OPEN(2) Linux Programmer's Manual OPEN(2)
open, creat - open and possibly create a file or device
int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char *pathname, mode_t mode);
Given a pathname for a file, open() returns a file descriptor, a small, nonnegative inte-
ger for use in subsequent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). 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. This entry records the file offset and the file status flags (modifiable via
the fcntl(2) F_SETFL operation). A file descriptor is a reference to one of these
entries; this reference is unaffected if pathname is subsequently removed or modified to
refer to a different file. The new open file description is initially not shared with any
other process, but sharing may arise via fork(2).
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, respec-
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_TRUNC, and O_TTY_INIT. The file status flags are all of the remaining flags
listed below. The distinction between these two groups of flags is that the file status
flags can be retrieved and (in some cases) modified using fcntl(2). The full list of file
creation flags and file status flags is as follows:
The file is opened in append mode. Before each write(2), the file offset is posi-
tioned at the end of the file, as if with lseek(2). 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.
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 descrip-
tor. This feature is available only for terminals, pseudoterminals, sockets, and
(since Linux 2.6) pipes and FIFOs. See fcntl(2) for further details.
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. Additionally, use of this flag is essential in some multithreaded
programs since 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 at the same time as another thread does a fork(2) plus execve(2).
If the file does not exist it will be created. The owner (user ID) of the file is
set to the effective user ID of the process. The group ownership (group ID) is set
either to the effective group ID of the process or to the group ID of the parent
directory (depending on filesystem type and mount options, and the mode of the par-
ent directory, see the mount options bsdgroups and sysvgroups described in
mode specifies the permissions to use in case a new file is created. This argument
must be supplied when O_CREAT is specified in flags; if O_CREAT is not specified,
then mode is ignored. The effective permissions are modified by the process's
umask in the usual way: The permissions of the created file are (mode & ~umask).
Note that this 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
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
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 syn-
chronously, but does not give the guarantees of the O_SYNC flag that data and nec-
essary 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
If pathname is not a directory, cause the open to fail. This flag is Linux-spe-
cific, and 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_EXCL Ensure that this call creates the file: if this flag is specified in conjunction
with O_CREAT, and pathname already exists, then open() will fail.
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 to.
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
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be repre-
sented 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 fea-
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 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
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.
If pathname is a symbolic link, then the open fails. This is a FreeBSD extension,
which was added to Linux in version 2.1.126. Symbolic links in earlier components
of the pathname will still be followed. See also O_NOPATH below.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode. Neither the open() nor any
subsequent operations on the file descriptor which is returned will cause the call-
ing process to wait. 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), mmap(2)) fail with the error
The following operations can be performed on the resulting file descriptor:
* close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux 3.6).
* 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(2) and the other
"*at()" system calls.
* 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_DIRECTORY and O_NOFOLLOW
If 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.
O_SYNC The file is opened for synchronous I/O. Any write(2)s on the resulting file
descriptor will block the calling process until the data has been physically writ-
ten to the underlying hardware. But see NOTES below.
If the file already exists and is a regular file and the open 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.
Some of these optional flags can be altered using fcntl(2) after the file has been opened.
creat() is equivalent to open() with flags equal to O_CREAT|O_WRONLY|O_TRUNC.
open() and creat() return the new file descriptor, or -1 if an error occurred (in which
case, errno is set appropriately).
EACCES The requested access to the file is not allowed, or search permission is denied for
one of the directories in the path prefix of pathname, or the file did not exist
yet and write access to the parent directory is not allowed. (See also path_reso-
EDQUOT Where O_CREAT is specified, the file does not exist, and the user's quota of disk
blocks or inodes on the filesystem has been exhausted.
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EFAULT pathname points outside your accessible address space.
EFBIG See EOVERFLOW.
EINTR While blocked waiting to complete an open of a slow device (e.g., a FIFO; see
fifo(7)), the call was interrupted by a signal handler; see signal(7).
EINVAL The filesystem does not support the O_DIRECT flag. See NOTES for more information.
EISDIR pathname refers to a directory and the access requested involved writing (that is,
O_WRONLY or O_RDWR is set).
ELOOP Too many symbolic links were encountered in resolving pathname, or O_NOFOLLOW was
specified but pathname was a symbolic link.
EMFILE The process already has the maximum number of files open.
pathname was too long.
ENFILE The system limit on the total number of open files has been reached.
ENODEV pathname refers to a device special file and no corresponding device exists. (This
is a Linux kernel bug; in this situation ENXIO must be returned.)
ENOENT O_CREAT is not set and the named file does not exist. Or, a directory component in
pathname does not exist or is a dangling symbolic link.
ENOMEM Insufficient kernel memory was available.
ENOSPC pathname was to be created but the device containing pathname has no room for the
A component used as a directory in pathname is not, in fact, a directory, or
O_DIRECTORY was specified and pathname was not a directory.
ENXIO O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no process has the file
open for reading. Or, the file is a device special file and no corresponding
pathname refers to a regular file that is too large to be opened. The usual sce-
nario here is that an application compiled on a 32-bit platform without
-D_FILE_OFFSET_BITS=64 tried to open a file whose size exceeds (2<<31)-1 bits; see
also O_LARGEFILE above. This is the error specified by POSIX.1-2001; in kernels
before 2.6.24, Linux gave the error EFBIG for this case.
EPERM The O_NOATIME flag was specified, but the effective user ID of the caller did not
match the owner of the file and the caller was not privileged (CAP_FOWNER).
EROFS pathname refers to a file on a read-only filesystem and write access was requested.
pathname refers to an executable image which is currently being executed and write
access was requested.
