chmod

CHMOD(2)                     BSD System Calls Manual                    CHMOD(2)

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 <fcntl.h>:

     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 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 non-standard S_ISTXT is a synonym for S_ISVTX.

     The FreeBSD VM system totally ignores the sticky bit (S_ISVTX) for
     executables.  On UFS-based file systems (FFS, LFS) the sticky bit may only
     be set upon directories.

     If mode S_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 sticky(7).

     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(7)

STANDARDS
     The chmod() system call is expected to conform to ISO/IEC 9945-1:1990
     (“POSIX.1”), except for the return of EFTYPE.  The S_ISVTX bit on
     directories is expected to conform to Version 3 of the Single UNIX
     Specification (“SUSv3”).  The fchmodat() system call is expected to conform
     to IEEE Std 1003.1-2008 (“POSIX.1”).

HISTORY
     The chmod() function appeared in Version 1 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.

BSD                             December 1, 2017                             BSD