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OPEN(2) Linux Programmer's Manual OPEN(2)
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_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). 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_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.
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_CREAT
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 sysvgroups 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 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
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 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_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 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 (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 file systems. 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 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 discussion 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.
O_TRUNC
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_resolution(7).)
EDQUOT Where O_CREAT is specified, the file does not exist, and the
user's quota of disk blocks or inodes on the file system 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).
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.
ENAMETOOLONG
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 new file.
ENOTDIR
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 device exists.
EOVERFLOW
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 privileged (CAP_FOWNER).
EROFS pathname refers to a file on a read-only file system and write
access was requested.
ETXTBSY
pathname refers to an executable image which is currently
being executed and write access was requested.
EWOULDBLOCK
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 (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 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
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 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 user-space 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
system-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.
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 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 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 3.51 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/.
Linux 2013-02-18 OPEN(2)
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