The O_NONBLOCK flag was specified, and an incompatible lease was held on the file
SVr4, 4.3BSD, POSIX.1-2001. The O_DIRECTORY, O_NOATIME, O_NOFOLLOW, and O_PATH flags are
Linux-specific, and one may need to define _GNU_SOURCE (before including any header files)
to obtain their definitions.
The O_CLOEXEC flag is not specified in POSIX.1-2001, but is specified in POSIX.1-2008.
O_DIRECT is not specified in POSIX; one has to define _GNU_SOURCE (before including any
header files) to get its definition.
Under Linux, the O_NONBLOCK flag indicates that one wants to open but does not necessarily
have the intention to read or write. This is typically used to open devices in order to
get a file descriptor for use with ioctl(2).
Unlike the other values that can be specified in flags, the access mode values O_RDONLY,
O_WRONLY, and O_RDWR, do not specify individual bits. Rather, they define the low order
two bits of flags, and are defined respectively as 0, 1, and 2. In other words, the com-
bination O_RDONLY | O_WRONLY is a logical error, and certainly does not have the same
meaning as O_RDWR. Linux reserves the special, nonstandard access mode 3 (binary 11) in
flags to mean: check for read and write permission on the file and return a descriptor
that can't be used for reading or writing. This nonstandard access mode is used by some
Linux drivers to return a descriptor that is to be used only for device-specific ioctl(2)
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many sys-
tems the file is actually truncated.
There are many infelicities in the protocol underlying NFS, affecting amongst others
O_SYNC and O_NDELAY.
POSIX provides for three different variants of synchronized I/O, corresponding to the
flags O_SYNC, O_DSYNC, and O_RSYNC. Currently (2.6.31), Linux implements only O_SYNC, but
glibc maps O_DSYNC and O_RSYNC to the same numerical value as O_SYNC. Most Linux filesys-
tems don't actually implement the POSIX O_SYNC semantics, which require all metadata
updates of a write to be on disk on returning to user space, but only the O_DSYNC seman-
tics, which require only actual file data and metadata necessary to retrieve it to be on
disk by the time the system call returns.
Note that open() can open device special files, but creat() cannot create them; use
On NFS filesystems with UID mapping enabled, open() may return a file descriptor but, for
example, read(2) requests are denied with EACCES. This is because the client performs
open() by checking the permissions, but UID mapping is performed by the server upon read
and write requests.
If the file is newly created, its st_atime, st_ctime, st_mtime fields (respectively, time
of last access, time of last status change, and time of last modification; see stat(2))
are set to the current time, and so are the st_ctime and st_mtime fields of the parent
directory. Otherwise, if the file is modified because of the O_TRUNC flag, its st_ctime
and st_mtime fields are set to the current time.
The O_DIRECT flag may impose alignment restrictions on the length and address of user-
space buffers and the file offset of I/Os. In Linux alignment restrictions vary by
filesystem and kernel version and might be absent entirely. However there is currently no
filesystem-independent interface for an application to discover these restrictions for a
given file or filesystem. Some filesystems provide their own interfaces for doing so, for
example the XFS_IOC_DIOINFO operation in xfsctl(3).
Under Linux 2.4, transfer sizes, and the alignment of the user buffer and the file offset
must all be multiples of the logical block size of the filesystem. Under Linux 2.6,
alignment to 512-byte boundaries suffices.
O_DIRECT I/Os should never be run concurrently with the fork(2) system call, if the memory
buffer is a private mapping (i.e., any mapping created with the mmap(2) MAP_PRIVATE flag;
this includes memory allocated on the heap and statically allocated buffers). Any such
I/Os, whether submitted via an asynchronous I/O interface or from another thread in the
process, should be completed before fork(2) is called. Failure to do so can result in
data corruption and undefined behavior in parent and child processes. This restriction
does not apply when the memory buffer for the O_DIRECT I/Os was created using shmat(2) or
mmap(2) with the MAP_SHARED flag. Nor does this restriction apply when the memory buffer
has been advised as MADV_DONTFORK with madvise(2), ensuring that it will not be available
to the child after fork(2).
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment restrictions similar
to those of Linux 2.4. IRIX has also a fcntl(2) call to query appropriate alignments, and
sizes. FreeBSD 4.x introduced a flag of the same name, but without alignment restric-
O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels sim-
ply ignore this flag. Some filesystems may not implement the flag and open() will fail
with EINVAL if it is used.
Applications should avoid mixing O_DIRECT and normal I/O to the same file, and especially
to overlapping byte regions in the same file. Even when the filesystem correctly handles
the coherency issues in this situation, overall I/O throughput is likely to be slower than
using either mode alone. Likewise, applications should avoid mixing mmap(2) of files with
direct I/O to the same files.
The behaviour of O_DIRECT with NFS will differ from local filesystems. Older kernels, or
kernels configured in certain ways, may not support this combination. The NFS protocol
does not support passing the flag to the server, so O_DIRECT I/O will bypass the page
cache only on the client; the server may still cache the I/O. The client asks the server
to make the I/O synchronous to preserve the synchronous semantics of O_DIRECT. Some
servers will perform poorly under these circumstances, especially if the I/O size is
small. Some servers may also be configured to lie to clients about the I/O having reached
stable storage; this will avoid the performance penalty at some risk to data integrity in
the event of server power failure. The Linux NFS client places no alignment restrictions
on O_DIRECT I/O.
In summary, O_DIRECT is a potentially powerful tool that should be used with caution. It
is recommended that applications treat use of O_DIRECT as a performance option which is
disabled by default.
"The thing that has always disturbed me about O_DIRECT is that the whole interface
is just stupid, and was probably designed by a deranged monkey on some serious
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when call-
ing open(); use fcntl(2) to enable this flag.
chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mmap(2),
mount(2), openat(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3),
fifo(7), path_resolution(7), symlink(7)
This page is part of release 3.55 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-08-09 OPEN(2)