The open() system call opens the file specified by pathname. If
the specified file does not exist, it may optionally (if O_CREAT
is specified in flags) be created by open().
The return value of open() is a file descriptor, a small,
nonnegative integer that is an index to an entry in the process's
table of open file descriptors. The file descriptor is used in
subsequent system calls (read(2), write(2), lseek(2), fcntl(2),
etc.) to refer to the open file. The file descriptor returned by
a successful call will be the lowest-numbered file descriptor not
currently open for the process.
By default, the new file descriptor is set to remain open across
an execve(2) (i.e., the FD_CLOEXEC file descriptor flag described
in fcntl(2) is initially disabled); the O_CLOEXEC flag, described
below, can be used to change this default. The file offset is
set to the beginning of the file (see lseek(2)).
A call to open() creates a new open file description, an entry in
the system-wide table of open files. The open file description
records the file offset and the file status flags (see below). A
file descriptor is a reference to an open file description; this
reference is unaffected if pathname is subsequently removed or
modified to refer to a different file. For further details on
open file descriptions, see NOTES.
The argument flags must include one of the following access
modes: O_RDONLY, O_WRONLY, or O_RDWR. These request opening the
file read-only, write-only, or read/write, respectively.
In addition, zero or more file creation flags and file status
flags can be bitwise-or'd in flags. The file creation flags are
O_CLOEXEC, O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW,
O_TMPFILE, and O_TRUNC. The file status flags are all of the
remaining flags listed below. The distinction between these two
groups of flags is that the file creation flags affect the
semantics of the open operation itself, while the file status
flags affect the semantics of subsequent I/O operations. The
file status flags can be retrieved and (in some cases) modified;
see fcntl(2) for details.
The full list of file creation flags and file status flags is as
follows:
O_APPEND
The file is opened in append mode. Before each write(2),
the file offset is positioned at the end of the file, as
if with lseek(2). The modification of the file offset and
the write operation are performed as a single atomic step.
O_APPEND may lead to corrupted files on NFS filesystems if
more than one process appends data to a file at once.
This is because NFS does not support appending to a file,
so the client kernel has to simulate it, which can't be
done without a race condition.
O_ASYNC
Enable signal-driven I/O: generate a signal (SIGIO by
default, but this can be changed via fcntl(2)) when input
or output becomes possible on this file descriptor. This
feature is available only for terminals, pseudoterminals,
sockets, and (since Linux 2.6) pipes and FIFOs. See
fcntl(2) for further details. See also BUGS, below.
O_CLOEXEC (since Linux 2.6.23)
Enable the close-on-exec flag for the new file descriptor.
Specifying this flag permits a program to avoid additional
fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.
Note that the use of this flag is essential in some
multithreaded programs, because using a separate fcntl(2)
F_SETFD operation to set the FD_CLOEXEC flag does not
suffice to avoid race conditions where one thread opens a
file descriptor and attempts to set its close-on-exec flag
using fcntl(2) at the same time as another thread does a
fork(2) plus execve(2). Depending on the order of
execution, the race may lead to the file descriptor
returned by open() being unintentionally leaked to the
program executed by the child process created by fork(2).
(This kind of race is in principle possible for any system
call that creates a file descriptor whose close-on-exec
flag should be set, and various other Linux system calls
provide an equivalent of the O_CLOEXEC flag to deal with
this problem.)
O_CREAT
If pathname does not exist, create it as a regular file.
The owner (user ID) of the new file is set to the
effective user ID of the process.
The group ownership (group ID) of the new file is set
either to the effective group ID of the process (System V
semantics) or to the group ID of the parent directory (BSD
semantics). On Linux, the behavior depends on whether the
set-group-ID mode bit is set on the parent directory: if
that bit is set, then BSD semantics apply; otherwise,
System V semantics apply. For some filesystems, the
behavior also depends on the bsdgroups and sysvgroups
mount options described in mount(8).
The mode argument specifies the file mode bits to be
applied when a new file is created. If neither O_CREAT
nor O_TMPFILE is specified in flags, then mode is ignored
(and can thus be specified as 0, or simply omitted). The
mode argument must be supplied if O_CREAT or O_TMPFILE is
specified in flags; if it is not supplied, some arbitrary
bytes from the stack will be applied as the file mode.
