CHMOD(2) Linux Programmer's Manual CHMOD(2)
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)):
_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
* 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 filesystem, the set-user-ID and set-group-ID exe-
cution 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 filesystems, 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 filesystems, 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 filesystem, 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_res-
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 CAP_FOWNER capability).
EROFS The named file resides on a read-only filesystem.
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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Linux 2010-09-26 CHMOD(2)