| NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | SEE ALSO | COLOPHON | The Linux Programming Interface |
CHMOD(2) Linux Programmer's Manual CHMOD(2)
chmod, fchmod - change permissions of a file
#include <sys/stat.h>
int chmod(const char *path, mode_t mode);
int fchmod(int fd, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fchmod():
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
These system calls change the permissions of a file. They differ only in how
the file is specified:
* chmod() changes the permissions of the file specified whose pathname is
given in path, which is dereferenced if it is a symbolic link.
* fchmod() changes the permissions of the file referred to by the open file
descriptor fd.
The new file permissions are specified in mode, which is a bit mask created by
ORing together zero or more of the following:
S_ISUID (04000) set-user-ID (set process effective user ID on execve(2))
S_ISGID (02000) set-group-ID (set process effective group ID on execve(2);
mandatory locking, as described in fcntl(2); take a new
file's group from parent directory, as described in chown(2)
and mkdir(2))
S_ISVTX (01000) sticky bit (restricted deletion flag, as described in
unlink(2))
S_IRUSR (00400) read by owner
S_IWUSR (00200) write by owner
S_IXUSR (00100) execute/search by owner ("search" applies for directories,
and means that entries within the directory can be accessed)
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.
On success, zero is returned. On error, -1 is returned, and errno is set
appropriately.
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 fd is not valid.
EIO See above.
EPERM See above.
EROFS See above.
4.4BSD, SVr4, POSIX.1-2001.
chown(2), execve(2), fchmodat(2), open(2), stat(2), path_resolution(7)
This page is part of release 3.32 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be found
at http://www.kernel.org/doc/man-pages/.
Linux 2010-09-26 CHMOD(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface