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open, creat — open and possibly create a file or device
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h>
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 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_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:
O_APPENDThe 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 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_ASYNCEnable 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.
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).
O_CREATIf 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 parent directory, see the mount options bsdgroups and
sysvgroups
described in mount(8)).
modespecifies the permissions to use in case a new file is created. This argument must be supplied whenO_CREATis specified inflags; ifO_CREATis not specified, thenmodeis ignored. The effective permissions are modified by the process'sumaskin 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; theopen() call that creates a read-only file may well return a read/write file descriptor.The following symbolic constants are provided for
mode:
S_IRWXU00700 user (file owner) has read, write and execute permission
S_IRUSR00400 user has read permission
S_IWUSR00200 user has write permission
S_IXUSR00100 user has execute permission
S_IRWXG00070 group has read, write and execute permission
S_IRGRP00040 group has read permission
S_IWGRP00020 group has write permission
S_IXGRP00010 group has execute permission
S_IRWXO00007 others have read, write and execute permission
S_IROTH00004 others have read permission
S_IWOTH00002 others have write permission
S_IXOTH00001 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 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_DIRECTORYIf 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.
O_EXCLEnsure 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 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 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_NOCTTYIf pathname
refers to a terminal device—see tty(4)\(emit will not
become the process's controlling terminal even if the
process does not have one.
O_NOFOLLOWIf 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_NDELAYWhen 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
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 EBADF.
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_GETFDandF_SETFD).Retrieving open file status flags using the fcntl(2)
F_GETFLoperation: the returned flags will include the bitO_PATH.Passing the file descriptor as the
dirfdargument of openat(2) and the other "*at()" system calls.Passing the file descriptor to another process via a UNIX domain socket (see
SCM_RIGHTSin unix(7)).
When O_PATH is
specified in flags, flag bits other
than O_DIRECTORY and
O_NOFOLLOW are
ignored.
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_SYNCThe 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.
O_TRUNCIf 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).
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).)
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.
pathname
already exists and O_CREAT and O_EXCL were used.
pathname
points outside your accessible address space.
See EOVERFLOW.
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).
The filesystem does not support the O_DIRECT flag. See NOTES for more information.
pathname
refers to a directory and the access requested involved
writing (that is, O_WRONLY or O_RDWR is set).
Too many symbolic links were encountered in
resolving pathname, or O_NOFOLLOW was specified but
pathname was a
symbolic link.
The process already has the maximum number of files open.
pathname was
too long.
The system limit on the total number of open files has been reached.
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.)
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.
Insufficient kernel memory was available.
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 directory.
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 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.
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).
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 (see fcntl(2)).
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 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 to be used only 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
implements only O_SYNC, but
glibc maps O_DSYNC and
O_RSYNC to the same numerical
value as O_SYNC. Most Linux
filesystems 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 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 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
restrictions.
O_DIRECT support was added
under Linux in kernel version 2.4.10. Older Linux kernels
simply 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 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), fifo(7), path_resolution(7), symlink(7)
This page is part of release 3.54 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man−pages/.
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This manpage is Copyright (C) 1992 Drew Eckhardt; and Copyright (C) 1993 Michael Haardt, Ian Jackson. and Copyright (C) 2008 Greg Banks %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified 1993-07-21 by Rik Faith <faithcs.unc.edu> Modified 1994-08-21 by Michael Haardt Modified 1996-04-13 by Andries Brouwer <aebcwi.nl> Modified 1996-05-13 by Thomas Koenig Modified 1996-12-20 by Michael Haardt Modified 1999-02-19 by Andries Brouwer <aebcwi.nl> Modified 1998-11-28 by Joseph S. Myers <jsm28hermes.cam.ac.uk> Modified 1999-06-03 by Michael Haardt Modified 2002-05-07 by Michael Kerrisk <mtk.manpagesgmail.com> Modified 2004-06-23 by Michael Kerrisk <mtk.manpagesgmail.com> 2004-12-08, mtk, reordered flags list alphabetically 2004-12-08, Martin Pool <mbpsourcefrog.net> (& mtk), added O_NOATIME 2007-09-18, mtk, Added description of O_CLOEXEC + other minor edits 2008-01-03, mtk, with input from Trond Myklebust <trond.myklebustfys.uio.no> and Timo Sirainen <tssiki.fi> Rewrite description of O_EXCL. 2008-01-11, Greg Banks <gnbmelbourne.sgi.com>: add more detail on O_DIRECT. 2008-02-26, Michael Haardt: Reorganized text for O_CREAT and mode FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and O_TTYINIT. Eventually these may need to be documented. --mtk FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC. |