chmod(2) - Linux manual page (original) (raw)
chmod(2) System Calls Manual chmod(2)
NAME top
chmod, fchmod, fchmodat - change permissions of a fileLIBRARY top
Standard C library (_libc_, _-lc_)SYNOPSIS top
**#include <sys/stat.h>**
**int chmod(const char ***_pathname_**, mode_t** _mode_**);**
**int fchmod(int** _fd_**, mode_t** _mode_**);**
**#include <fcntl.h>** /* Definition of AT_* constants */
**#include <sys/stat.h>**
**int fchmodat(int** _dirfd_**, const char ***_pathname_**, mode_t** _mode_**, int** _flags_**);**Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
**fchmod**():
Since glibc 2.24:
_POSIX_C_SOURCE >= 199309L
glibc 2.19 to glibc 2.23
_POSIX_C_SOURCE
glibc 2.16 to glibc 2.19:
_BSD_SOURCE || _POSIX_C_SOURCE
glibc 2.12 to glibc 2.16:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
|| _POSIX_C_SOURCE >= 200809L
glibc 2.11 and earlier:
_BSD_SOURCE || _XOPEN_SOURCE >= 500
**fchmodat**():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCEDESCRIPTION top
The **chmod**() and **fchmod**() system calls change a file's mode bits.
(The file mode consists of the file permission bits plus the set-
user-ID, set-group-ID, and sticky bits.) These system calls
differ only in how the file is specified:
• **chmod**() changes the mode of the file specified whose pathname
is given in _pathname_, which is dereferenced if it is a symbolic
link.
• **fchmod**() changes the mode of the file referred to by the open
file descriptor _fd_.
The new file mode is 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)](../man2/execve.2.html))
**S_ISGID** (02000)
set-group-ID (set process effective group ID on [execve(2)](../man2/execve.2.html);
mandatory locking, as described in [fcntl(2)](../man2/fcntl.2.html); take a new
file's group from parent directory, as described in
[chown(2)](../man2/chown.2.html) and [mkdir(2)](../man2/mkdir.2.html))
**S_ISVTX** (01000)
sticky bit (restricted deletion flag, as described in
[unlink(2)](../man2/unlink.2.html))
**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 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 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 [inode(7)](../man7/inode.7.html).
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.fchmodat() The fchmodat() system call operates in exactly the same way as chmod(), except for the differences described here.
If the pathname given in _pathname_ is relative, then it is
interpreted relative to the directory referred to by the file
descriptor _dirfd_ (rather than relative to the current working
directory of the calling process, as is done by **chmod**() for a
relative pathname).
If _pathname_ is relative and _dirfd_ is the special value **AT_FDCWD**,
then _pathname_ is interpreted relative to the current working
directory of the calling process (like **chmod**()).
If _pathname_ is absolute, then _dirfd_ is ignored.
_flags_ can either be 0, or include the following flag:
**AT_SYMLINK_NOFOLLOW**
If _pathname_ is a symbolic link, do not dereference it:
instead operate on the link itself. This flag is not
currently implemented.
See [openat(2)](../man2/openat.2.html) for an explanation of the need for **fchmodat**().RETURN VALUE top
On success, zero is returned. On error, -1 is returned, and _[errno](../man3/errno.3.html)_
is set to indicate the error.ERRORS top
Depending on the filesystem, errors other than those listed below
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)](../man7/path%5Fresolution.7.html).)
**EBADF** (**fchmod**()) The file descriptor _fd_ is not valid.
**EBADF** (**fchmodat**()) _pathname_ is relative but _dirfd_ is neither
**AT_FDCWD** nor a valid file descriptor.
**EFAULT** _pathname_ points outside your accessible address space.
**EINVAL** (**fchmodat**()) Invalid flag specified in _flags_.
**EIO** An I/O error occurred.
**ELOOP** Too many symbolic links were encountered in resolving
_pathname_.
**ENAMETOOLONG**
_pathname_ 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.
**ENOTDIR**
(**fchmodat**()) _pathname_ is relative and _dirfd_ is a file
descriptor referring to a file other than a directory.
**ENOTSUP**
(**fchmodat**()) _flags_ specified **AT_SYMLINK_NOFOLLOW**, which is
not supported.
**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).
**EPERM** The file is marked immutable or append-only. (See
[FS_IOC_SETFLAGS(2const)](../man2/FS%5FIOC%5FSETFLAGS.2const.html).)
**EROFS** The named file resides on a read-only filesystem.VERSIONS top
C library/kernel differences The GNU C library fchmodat() wrapper function implements the POSIX-specified interface described in this page. This interface differs from the underlying Linux system call, which does not have a flags argument.
glibc notes On older kernels where fchmodat() is unavailable, the glibc wrapper function falls back to the use of chmod(). When pathname is a relative pathname, glibc constructs a pathname based on the symbolic link in /proc/self/fd that corresponds to the dirfd argument.
STANDARDS top
POSIX.1-2008.HISTORY top
**chmod**()
**fchmod**()
4.4BSD, SVr4, POSIX.1-2001.
**fchmodat**()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.SEE ALSO top
[chmod(1)](../man1/chmod.1.html), [chown(2)](../man2/chown.2.html), [execve(2)](../man2/execve.2.html), [open(2)](../man2/open.2.html), [stat(2)](../man2/stat.2.html), [inode(7)](../man7/inode.7.html),
[path_resolution(7)](../man7/path%5Fresolution.7.html), [symlink(7)](../man7/symlink.7.html)COLOPHON top
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man-pages@man7.orgLinux man-pages 6.10 2024-07-23 chmod(2)
Pages that refer to this page:chmod(1), access(2), chown(2), execve(2), fcntl(2), mkdir(2), mknod(2), open(2), rename(2), rmdir(2), stat(2), statx(2), syscalls(2), umask(2), unlink(2), euidaccess(3), mode_t(3type), shm_open(3), capabilities(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), spufs(7), symlink(7), unix(7), logrotate(8), xfs_db(8)