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 cur-
rently 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 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 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_CLOEXEC,
O_CREAT, O_DIRECTORY, O_EXCL, O_NOCTTY, O_NOFOLLOW, O_TRUNC, and
O_TTY_INIT. 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) modi-
fied 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 filesys-
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
fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag. Addi-
tionally, 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 condi-
tions 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 direc-
tory (depending on filesystem 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 cre-
ated. 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 applies only 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
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.
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.
O_EXCL Ensure that this call creates the file: if this flag is speci-
fied in conjunction with O_CREAT, and pathname already exists,
then open() will fail.
When these two flags are specified, symbolic links are not fol-
lowed: if pathname is a symbolic link, then open() fails regard-
less 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 supported only 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 filesystem (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 effec-
tive 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 process
does not have one.
mandatory file locks and with file leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two purposes: to
indicate a location in the filesystem tree and to perform opera-
tions that act purely at the file descriptor level. The file
itself is not opened, and other file operations (e.g., read(2),
write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2)) fail with
the error EBADF.
The following operations can be performed on the resulting file
* close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux
* Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,
* Getting and setting file descriptor flags (fcntl(2) F_GETFD
* Retrieving open file status flags using the fcntl(2) F_GETFL
operation: the returned flags will include the bit O_PATH.
* Passing the file descriptor as the dirfd argument of ope-
nat(2) and the other "*at()" system calls.
* Passing the file descriptor to another process via a UNIX
domain socket (see SCM_RIGHTS in unix(7)).
When O_PATH is specified in flags, flag bits other than O_DIREC-
TORY and O_NOFOLLOW are ignored.
If the O_NOFOLLOW flag is also specified, then the call returns
a file descriptor referring to the symbolic link. This file
descriptor can be used as the dirfd argument in calls to fchow-
nat(2), fstatat(2), linkat(2), and readlinkat(2) with an empty
pathname to have the calls operate on the symbolic link.
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
of pathname, or the file did not exist yet and write access to
the parent directory is not allowed. (See also path_resolu-
EDQUOT Where O_CREAT is specified, the file does not exist, and the
user's quota of disk blocks or inodes on the filesystem has been
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 sig-
nal handler; see signal(7).
EINVAL The filesystem does not support the O_DIRECT flag. See NOTES for
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
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_DIRECTORY, O_NOATIME, O_NOFOLLOW,
and O_PATH flags are Linux-specific, and one may need to define
_GNU_SOURCE (before including any header files) to obtain their defini-
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 typ-
ically used to open devices in order to get a file descriptor for use
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 to be used only for device-specific
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementa-
tions. 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, corre-
sponding to the flags O_SYNC, O_DSYNC, and O_RSYNC. Currently
(2.6.31), Linux implements only O_SYNC, but glibc maps O_DSYNC and
O_RSYNC to the same numerical value as O_SYNC. Most Linux filesystems
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 user-space buffers and the file offset of I/Os. In Linux
alignment restrictions vary by filesystem and kernel version and might
be absent entirely. However there is currently no filesystem-indepen-
dent interface for an application to discover these restrictions for a
given file or filesystem. Some filesystems provide their own inter-
faces for doing so, for example the XFS_IOC_DIOINFO operation in
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 filesystem. Under Linux 2.6, alignment to 512-byte boundaries suf-
O_DIRECT I/Os should never be run concurrently with the fork(2) system
call, if the memory buffer is a private mapping (i.e., any mapping cre-
ated with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
on the heap and statically allocated buffers). Any such I/Os, whether
submitted via an asynchronous I/O interface or from another thread in
the process, should be completed before fork(2) is called. Failure to
do so can result in data corruption and undefined behavior in parent
and child processes. This restriction does not apply when the memory
buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with
the MAP_SHARED flag. Nor does this restriction apply when the memory
buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that
it will not be available to the child after fork(2).
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 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 filesystems may not imple-
ment 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 filesystem 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.
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 sub-
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),
This page is part of release 3.54 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 2013-08-09 OPEN(2)
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