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

fchmod(fd, mode)
int fd, mode;

ISUID 04000 set user ID on execution
ISGID 02000 set group ID on execution
ISVTX 01000 `sticky bit' (see below)
IREAD 00400 read by owner
IWRITE 00200 write by owner
IEXEC 00100 execute (search on directory) by owner
 00070 read, write, execute (search) by group
 00007 read, 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.

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

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

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

15 [ENOENT] The named file does not exist.

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

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

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

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

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

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

Fchmod will fail if:

15 [EBADF] The descriptor is not valid.

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

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

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