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chmod, fchmod - change permissions of a file
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
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
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
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
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
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
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.
path is too long.
ENOENT The file does not exist.
ENOMEM Insufficient kernel memory was available.
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
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)
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