OPEN(2) Linux Programmer's Manual OPEN(2)
open, openat, creat - open and possibly create a file
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);
int openat(int dirfd, const char *pathname, int flags);
int openat(int dirfd, const char *pathname, int flags, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
The open() system call opens the file specified by pathname. If the
specified file does not exist, it may optionally (if O_CREAT is speci‐
fied in flags) be created by open().
The return value of open() is a file descriptor, a small, nonnegative
integer that is used in subsequent system calls (read(2), write(2),
lseek(2), fcntl(2), etc.) to refer to the open file. The file descrip‐
tor 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. The open file description records the
file offset and the file status flags (see below). A file descriptor
is a reference to an open file description; this reference is unaf‐
fected if pathname is subsequently removed or modified to refer to a
different file. For further details on open file descriptions, see
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_TMPFILE, 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 creation flags affect the semantics of the open operation itself,
while the file status flags affect the semantics of subsequent I/O
operations. The file status flags can be retrieved and (in some cases)
modified; see fcntl(2) for details.
The full list of file creation flags and file status flags is as fol‐
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). The modification of the file offset and the write
operation are performed as a single atomic step.
O_APPEND may lead to corrupted files on NFS filesystems 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
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
available only for terminals, pseudoterminals, sockets, and
(since Linux 2.6) pipes and FIFOs. See fcntl(2) for further
details. See also BUGS, below.
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.
Note that the use of this flag is essential in some multi‐
threaded programs, because 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 and
attempts to set its close-on-exec flag using fcntl(2) at the
same time as another thread does a fork(2) plus execve(2).
Depending on the order of execution, the race may lead to the
file descriptor returned by open() being unintentionally leaked
to the program executed by the child process created by fork(2).
(This kind of race is in principle possible for any system call
that creates a file descriptor whose close-on-exec flag should
be set, and various other Linux system calls provide an equiva‐
lent of the O_CLOEXEC flag to deal with this problem.)
If pathname does not exist, create it as a regular file.
The owner (user ID) of the new file is set to the effective user
ID of the process.
The group ownership (group ID) of the new file is set either to
the effective group ID of the process (System V semantics) or to
the group ID of the parent directory (BSD semantics). On Linux,
the behavior depends on whether the set-group-ID mode bit is set
on the parent directory: if that bit is set, then BSD semantics
apply; otherwise, System V semantics apply. For some filesys‐
tems, the behavior also depends on the bsdgroups and sysvgroups
mount options described in mount(8)).
The mode argument specifies the file mode bits be applied when a
new file is created. This argument must be supplied when
O_CREAT or O_TMPFILE is specified in flags; if neither O_CREAT
nor O_TMPFILE is specified, then mode is ignored. The effective
mode is modified by the process's umask in the usual way: in the
absence of a default ACL, the mode of the created file is
(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
According to POSIX, the effect when other bits are set in mode
is unspecified. On Linux, the following bits are also honored
S_ISUID 0004000 set-user-ID bit
S_ISGID 0002000 set-group-ID bit (see inode(7)).
S_ISVTX 0001000 sticky bit (see inode(7)).
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 an effort to transfer data
synchronously, but does not give the guarantees of the O_SYNC
flag that data and necessary metadata are transferred. To guar‐
antee synchronous I/O, 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 was added in kernel version 2.1.126, to avoid denial-of-
service problems if opendir(3) is called on a FIFO or tape
Write operations on the file will complete according to the
requirements of synchronized I/O data integrity completion.
By the time write(2) (and similar) return, the output data has
been transferred to the underlying hardware, along with any file
metadata that would be required to retrieve that data (i.e., as
though each write(2) was followed by a call to fdatasync(2)).
See NOTES below.
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() fails with the error EEXIST.
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.
