open() - Unix, Linux System Call
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, 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.
The new file descriptor is set to remain open across an
FD_CLOEXEC file descriptor flag described in
is initially disabled).
The file offset is set to the beginning of the file (see
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() 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
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,
In addition, zero or more file creation flags and file status flags
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(), the file offset is positioned at the end of the file,
as if with
lseek(). 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
cant be done without a race condition.
Enable signal-driven I/O:
generate a signal (SIGIO by default, but this can be changed via
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.
for further details.
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 filesystem type and mount options,
and the mode of the parent directory, see, e.g., the mount options
sysvgroups of the ext2 filesystem, as described in
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, i.e., at the completion of a
data is guaranteed to have been transferred.
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 must fit the block
size of the device.
A semantically similar (but deprecated) interface for block devices
is described in
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.
When used with
O_CREAT, if the file already exists it is an error and the
open() will fail. In this context, a symbolic link exists, regardless
of where it points to.
O_EXCL is broken on NFS file systems; programs which rely on it for performing
locking tasks will contain a race condition. The solution for performing
atomic file locking using a lockfile is to create a unique file on
the same file system (e.g., incorporating hostname and pid), use
to make a link to the lockfile. If link() returns 0, the lock is
successful. Otherwise, use
on the unique file to check if its link count has increased to 2,
in which case the lock is also successful.
Allow files whose sizes cannot be represented in an
off_t (but can be represented in an
off64_t) to be opened.
(Since Linux 2.6.8)
Do not update the file last access time (st_atime in the inode)
when the file is
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.
pathname refers to a terminal device see
it will not become the processs 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 be
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
For a discussion of the effect of
O_NONBLOCK in conjunction with mandatory file locks and with file leases, see
The file is opened for synchronous I/O. Any
write()s on the resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
But see RESTRICTIONS 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() after the file has been opened.
mode specifies the permissions to use in case a new file is created. It is
modified by the processs
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
The following symbolic constants are provided for
00700 user (file owner) has read, write and execute permission
00400 user has read permission
00200 user has write permission
00100 user has execute permission
00070 group has read, write and execute permission
00040 group has read permission
00020 group has write permission
00010 group has execute permission
00007 others have read, write and execute permission
00004 others have read permission
00002 others have write permission
00001 others have execute permission
mode must be specified when
O_CREAT is in the
flags, and is ignored otherwise.
creat() is equivalent to
flags equal to
open() and creat() return the new file descriptor, or -1 if an error occurred
(in which case, errno is set appropriately).
Note that open() can open device special files, but
creat() cannot create them; use
On NFS file systems with UID mapping enabled, open() may
return a file descriptor but e.g. read(2) requests are denied
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 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.
pathname already exists and
O_CREAT and O_EXCL were used.
pathname points outside your accessible address space.
pathname refers to a directory and the access requested involved writing
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.
pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.
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
pathname refers to a file on a read-only filesystem and write access was
pathname refers to an executable image which is currently being executed and
write access was requested.
O_NONBLOCK flag was specified, and an incompatible lease was held on the file
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).
SVr4, 4.3BSD, POSIX.1-2001. The
O_NOATIME, O_NOFOLLOW, and
O_DIRECTORY flags are Linux-specific.
One may have to define the
_GNU_SOURCE macro to get their definitions.
The (undefined) effect of
O_RDONLY | O_TRUNC varies among implementations. On many systems the file is actually
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 same name, but without alignment restrictions.
Support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.
One may have to define the _GNU_SOURCE macro to get its definition.
"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.
There are many infelicities in the protocol underlying NFS, affecting amongst others O_SYNC and O_NDELAY.
POSIX provides for three different variants of synchronised I/O, corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.