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 integer 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 Linux-specific 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, 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_CREAT,
O_EXCL, O_NOCTTY, 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 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 positioned at the end of the file, as if with lseek(2).
O_APPEND may lead to corrupted files on NFS file systems 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 descriptor. This feature is only available for terminals, pseudo-terminals, 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 file system type and mount options, and the mode of the parent directory, see the mount options bsdgroups and sysv-
groups described in mount(8)).
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 only applies 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
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 spe-
cial 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 at an effort to transfer data synchronously, but does not give the guarantees of the O_SYNC that data
and necessary metadata are transferred. To guarantee synchronous I/O the 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).
If pathname is not a directory, cause the open to fail. This flag is Linux-specific, 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, but should not be used outside of the implementa-
tion of opendir(3).
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. The behavior of O_EXCL is undefined if O_CREAT is not specified.
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.
On NFS, O_EXCL is only supported 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 per-
form 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 file system (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.
(LFS) Allow files whose sizes cannot be represented in an off_t (but can be represented in an off64_t) to be opened. The _LARGE-
FILE64_SOURCE macro must be defined 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 obtaining 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 file
systems. One example is NFS, where the server maintains the access time.
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.
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 calling process to wait. For the handling of FIFOs (named pipes), see also fifo(7). For a discus-
sion of the effect of O_NONBLOCK in conjunction with mandatory file locks and with file leases, see fcntl(2).
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 written 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
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_resolution(7).)
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 han-
dler; see signal(7).
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 sym-
ENOMEM Insufficient kernel memory was available.
ENOSPC pathname was to be created but the device containing pathname has no room for the new file.
A component used as a directory in pathname is not, in fact, a directory, or O_DIRECTORY was specified and pathname was not a direc-
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 spe-
cial file and no corresponding device exists.
pathname refers to a regular file that is too large to be opened. The usual scenario 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
EROFS pathname refers to a file on a read-only file system 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 (see fcntl(2)).
SVr4, 4.3BSD, POSIX.1-2001. The O_DIRECTORY, O_NOATIME, and O_NOFOLLOW flags are Linux-specific, and one may need to define _GNU_SOURCE 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 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 combination
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 only to be used for device-
specific ioctl(2) operations.
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementations. On many systems 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 only implements O_SYNC, but glibc maps O_DSYNC and O_RSYNC to the same numerical value as O_SYNC Most Linux file systems
don't actually implement the POSIX O_SYNC semantics, which require all metadata updates of a write to be on disk on returning to userspace,
but only the O_DSYNC semantics, 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 mknod(2) instead.
On NFS file systems with UID mapping enabled, open() may return a file descriptor but, for example, read(2) requests are denied with EAC-
CES. This is because the client performs open() by checking the permissions, but UID mapping is performed by the server upon read and
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 userspace buffers and the file offset of I/Os. In Linux
alignment restrictions vary by file system and kernel version and might be absent entirely. However there is currently no file sys-
tem-independent interface for an application to discover these restrictions for a given file or file system. Some file systems 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 file system. Under Linux 2.6, alignment to 512-byte boundaries suffices.
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 restrictions.
O_DIRECT support was added under Linux in kernel version 2.4.10. Older Linux kernels simply ignore this flag. Some file systems 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 file system 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 file systems. Older kernels, or kernels configured in certain ways, may not sup-
port this combination. The NFS protocol does not support passing the flag to the server, so O_DIRECT I/O will only bypass the page cache
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 mind-controlling substances." -- Linus
Currently, it is not possible to enable signal-driven I/O by specifying O_ASYNC when calling 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), feature_test_macros(7), fifo(7), path_resolution(7), symlink(7)
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Linux 2010-06-14 OPEN(2)