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 can be employed only if one of the following condi‐
tions is true:
* The effective UID of the process matches the owner UID of the
* The calling process has the CAP_FOWNER capability in its user
namespace and the owner UID of the file has a mapping in the
This flag is intended for use by indexing or backup programs,
where its use can significantly reduce the amount of disk activ‐
ity. 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 process
does not have one.
If pathname is a symbolic link, then the open fails, with the
error ELOOP. Symbolic links in earlier components of the path‐
name will still be followed. (Note that the ELOOP error that
can occur in this case is indistinguishable from the case where
an open fails because there are too many symbolic links found
while resolving components in the prefix part of the pathname.)
This flag is a FreeBSD extension, which was added to Linux in
version 2.1.126, and has subsequently been standardized in
See also O_PATH below.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode. Neither
the open() nor any subsequent I/O operations on the file
descriptor which is returned will cause the calling process to
Note that the setting of this flag has no effect on the opera‐
tion of poll(2), select(2), epoll(7), and similar, since those
interfaces merely inform the caller about whether a file
descriptor is "ready", meaning that an I/O operation performed
on the file descriptor with the O_NONBLOCK flag clear would not
Note that this flag has no effect for regular files and block
devices; that is, I/O operations will (briefly) block when
device activity is required, regardless of whether O_NONBLOCK is
set. Since O_NONBLOCK semantics might eventually be imple‐
mented, applications should not depend upon blocking behavior
when specifying this flag for regular files and block devices.
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_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), ioctl(2), mmap(2))
fail with the error EBADF.
The following operations can be performed on the resulting file
* fchdir(2), if the file descriptor refers to a directory
(since Linux 3.5).
* fstat(2) (since Linux 3.6).
* fstatfs(2) (since Linux 3.12).
* 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 openat()
and the other "*at()" system calls. This includes linkat(2)
with AT_EMPTY_PATH (or via procfs using AT_SYMLINK_FOLLOW)
even if the file is not a directory.
* 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_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
Opening a file or directory with the O_PATH flag requires no
permissions on the object itself (but does require execute per‐
mission on the directories in the path prefix). Depending on
the subsequent operation, a check for suitable file permissions
may be performed (e.g., fchdir(2) requires execute permission on
the directory referred to by its file descriptor argument). By
contrast, obtaining a reference to a filesystem object by open‐
ing it with the O_RDONLY flag requires that the caller have read
permission on the object, even when the subsequent operation
(e.g., fchdir(2), fstat(2)) does not require read permission on
If pathname is a symbolic link and 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 fchownat(2), fstatat(2), linkat(2),
and readlinkat(2) with an empty pathname to have the calls oper‐
ate on the symbolic link.
If pathname refers to an automount point that has not yet been
triggered, so no other filesystem is mounted on it, then the
call returns a file descriptor referring to the automount direc‐
tory without triggering a mount. fstatfs(2) can then be used to
determine if it is, in fact, an untriggered automount point
(.f_type == AUTOFS_SUPER_MAGIC).
One use of O_PATH for regular files is to provide the equivalent
of POSIX.1's O_EXEC functionality. This permits us to open a
file for which we have execute permission but not read permis‐
sion, and then execute that file, with steps something like the
fd = open("some_prog", O_PATH);
snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
execl(buf, "some_prog", (char *) NULL);
An O_PATH file descriptor can also be passed as the argument of
O_SYNC Write operations on the file will complete according to the
requirements of synchronized I/O file integrity completion (by
contrast with the synchronized I/O data integrity completion
provided by O_DSYNC.)
By the time write(2) (or similar) returns, the output data and
associated file metadata have been transferred to the underlying
hardware (i.e., as though each write(2) was followed by a call
to fsync(2)). See NOTES below.
O_TMPFILE (since Linux 3.11)
Create an unnamed temporary regular file. The pathname argument
specifies a directory; an unnamed inode will be created in that
directory's filesystem. Anything written to the resulting file
will be lost when the last file descriptor is closed, unless the
file is given a name.
