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CHMOD(2) System Calls Manual CHMOD(2)

chmod, lchmod, fchmod, fchmodatchange mode of file

library “libc”

#include <sys/stat.h>

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

int
lchmod(const char *path, mode_t mode);

int
fchmod(int fd, mode_t mode);

#include <sys/stat.h>
#include <fcntl.h>

int
fchmodat(int fd, const char *path, mode_t mode, int flag);

The function chmod() sets the file permission bits of the file specified by the pathname path to mode. fchmod() sets the permission bits of the specified file descriptor fd. lchmod() is like chmod() except in the case where the named file is a symbolic link, in which case lchmod() sets the permission bits of the link, while chmod() sets the bits of the file the link references.

fchmodat() works the same way as chmod() (or lchmod() if AT_SYMLINK_NOFOLLOW is set in flag) except if path is relative. In that case, it is looked up from a directory whose file descriptor was passed as fd. Search permission is required on this directory. fd except if that file descriptor was opened with the O_SEARCH flag. fd can be set to AT_FDCWD in order to specify the current directory.

chmod() verifies that the process owner (user) either owns the file specified by path (or fd), or is the super-user. A mode is created from or'd permission bit masks defined in <sys/stat.h>:

#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 */

The mode ISVTX (the ‘sticky bit’) can be set on regular files, but has no effect. For historical reasons this can be done only by 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. The sticky bit may be set by any user on a directory which the user owns or has appropriate permissions.

For more information about the history and properties of the sticky bit, see sticky(7).

Changing the owner of a file turns off the set-user-id and set-group-id bits; writing to 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.

The chmod(), lchmod(), fchmod(), and fchmodat() functions return the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

chmod(), lchmod() and fchmodat() will fail and the file mode will be unchanged if:

[]
Search permission is denied for a component of the path prefix.
[]
path points outside the process's allocated address space.
[]
The effective user ID is not the super-user, the mode includes the sticky bit (S_ISVTX), and path does not refer to a directory.
[]
An I/O error occurred while reading from or writing to the file system.
[]
Too many symbolic links were encountered in translating the pathname.
[]
A component of a pathname exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters.
[]
The named file does not exist.
[]
A component of the path prefix is not a directory.
[]
The effective user ID does not match the owner of the file and the effective user ID is not the super-user; or the mode includes the setgid bit (S_ISGID) but the file's group is neither the effective group ID nor is it in the group access list.
[]
The named file resides on a read-only file system.

In addition, fchmodat() will fail if:

[]
path does not specify an absolute path and fd is neither AT_FDCWD nor a valid file descriptor open for reading or searching.
[]
path is not an absolute path and fd is a file descriptor associated with a non-directory file.

fchmod() will fail if:

[]
The descriptor is not valid.
[]
The effective user ID is not the super-user, the mode includes the sticky bit (S_ISVTX), and fd does not refer to a directory.
[]
fd refers to a socket, not to a file.
[]
An I/O error occurred while reading from or writing to the file system.
[]
The effective user ID does not match the owner of the file and the effective user ID is not the super-user; or the mode includes the setgid bit (S_ISGID) but the file's group is neither the effective group ID nor is it in the group access list.
[]
The file resides on a read-only file system.

chmod(1), chflags(2), chown(2), open(2), stat(2), getmode(3), setmode(3), sticky(7), symlink(7)

The chmod() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”). fchmodat() function conforms to IEEE Std 1003.1-2008 (“POSIX.1”).

The fchmod() function call appeared in 4.2BSD. The lchmod() function call appeared in NetBSD 1.3.

July 29, 2013 NetBSD-7.0.1