The effective mode is modified by the process's umask in
the usual way: in the absence of a default ACL, the mode
of the created file is (mode & ~umask).
Note that mode applies only to future accesses of the
newly created file; the open() call that creates a read-
only file may well return a read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU 00700 user (file owner) has read, write, and
execute permission
S_IRUSR 00400 user has read permission
S_IWUSR 00200 user has write permission
S_IXUSR 00100 user has execute permission
S_IRWXG 00070 group has read, write, and execute
permission
S_IRGRP 00040 group has read permission
S_IWGRP 00020 group has write permission
S_IXGRP 00010 group has execute permission
S_IRWXO 00007 others have read, write, and execute
permission
S_IROTH 00004 others have read permission
S_IWOTH 00002 others have write permission
S_IXOTH 00001 others have execute permission
According to POSIX, the effect when other bits are set in
mode is unspecified. On Linux, the following bits are
also honored in mode:
S_ISUID 0004000 set-user-ID bit
S_ISGID 0002000 set-group-ID bit (see inode(7)).
S_ISVTX 0001000 sticky bit (see inode(7)).
O_DIRECT (since Linux 2.4.10)
Try to minimize cache effects of the I/O to and from this
file. In general this will degrade performance, but it is
useful in special situations, such as when applications do
their own caching. File I/O is done directly to/from
user-space buffers. The O_DIRECT flag on its own makes an
effort to transfer data synchronously, but does not give
the guarantees of the O_SYNC flag that data and necessary
metadata are transferred. To guarantee synchronous I/O,
O_SYNC must be used in addition to O_DIRECT. See NOTES
below for further discussion.
A semantically similar (but deprecated) interface for
block devices is described in raw(8).
O_DIRECTORY
If pathname is not a directory, cause the open to fail.
This flag was added in kernel version 2.1.126, to avoid
denial-of-service problems if opendir(3) is called on a
FIFO or tape device.
O_DSYNC
Write operations on the file will complete according to
the requirements of synchronized I/O data integrity
completion.
By the time write(2) (and similar) return, the output data
has been transferred to the underlying hardware, along
with any file metadata that would be required to retrieve
that data (i.e., as though each write(2) was followed by a
call to fdatasync(2)). See NOTES below.
O_EXCL Ensure that this call creates the file: if this flag is
specified in conjunction with O_CREAT, and pathname
already exists, then open() fails with the error EEXIST.
When these two flags are specified, symbolic links are not
followed: if pathname is a symbolic link, then open()
fails regardless of where the symbolic link points.
In general, the behavior of O_EXCL is undefined if it is
used without O_CREAT. There is one exception: on Linux
2.6 and later, O_EXCL can be used without O_CREAT if
pathname refers to a block device. If the block device is
in use by the system (e.g., mounted), open() fails with
the error EBUSY.
On NFS, O_EXCL is supported only when using NFSv3 or later
on kernel 2.6 or later. In NFS environments where O_EXCL
support is not provided, programs that rely on it for
performing locking tasks will contain a race condition.
Portable programs that want to perform atomic file locking
using a lockfile, and need to avoid reliance on NFS
support for O_EXCL, can create a unique file on the same
filesystem (e.g., incorporating hostname and PID), and use
link(2) to make a link to the lockfile. If link(2)
returns 0, the lock is successful. Otherwise, use stat(2)
on the unique file to check if its link count has
increased to 2, in which case the lock is also successful.
O_LARGEFILE
(LFS) Allow files whose sizes cannot be represented in an
off_t (but can be represented in an off64_t) to be opened.
The _LARGEFILE64_SOURCE macro must be defined (before
including any header files) in order to obtain this
definition. Setting the _FILE_OFFSET_BITS feature test
macro to 64 (rather than using O_LARGEFILE) is the
preferred method of accessing large files on 32-bit
systems (see feature_test_macros(7)).
O_NOATIME (since Linux 2.6.8)
Do not update the file last access time (st_atime in the
inode) when the file is read(2).
This flag can be employed only if one of the following
conditions is true:
* The effective UID of the process matches the owner UID
of the file.
* The calling process has the CAP_FOWNER capability in
its user namespace and the owner UID of the file has a
mapping in the namespace.
