NAME
chmod, fchmod, lchmod, fchmodat - change mode of file
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/stat.h>
int
chmod(const char *path, mode_t mode);
int
fchmod(int fd, mode_t mode);
int
lchmod(const char *path, mode_t mode);
int
fchmodat(int fd, const char *path, mode_t mode, int flag);
DESCRIPTION
The file permission bits of the file named specified by path or
referenced by the file descriptor fd are changed to mode. The chmod()
system call verifies that the process owner (user) either owns the file
specified by path (or fd), or is the super-user. The chmod() system call
follows symbolic links to operate on the target of the link rather than
the link itself.
The lchmod() system call is similar to chmod() but does not follow
symbolic links.
The fchmodat() is equivalent to either chmod() or lchmod() depending on
the flag except in the case where path specifies a relative path. In
this case the file to be changed is determined relative to the directory
associated with the file descriptor fd instead of the current working
directory. The values for the flag are constructed by a bitwise-
inclusive OR of flags from the following list, defined in
AT_SYMLINK_NOFOLLOW
If path names a symbolic link, then the mode of the symbolic link
is changed.
If fchmodat() is passed the special value AT_FDCWD in the fd parameter,
the current working directory is used. If also flag is zero, the
behavior is identical to a call to chmod().
A mode is created from ord permission bit masks defined in
#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 */
#ifndef __BSD_VISIBLE
#define S_ISTXT 0001000 /* sticky bit */
#endif
The FreeBSD VM system totally ignores the sticky bit (ISTXT) for
executables. On UFS-based file systems (FFS, LFS) the sticky bit may
only be set upon directories.
If mode ISTXT (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 sticky(8).
If mode ISUID (set UID) is set on a directory, and the MNT_SUIDDIR option
was used in the mount of the file system, then the owner of any new files
and sub-directories created within this directory are set to be the same
as the owner of that directory. If this function is enabled, new
directories will inherit the bit from their parents. Execute bits are
removed from the file, and it will not be given to root. This behavior
does not change the requirements for the user to be allowed to write the
file, but only the eventual owner after it has been created. Group
inheritance is not affected.
This feature is designed for use on fileservers serving PC users via ftp,
SAMBA, or netatalk. It provides security holes for shell users and as
such should not be used on shell machines, especially on home
directories. This option requires the SUIDDIR option in the kernel to
work. Only UFS file systems support this option. For more details of
the suiddir mount option, see mount(8).
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.
RETURN VALUES
Upon successful completion, the value 0 is returned; otherwise the
value -1 is returned and the global variable errno is set to indicate the
error.
ERRORS
The chmod() system call will fail and the file mode will be unchanged if:
[ENOTDIR] A component of the path prefix is not a directory.
[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.
[EPERM] The effective user ID is not the super-user, the
effective user ID do match the owner of the file, but
the group ID of the file does not match the effective
group ID nor one of the supplementary group IDs.
[EPERM] The named file has its immutable or append-only flag
set, see the chflags(2) manual page for more
information.
[EROFS] The named file resides on a read-only file system.
[EFAULT] The path argument points outside the process’s
allocated address space.
[EIO] An I/O error occurred while reading from or writing to
the file system.
[EFTYPE] 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.
The fchmod() system call will fail if:
[EBADF] The descriptor is not valid.
[EINVAL] The fd argument refers to a socket, not to a file.
[EROFS] The file resides on a read-only file system.
[EIO] An I/O error occurred while reading from or writing to
the file system.
In addition to the chmod() errors, fchmodat() fails if:
[EBADF] The path argument does not specify an absolute path
and the fd argument is neither AT_FDCWD nor a valid
file descriptor open for searching.
[EINVAL] The value of the flag argument is not valid.
[ENOTDIR] The path argument is not an absolute path and fd is
neither AT_FDCWD nor a file descriptor associated with
a directory.
SEE ALSO
chmod(1), chflags(2), chown(2), open(2), stat(2), sticky(8)
STANDARDS
The chmod() system call is expected to conform to ISO/IEC 9945-1:1990
(“POSIX.1”), except for the return of EFTYPE and the use of S_ISTXT. The
fchmodat() system call follows The Open Group Extended API Set 2
specification.
HISTORY
The chmod() function appeared in Version 7 AT&T UNIX. The fchmod()
system call appeared in 4.2BSD. The lchmod() system call appeared in
FreeBSD 3.0. The fchmodat() system call appeared in FreeBSD 8.0.