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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_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_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 only available
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 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 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 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 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).)
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 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 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 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 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), fifo(7), path_resolution(7),
symlink(7)
This page is part of release 3.41 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 2012-05-01 OPEN(2)
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The Linux Programming Interface,
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