The open() system call is used to convert a pathname into a file
descriptor (a small, non-negative integer for use in subsequent I/O as with
read, write, etc.). When the call is successful, the file
descriptor returned will be the lowest file descriptor not currently open for
the process. This call creates a new open file, not shared with any other
process. (But shared open files may arise via the fork(2) system call.)
The new file descriptor is set to remain open across exec functions (see
fcntl(2)). The file offset is set to the beginning of the file.
The parameter flags is one of O_RDONLY, O_WRONLY or
O_RDWR which request opening the file read-only, write-only or
read/write, respectively, bitwise-or'd with zero or more of the
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 bsdgroups and sysvgroups of the
ext2 filesystem, as described in mount(8)).
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 its 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 fs (e.g.,
incorporating hostname and pid), use link(2) to make a link to the
lockfile. If link() 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.
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 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
The file is opened in append mode. Before each
write, the file pointer 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 can't be done without a race condition.
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(4). This mode
need not have any effect on files other than FIFOs.
The file is opened for synchronous I/O. Any writes
on the resulting file descriptor will block the calling process until the
data has been physically written to the underlying hardware. See
RESTRICTIONS below, though.
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. The headers from glibc 2.0.100 and later include a definition of
this flag; kernels before 2.1.126 will ignore it ifused.
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.
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 the read(2) or write(2) system
call, 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 to 512-byte boundaries suffices.
A semantically similar interface for block devices is described in
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, and sockets. See fcntl(2) for further
(LFS) Allow files whose sizes cannot be represented in an
off_t (but can be represented in an off64_t) to be
Some of these optional flags can be altered using fcntl after the file
has been opened.
The argument mode specifies the permissions to use in case a new file is
created. It is 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
The following symbolic constants are provided for mode:
00700 user (file owner) has read, write and execute
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 permisson
00001 others have execute permission
mode must be specified when O_CREAT is in the flags, and is
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). 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 e.g. 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 atime, ctime, mtime fields are set to the
current time, and so are the ctime and mtime fields of the parent directory.
Otherwise, if the file is modified because of the O_TRUNC flag, its ctime and
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. (See also
pathname already exists and O_CREAT and
O_EXCL were used.
pathname points outside your accessible address
pathname refers to a directory and the access
requested involved writing (that is, O_WRONLY or O_RDWR is
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
pathname was too long.
The system limit on the total number of open files has been
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
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.
pathname refers to a file on a read-only filesystem
and write access was requested.
pathname refers to an executable image which is
currently being executed and write access was requested.
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, SVID, POSIX, X/OPEN, BSD 4.3. The 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 various among
implementations. On many systems the file is actually truncated.
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 same
name, but without alignment restrictions. Support was added under Linux in
kernel version 2.4.10. Older Linux kernels simply ignore this flag.