This flag is intended for use by indexing or backup
programs, where its use can significantly reduce the
amount of disk activity. This flag may not be effective
on all filesystems. One example is NFS, where the server
maintains the access time.
O_NOCTTY
If pathname refers to a terminal device—see tty(4)—it will
not become the process's controlling terminal even if the
process does not have one.
O_NOFOLLOW
If the trailing component (i.e., basename) of pathname is
a symbolic link, then the open fails, with the error
ELOOP. Symbolic links in earlier components of the
pathname will still be followed. (Note that the ELOOP
error that can occur in this case is indistinguishable
from the case where an open fails because there are too
many symbolic links found while resolving components in
the prefix part of the pathname.)
This flag is a FreeBSD extension, which was added to Linux
in version 2.1.126, and has subsequently been standardized
in POSIX.1-2008.
See also O_PATH below.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode.
Neither the open() nor any subsequent I/O operations on
the file descriptor which is returned will cause the
calling process to wait.
Note that the setting of this flag has no effect on the
operation of poll(2), select(2), epoll(7), and similar,
since those interfaces merely inform the caller about
whether a file descriptor is "ready", meaning that an I/O
operation performed on the file descriptor with the
O_NONBLOCK flag clear would not block.
Note that this flag has no effect for regular files and
block devices; that is, I/O operations will (briefly)
block when device activity is required, regardless of
whether O_NONBLOCK is set. Since O_NONBLOCK semantics
might eventually be implemented, applications should not
depend upon blocking behavior when specifying this flag
for regular files and block devices.
For the handling of FIFOs (named pipes), see also fifo(7).
For a discussion of the effect of O_NONBLOCK in
conjunction with mandatory file locks and with file
leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two
purposes: to indicate a location in the filesystem tree
and to perform operations that act purely at the file
descriptor level. The file itself is not opened, and
other file operations (e.g., read(2), write(2), fchmod(2),
fchown(2), fgetxattr(2), ioctl(2), mmap(2)) fail with the
error EBADF.
The following operations can be performed on the resulting
file descriptor:
* close(2).
* fchdir(2), if the file descriptor refers to a directory
(since Linux 3.5).
* fstat(2) (since Linux 3.6).
* fstatfs(2) (since Linux 3.12).
* Duplicating the file descriptor (dup(2), fcntl(2)
F_DUPFD, etc.).
* Getting and setting file descriptor flags (fcntl(2)
F_GETFD and F_SETFD).
* Retrieving open file status flags using the fcntl(2)
F_GETFL operation: the returned flags will include the
bit O_PATH.
* Passing the file descriptor as the dirfd argument of
openat() and the other "*at()" system calls. This
includes linkat(2) with AT_EMPTY_PATH (or via procfs
using AT_SYMLINK_FOLLOW) even if the file is not a
directory.
* Passing the file descriptor to another process via a
UNIX domain socket (see SCM_RIGHTS in unix(7)).
When O_PATH is specified in flags, flag bits other than
O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
Opening a file or directory with the O_PATH flag requires
no permissions on the object itself (but does require
execute permission on the directories in the path prefix).
Depending on the subsequent operation, a check for
suitable file permissions may be performed (e.g.,
fchdir(2) requires execute permission on the directory
referred to by its file descriptor argument). By
contrast, obtaining a reference to a filesystem object by
opening it with the O_RDONLY flag requires that the caller
have read permission on the object, even when the
subsequent operation (e.g., fchdir(2), fstat(2)) does not
require read permission on the object.
If pathname is a symbolic link and the O_NOFOLLOW flag is
also specified, then the call returns a file descriptor
referring to the symbolic link. This file descriptor can
be used as the dirfd argument in calls to fchownat(2),
fstatat(2), linkat(2), and readlinkat(2) with an empty
pathname to have the calls operate on the symbolic link.
If pathname refers to an automount point that has not yet
been triggered, so no other filesystem is mounted on it,
then the call returns a file descriptor referring to the
automount directory without triggering a mount.
fstatfs(2) can then be used to determine if it is, in
fact, an untriggered automount point (.f_type ==
AUTOFS_SUPER_MAGIC).
