NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON
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, 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 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,
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).
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 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).
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. 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 only supported on NFS when using NFSv3 or later on kernel 2.6
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 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 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_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 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, non-standard 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 non-standard 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 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 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.
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), feature_test_macros(7), fifo(7),
path_resolution(7), symlink(7)
This page is part of release 3.23 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 2009-09-20 OPEN(2)