CHMOD(2)

NAME
chmod, fchmod - change permissions of a file

SYNOPSIS
#include <sys/types.h>
#include <sys/stat.h>

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

DESCRIPTION
The mode of the file given by path or referenced by fildes is changed.

Modes are specified by or'ing the following:

    S_ISUID
        04000 set user ID on execution

    S_ISGID
        02000 set group ID on execution

    S_ISVTX
        01000 sticky bit

    S_IRUSR
        00400 read by owner

    S_IWUSR
        00200 write by owner

    S_IXUSR
        00100 execute/search by owner

    S_IRGRP
        00040 read by group

    S_IWGRP
        00020 write by group

    S_IXGRP
        00010 execute/search by group

    S_IROTH
        00004 read by others

    S_IWOTH
        00002 write by others

    S_IXOTH
        00001 execute/search by others

The effective UID of the calling process must match the owner of the file, or the process must be privileged (Linux: it must have the CAP_FOWNER capability).

If the calling process is not privileged (Linux: does not have the CAP_FSETID capability), and the group of the file does not match the effective group ID of the process or one of its supplementary group IDs, the S_ISGID bit will be turned off, but this will not cause an error to be returned.

As a security measure, depending on the file system, the set-user-ID and set-group-ID execution bits may be turned off if a file is written. (On Linux this occurs if the writing process does not have the CAP_FSETID capability.) On some file systems, only the superuser can set the sticky bit, which may have a special meaning. For the sticky bit, and for set-user-ID and set-group-ID bits on directories, see stat(2).

On NFS file systems, restricting the permissions will immediately influence already open files, because the access control is done on the server, but open files are maintained by the client. Widening the permissions may be delayed for other clients if attribute caching is enabled on them.

RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.

ERRORS
Depending on the file system, other errors can be returned. The more general errors for chmod() are listed below:

EACCES
    Search permission is denied on a component of the path prefix. (See also path_resolution(2).)

EFAULT
    path points outside your accessible address space.

EIO An I/O error occurred.

ELOOP
    Too many symbolic links were encountered in resolving path.

ENAMETOOLONG
    path is too long.

ENOENT
    The file does not exist.

ENOMEM
    Insufficient kernel memory was available.

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

EPERM
    The effective UID does not match the owner of the file, and the process is not privileged (Linux: it does not have the CAP_FOWNER capability).

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

The general errors for fchmod() are listed below:

EBADF
    The file descriptor fildes is not valid.

EIO See above.

EPERM
    See above.

EROFS
    See above.

CONFORMING TO
4.4BSD, SVr4, POSIX.1-2001.

SEE ALSO
chown(2), execve(2), fchmodat(2), open(2), path_resolution(2), stat(2)