Linux Blog

CHMOD

Section: Linux Programmer's Manual (2)
Updated: 2004-06-23
Index Return to Main Contents
 

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(7).)
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), stat(2), path_resolution(7)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
CONFORMING TO
SEE ALSO




Random Man Pages:
gnupg
lp
swirl
feature_test_macros