O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and,
optionally, O_EXCL. If O_EXCL is not specified, then linkat(2)
can be used to link the temporary file into the filesystem, mak‐
ing it permanent, using code like the following:
fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
S_IRUSR | S_IWUSR);
/* File I/O on 'fd'... */
snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
In this case, the open() mode argument determines the file per‐
mission mode, as with O_CREAT.
Specifying O_EXCL in conjunction with O_TMPFILE prevents a tem‐
porary file from being linked into the filesystem in the above
manner. (Note that the meaning of O_EXCL in this case is dif‐
ferent from the meaning of O_EXCL otherwise.)
There are two main use cases for O_TMPFILE:
* Improved tmpfile(3) functionality: race-free creation of tem‐
porary files that (1) are automatically deleted when closed;
(2) can never be reached via any pathname; (3) are not sub‐
ject to symlink attacks; and (4) do not require the caller to
devise unique names.
* Creating a file that is initially invisible, which is then
populated with data and adjusted to have appropriate filesys‐
tem attributes (fchown(2), fchmod(2), fsetxattr(2), etc.)
before being atomically linked into the filesystem in a fully
formed state (using linkat(2) as described above).
O_TMPFILE requires support by the underlying filesystem; only a
subset of Linux filesystems provide that support. In the ini‐
tial implementation, support was provided in the ext2, ext3,
ext4, UDF, Minix, and shmem filesystems. Support for other
filesystems has subsequently been added as follows: XFS (Linux
3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and ubifs (Linux
If the file already exists and is a regular file and the access
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.
A call to creat() is equivalent to calling open() with flags equal to
The openat() system call operates in exactly the same way as open(),
except for the differences described here.
If the pathname given in pathname is relative, then it is interpreted
relative to the directory referred to by the file descriptor dirfd
(rather than relative to the current working directory of the calling
process, as is done by open() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then
pathname is interpreted relative to the current working directory of
the calling process (like open()).
If pathname is absolute, then dirfd is ignored.
open(), openat(), and creat() return the new file descriptor, or -1 if
an error occurred (in which case, errno is set appropriately).
open(), openat(), and creat() can fail with the following errors:
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‐
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 more information.
EINVAL Invalid value in flags.
EINVAL O_TMPFILE was specified in flags, but neither O_WRONLY nor
O_RDWR was specified.
EINVAL O_CREAT was specified in flags and the final component ("base‐
name") of the new file's pathname is invalid (e.g., it contains
characters not permitted by the underlying filesystem).
EISDIR pathname refers to a directory and the access requested involved
writing (that is, O_WRONLY or O_RDWR is set).
EISDIR pathname refers to an existing directory, O_TMPFILE and one of
O_WRONLY or O_RDWR were specified in flags, but this kernel ver‐
sion does not provide the O_TMPFILE functionality.
ELOOP Too many symbolic links were encountered in resolving pathname.
ELOOP pathname was a symbolic link, and flags specified O_NOFOLLOW but
EMFILE The per-process limit on the number of open file descriptors has
been reached (see the description of RLIMIT_NOFILE in getr‐
pathname was too long.
ENFILE The system-wide 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.
ENOENT A directory component in pathname does not exist or is a dan‐
gling symbolic link.
ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
O_WRONLY or O_RDWR were specified in flags, but this kernel ver‐
sion does not provide the O_TMPFILE functionality.
ENOMEM The named file is a FIFO, but memory for the FIFO buffer can't
be allocated because the per-user hard limit on memory alloca‐
tion for pipes has been reached and the caller is not privi‐
leged; see pipe(7).
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 FIFO open for reading.
ENXIO The file is a device special file and no corresponding device
ENXIO The file is a UNIX domain socket.
The filesystem containing pathname does not support O_TMPFILE.
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 (1<<31)-1 bytes; see also
O_LARGEFILE above. This is the error specified by POSIX.1; in
kernels before 2.6.24, Linux gave the error EFBIG for this case.
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.
