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, non-negative 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 parameter 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:
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 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.
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 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 an additional fcntl(2) F_SETFD operation 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, for example, the mount
options bsdgroups and
sysvgroups of
the ext2 filesystem, as 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 only applies 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 I/O is
synchronous, that is, at the completion of a read(2) or write(2), data is
guaranteed to have been transferred. 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, but should not be used
outside of the implementation of opendir(3).
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.
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.
O_EXCL is not
supported on NFSv2 or on Linux before kernel 2.6; it is
supported on Linux 2.6 and later, with NFSv3 or later.
In environments where NFS 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 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.
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 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 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) — it
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.
O_NONBLOCK or O_NDELAYWhen possible, the file is opened in non-blocking
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_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).)
pathname
already exists and O_CREAT and O_EXCL were used.
pathname
points outside your accessible address space.
pathname
refers to a regular file, too large to be opened; see
O_LARGEFILE above.
(POSIX.1-2001 specifies the error EOVERFLOW for this case.)
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.
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_CLOEXEC, O_DIRECTORY, O_NOATIME, and O_NOFOLLOW flags are Linux-specific. One
may have to define the _GNU_SOURCE macro to get their definitions.
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).
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 (2.1.130) these are all
synonymous under Linux.
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 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
userspace 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 user buffer and 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 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 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.
close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mount(2), mmap(2), openat(2), read(2), socket(2), stat(2), umask(2), unlink(2), write(2), fopen(3), fifo(7), feature_test_macros(7), path_resolution(7)
This page is part of release 2.79 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/.
|
This manpage is Copyright (C) 1992 Drew Eckhardt; 1993 Michael Haardt, Ian Jackson. 2008 Greg Banks 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. 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: Reorganised text for O_CREAT and mode |