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open, creat - open and possibly create a file or device
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
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:
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
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).
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
The following symbolic constants are provided for mode:
S_IRWXU 00700 user (file owner) has read, write and execute
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).
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.
(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.
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.
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
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.
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
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
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.
pathname was too long.
ENFILE The system limit on the total number of open files has been
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
ENOMEM Insufficient kernel memory was available.
ENOSPC 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
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.
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
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.
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, 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
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
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
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
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
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
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),
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