EPERM The operation was prevented by a file seal; see fcntl(2).
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.
pathname refers to a file that is currently in use as a swap
file, and the O_TRUNC flag was specified.
pathname refers to a file that is currently being read by the
kernel (e.g. for module/firmware loading), and write access was
The O_NONBLOCK flag was specified, and an incompatible lease was
held on the file (see fcntl(2)).
The following additional errors can occur for openat():
EBADF dirfd is not a valid file descriptor.
pathname is a relative pathname and dirfd is a file descriptor
referring to a file other than a directory.
openat() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-spe‐
cific. One must define _GNU_SOURCE to obtain their definitions.
The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified in
POSIX.1-2001, but are specified in POSIX.1-2008. Since glibc 2.12, one
can obtain their definitions by defining either _POSIX_C_SOURCE with a
value greater than or equal to 200809L or _XOPEN_SOURCE with a value
greater than or equal to 700. In glibc 2.11 and earlier, one obtains
the definitions by defining _GNU_SOURCE.
As noted in feature_test_macros(7), feature test macros such as
_POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined before
including any header files.
Under Linux, the O_NONBLOCK flag is sometimes used in cases where one
wants to open but does not necessarily have the intention to read or
write. For example, this may be used to open a device in order to get
a file descriptor for use with ioctl(2).
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementa‐
tions. On many systems the file is actually truncated.
Note that open() can open device special files, but creat() cannot cre‐
ate them; use mknod(2) instead.
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 files in the /proc/[pid]/fd directory show the open file descrip‐
tors of the process with the PID pid. The files in the
/proc/[pid]/fdinfo directory show even more information about these
file descriptors. See proc(5) for further details of both of these
The Linux header file <asm/fcntl.h> doesn't define O_ASYNC; the (BSD-
derived) FASYNC synonym is defined instead.
Open file descriptions
The term open file description is the one used by POSIX to refer to the
entries in the system-wide table of open files. In other contexts,
this object is variously also called an "open file object", a "file
handle", an "open file table entry", or—in kernel-developer parlance—a
When a file descriptor is duplicated (using dup(2) or similar), the
duplicate refers to the same open file description as the original file
descriptor, and the two file descriptors consequently share the file
offset and file status flags. Such sharing can also occur between pro‐
cesses: a child process created via fork(2) inherits duplicates of its
parent's file descriptors, and those duplicates refer to the same open
Each open() of a file creates a new open file description; thus, there
may be multiple open file descriptions corresponding to a file inode.
On Linux, one can use the kcmp(2) KCMP_FILE operation to test whether
two file descriptors (in the same process or in two different pro‐
cesses) refer to the same open file description.
The POSIX.1-2008 "synchronized I/O" option specifies different variants
of synchronized I/O, and specifies the open() flags O_SYNC, O_DSYNC,
and O_RSYNC for controlling the behavior. Regardless of whether an
implementation supports this option, it must at least support the use
of O_SYNC for regular files.
Linux implements O_SYNC and O_DSYNC, but not O_RSYNC. Somewhat incor‐
rectly, glibc defines O_RSYNC to have the same value as O_SYNC.
(O_RSYNC is defined in the Linux header file <asm/fcntl.h> on HP PA-
RISC, but it is not used.)
O_SYNC provides synchronized I/O file integrity completion, meaning
write operations will flush data and all associated metadata to the
underlying hardware. O_DSYNC provides synchronized I/O data integrity
completion, meaning write operations will flush data to the underlying
hardware, but will only flush metadata updates that are required to
allow a subsequent read operation to complete successfully. Data
integrity completion can reduce the number of disk operations that are
required for applications that don't need the guarantees of file
To understand the difference between the two types of completion, con‐
sider two pieces of file metadata: the file last modification timestamp
(st_mtime) and the file length. All write operations will update the
last file modification timestamp, but only writes that add data to the
end of the file will change the file length. The last modification
timestamp is not needed to ensure that a read completes successfully,
but the file length is. Thus, O_DSYNC would only guarantee to flush
updates to the file length metadata (whereas O_SYNC would also always
flush the last modification timestamp metadata).
Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
However, when that flag was specified, most filesystems actually pro‐
vided the equivalent of synchronized I/O data integrity completion
(i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).
Since Linux 2.6.33, proper O_SYNC support is provided. However, to
ensure backward binary compatibility, O_DSYNC was defined with the same
value as the historical O_SYNC, and O_SYNC was defined as a new (two-
bit) flag value that includes the O_DSYNC flag value. This ensures
that applications compiled against new headers get at least O_DSYNC
semantics on pre-2.6.33 kernels.
C library/kernel differences
Since version 2.26, the glibc wrapper function for open() employs the
openat() system call, rather than the kernel's open() system call. For
certain architectures, this is also true in glibc versions before 2.26.
There are many infelicities in the protocol underlying NFS, affecting
amongst others O_SYNC and O_NDELAY.
On NFS filesystems 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
Opening the read or write end of a FIFO blocks until the other end is
also opened (by another process or thread). See fifo(7) for further
File access mode
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 file descriptor that can't be used for reading or writing.
This nonstandard access mode is used by some Linux drivers to return a
file descriptor that is to be used only for device-specific ioctl(2)
Rationale for openat() and other directory file descriptor APIs
openat() and the other system calls and library functions that take a
directory file descriptor argument (i.e., execveat(2), faccessat(2),
fanotify_mark(2), fchmodat(2), fchownat(2), fstatat(2), futimesat(2),
linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2),
renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2), mkfi‐
foat(3), and scandirat(3)) address two problems with the older inter‐
faces that preceded them. Here, the explanation is in terms of the
openat() call, but the rationale is analogous for the other interfaces.
First, openat() allows an application to avoid race conditions that
could occur when using open() to open files in directories other than
the current working directory. These race conditions result from the
fact that some component of the directory prefix given to open() could
be changed in parallel with the call to open(). Suppose, for example,
that we wish to create the file dir1/dir2/xxx.dep if the file
dir1/dir2/xxx exists. The problem is that between the existence check
and the file-creation step, dir1 or dir2 (which might be symbolic
links) could be modified to point to a different location. Such races
can be avoided by opening a file descriptor for the target directory,
and then specifying that file descriptor as the dirfd argument of (say)
fstatat(2) and openat(). The use of the dirfd file descriptor also has
* the file descriptor is a stable reference to the directory, even if
the directory is renamed; and
* the open file descriptor prevents the underlying filesystem from
being dismounted, just as when a process has a current working
directory on a filesystem.
Second, openat() allows the implementation of a per-thread "current
working directory", via file descriptor(s) maintained by the applica‐
tion. (This functionality can also be obtained by tricks based on the
use of /proc/self/fd/dirfd, but less efficiently.)
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. Since Linux 2.6.0, alignment to the logical block size
of the underlying storage (typically 512 bytes) suffices. The logical
block size can be determined using the ioctl(2) BLKSSZGET operation or
from the shell using the command:
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, in which case open() fails with the error EINVAL if it
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.
The behavior of O_DIRECT with NFS will differ from local filesystems.
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 bypass the page cache only 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 config‐
ured 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.
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.
One must check for two different error codes, EISDIR and ENOENT, when
trying to determine whether the kernel supports O_TMPFILE functional‐
When both O_CREAT and O_DIRECTORY are specified in flags and the file
specified by pathname does not exist, open() will create a regular file
(i.e., O_DIRECTORY is ignored).
chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2),
mknod(2), mmap(2), mount(2), open_by_handle_at(2), read(2), socket(2),
stat(2), umask(2), unlink(2), write(2), fopen(3), acl(5), fifo(7),
inode(7), path_resolution(7), symlink(7)
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latest version of this page, can be found at
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