change mode of file (includes AFS extensions)

SYNOPSIS

chmod(path, mode)
char ∗path;
int mode;

fchmod(fd, mode)
int fd, mode;

DESCRIPTION

The file whose name is given by path or referenced by the descriptor fd has its mode changed to mode. Modes are constructed by or’ing together some combination of the following, defined in <sys/inode.h>:

ISUID04000set user ID on execution
ISGID02000set group ID on execution
ISVTX01000‘sticky bit’ (see below)
IREAD00400read by owner
IWRITE00200write by owner
IEXEC00100execute (search on directory) by owner
00070read, write, execute (search) by group
00007read, write, execute (search) by others

If an executable file is set up for sharing (this is the default) then mode ISVTX (the ‘sticky bit’) prevents the system from abandoning the swap-space image of the program-text portion of the file when its last user terminates. Ability to set this bit on executable files is restricted to the super-user.

If mode ISVTX (the ‘sticky bit’) is set on a directory, an unprivileged user may not delete or rename files of other users in that directory. For more details of the properties of the sticky bit, see sticky(8).

Only the owner of a file (or the super-user) may change the mode.

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.

CHMOD ON ANDREW

The effect of chmod has been changed somewhat to work with the Andrew File System’s protection scheme. Although all 12 mode bits for a file are implemented and can be set by chmod, for nearly all operations, the low order six protection bits (the "group" and "world" protection bits) are completely ignored. All protection is governed by the "owner" rwx bits and the access control list associated with the file’s immediate parent directory. (For information about access control lists in the Andrew File System, see the file "libal.vdoc" in /usr/andrew/doc/vdoc.) There is one exception: if a file is to be made executable, it must have all the "x" bits turned on, and if it is not to be executable, it should have none of them turned on.

When accessing a file on Andrew, first the access control list is consulted, resulting in certain rights being granted to the user. Next, the owner mode bits are checked. If the "r" bit is not set in the owner mode bits, read access is removed from the rights granted via the access control list. Similary, if the owner’s "w" bit is not set, the write access is removed from the rights granted via the access control list. All other mode bits operate as described below.

A user who has write access to the file on the access list is considered the owner of the file and is allowed to change the mode bits. Therefore, if the "r" and "w" bits are not set in the owner mode bits, a person who has write access to the file on the access list can use chmod(1v) to change the owner bits and enable himself to read or write the file.

RETURN VALUE

Upon successful completion, a value of 0 is returned. Otherwise, a value of −1 is returned and errno is set to indicate the error.

ERRORS

Chmod will fail and the file mode will be unchanged if:

[ENOTDIR] A component of the path prefix is not a directory.

[EINVAL] The pathname contains a character with the high-order bit set.

[ENAMETOOLONG]
A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.

[ENOENT] The named file does not exist.

[EACCES] Search permission is denied for a component of the path prefix.

[ELOOP] Too many symbolic links were encountered in translating the pathname.

[EPERM] The effective user ID does not match the owner of the file and the effective user ID is not the super-user.

[EROFS] The named file resides on a read-only file system.

[EFAULT] Path points outside the process’s allocated address space.

[EIO] An I/O error occurred while reading from or writing to the file system.

Fchmod will fail if:

[EBADF] The descriptor is not valid.

[EINVAL] Fd refers to a socket, not to a file.

[EROFS] The file resides on a read-only file system.