OPEN(2) Linux Programmer's Manual OPEN(2)
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 cur-
rently open for the process.
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 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 opera-
tion). 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 ini-
tially not shared with any other process, but sharing may arise via
The parameter 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) mod-
ified 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 sys-
tems 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, pseudo-terminals, sockets, and (since
Linux 2.6) pipes and FIFOs. See fcntl(2) 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 direc-
tory (depending on filesystem type and mount options, and the
mode of the parent directory, see, for example, the mount
options bsdgroups and sysvgroups of the ext2 filesystem, as
described in mount(8)).
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, that is, at the completion of a read(2)
or write(2), 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 bound-
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 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 host-
name and pid), 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 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)).
(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 filesystems. 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 pro-
cess 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 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.
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.
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 file descriptor.
The following symbolic constants are provided for mode:
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
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 per-
mission 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_resolu-
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EFAULT pathname points outside your accessible address space.
EFBIG pathname refers to a regular file, too large to be opened; see
O_LARGEFILE above. (POSIX.1-2001 specifies the error EOVERFLOW
for this case.)
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.
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 filesystem 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_NOATIME, O_NOFOLLOW, and O_DIRECTORY
flags are Linux specific. One may have to define the _GNU_SOURCE macro
to get their definitions.
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 typ-
ically used to open devices in order to get a file descriptor for use
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementa-
tions. 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 intro-
duced 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.
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, corre-
sponding to the flags O_SYNC, O_DSYNC and O_RSYNC. Currently (2.1.130)
these are all synonymous under Linux.
Note that open() can open device special files, but creat() cannot cre-
ate 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 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.
close(2), dup(2), fcntl(2), link(2), lseek(2), mknod(2), mount(2),
mmap(2), openat(2), read(2), socket(2), stat(2), umask(2), unlink(2),
write(2), fopen(3), fifo(7), feature_test_macros(7), path_resolution(7)
Linux 2005-06-22 OPEN(2)