One use of O_PATH for regular files is to provide the
equivalent of POSIX.1's O_EXEC functionality. This
permits us to open a file for which we have execute
permission but not read permission, and then execute that
file, with steps something like the following:
char buf[PATH_MAX];
fd = open("some_prog", O_PATH);
snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
execl(buf, "some_prog", (char *) NULL);
An O_PATH file descriptor can also be passed as the
argument of fexecve(3).
O_SYNC Write operations on the file will complete according to
the requirements of synchronized I/O file integrity
completion (by contrast with the synchronized I/O data
integrity completion provided by O_DSYNC.)
By the time write(2) (or similar) returns, the output data
and associated file metadata have been transferred to the
underlying hardware (i.e., as though each write(2) was
followed by a call to fsync(2)). See NOTES below.
O_TMPFILE (since Linux 3.11)
Create an unnamed temporary regular file. The pathname
argument specifies a directory; an unnamed inode will be
created in that directory's filesystem. Anything written
to the resulting file will be lost when the last file
descriptor is closed, unless the file is given a name.
O_TMPFILE must be specified with one of O_RDWR or O_WRONLY
and, optionally, O_EXCL. If O_EXCL is not specified, then
linkat(2) can be used to link the temporary file into the
filesystem, making it permanent, using code like the
following:
char path[PATH_MAX];
fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
S_IRUSR | S_IWUSR);
/* File I/O on 'fd'... */
linkat(fd, "", AT_FDCWD, "/path/for/file", AT_EMPTY_PATH);
/* If the caller doesn't have the CAP_DAC_READ_SEARCH
capability (needed to use AT_EMPTY_PATH with linkat(2)),
and there is a proc(5) filesystem mounted, then the
linkat(2) call above can be replaced with:
snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
AT_SYMLINK_FOLLOW);
*/
In this case, the open() mode argument determines the file
permission mode, as with O_CREAT.
Specifying O_EXCL in conjunction with O_TMPFILE prevents a
temporary file from being linked into the filesystem in
the above manner. (Note that the meaning of O_EXCL in
this case is different from the meaning of O_EXCL
otherwise.)
There are two main use cases for O_TMPFILE:
* Improved tmpfile(3) functionality: race-free creation
of temporary files that (1) are automatically deleted
when closed; (2) can never be reached via any pathname;
(3) are not subject to symlink attacks; and (4) do not
require the caller to devise unique names.
* Creating a file that is initially invisible, which is
then populated with data and adjusted to have
appropriate filesystem attributes (fchown(2),
fchmod(2), fsetxattr(2), etc.) before being atomically
linked into the filesystem in a fully formed state
(using linkat(2) as described above).
O_TMPFILE requires support by the underlying filesystem;
only a subset of Linux filesystems provide that support.
In the initial implementation, support was provided in the
ext2, ext3, ext4, UDF, Minix, and tmpfs filesystems.
Support for other filesystems has subsequently been added
as follows: XFS (Linux 3.15); Btrfs (Linux 3.16); F2FS
(Linux 3.16); and ubifs (Linux 4.9)
O_TRUNC
If the file already exists and is a regular file and the
access mode allows writing (i.e., is O_RDWR or O_WRONLY)
it will be truncated to length 0. If the file is a FIFO
or terminal device file, the O_TRUNC flag is ignored.
Otherwise, the effect of O_TRUNC is unspecified.
creat()
A call to creat() is equivalent to calling open() with flags
equal to O_CREAT|O_WRONLY|O_TRUNC.
openat()
The openat() system call operates in exactly the same way as
open(), except for the differences described here.
The dirfd argument is used in conjunction with the pathname
argument as follows:
* If the pathname given in pathname is absolute, then dirfd is
ignored.
* If the pathname given in 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
open()).
* 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 open() for a
relative pathname). In this case, dirfd must be a directory
that was opened for reading (O_RDONLY) or using the O_PATH
flag.
If the pathname given in pathname is relative, and dirfd is not a
valid file descriptor, an error (EBADF) results. (Specifying an
invalid file descriptor number in dirfd can be used as a means to
ensure that pathname is absolute.)
openat2(2)
The openat2(2) system call is an extension of openat(), and
provides a superset of the features of openat(). It is
documented separately, in openat2(2).
