open, openat, creat - open and possibly create a file
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> 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); /* Documented separately, in openat2(2): */ int openat2(int dirfd, const char *pathname, const struct open_how *how, size_t size);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
- Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
- Before glibc 2.10:
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 descriptor 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 unaffected if
subsequently removed or modified to refer to a different file. For
further details on open file descriptions, see NOTES.
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-
flags. The file creation
flags are O_CLOEXEC, O_CREAT,
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 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). 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 condition.
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.
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 multithreaded 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 equivalent of the O_CLOEXEC flag to deal with this problem.)
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 filesystems, the behavior also
depends on the
sysvgroups mount options
described in mount(8).
mode argument specifies the file mode bits to be applied
when a new file is created. If neither O_CREAT nor
O_TMPFILE is specified in
mode is ignored (and can thus be specified as 0, or simply
mode argument must be supplied
if O_CREAT or O_TMPFILE is specified
flags; if it is not supplied, some arbitrary bytes from the
stack will be applied as the file mode.
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
(mode & ~umask).
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
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
According to POSIX, the effect when other bits are set in
mode is unspecified. On Linux, the following bits are also
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 guarantee 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).
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
See NOTES below.
Ensure that this call creates the file: if this flag is specified in
conjunction with O_CREAT, and
exists, then open() fails with the error
When these two flags are specified, symbolic links are not followed:
pathname is a symbolic link, then open()
fails regardless 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
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
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
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 conditions is true:
The effective UID of the process matches the owner UID of the file.
The calling process has the CAP_FOWNER capability in its user namespace and the owner UID of the file has a mapping in the namespace.
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
tty(4)—it will not become the process's controlling
terminal even if the process does not have one.
If the trailing component (i.e., basename) of
pathname is a
symbolic link, then the open fails, with the error
ELOOP. Symbolic links in earlier components of the
pathname 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 POSIX.1-2008.
See also O_PATH below.
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 wait.
Note that the setting of this flag has no effect on the operation of
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
clear would not block.
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 implemented, applications should not depend upon blocking behavior when specifying this flag for regular files and block devices.
Obtain a file descriptor that can be used for two purposes: to indicate a location in the filesystem tree and to perform operations 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
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).
Getting and setting file descriptor flags (fcntl(2) F_GETFD and F_SETFD).
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
bits other than O_CLOEXEC,
O_DIRECTORY, and O_NOFOLLOW are
Opening a file or directory with the O_PATH flag requires no permissions on the object itself (but does require execute permission 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 opening 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 the object.
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
linkat(2), and readlinkat(2) with an
empty pathname to have the calls operate on the symbolic link.
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 directory 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 permission, and then execute that file, with steps something like the following:
char buf[PATH_MAX]; 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 fexecve(3).
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.
Create an unnamed temporary regular file. The
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
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, making it permanent, using code like the following:
char path[PATH_MAX]; fd = open("/path/to/dir", O_TMPFILE | O_RDWR, S_IRUSR | S_IWUSR); /* File I/O on 'fd'... */ linkat(fd, NULL, AT_FDCWD, "/path/for/file", AT_EMPTY_PATH); /* If the caller doesn't have the CAP_DAC_READ_SEARCH capability (needed to use AT_EMPTY_PATH with linkat(2)), and there is a proc(5) filesystem mounted, then the linkat(2) call above can be replaced with: snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd); linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file", AT_SYMLINK_FOLLOW); */
In this case, the open()
determines the file permission mode, as with
Specifying O_EXCL in conjunction with O_TMPFILE prevents a temporary file from being linked into the filesystem in the above manner. (Note that the meaning of O_EXCL in this case is different from the meaning of O_EXCL otherwise.)
There are two main use cases for O_TMPFILE:
Improved tmpfile(3) functionality: race-free creation of temporary files that (1) are automatically deleted when closed; (2) can never be reached via any pathname; (3) are not subject 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 filesystem 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 initial 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 4.9)
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.
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
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
pathname is absolute, then
dirfd is ignored.
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
Where O_CREAT is specified, the
protected_regular sysctl is
enabled, the file already exists and is a FIFO or regular file, the
owner of the file is neither the current user nor the owner of the
containing directory, and the containing directory is both world- or
group-writable and sticky. For details, see the descriptions of
/proc/sys/fs/protected_regular in proc(5).
O_EXCL was specified in
pathname refers to a block device that is in use by the system
(e.g., it is mounted).
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 exhausted.
pathname already exists and O_CREAT and
O_EXCL were used.
pathname points outside your accessible address space.
The filesystem does not support the O_DIRECT flag. See NOTES for more information.
Invalid value in
O_TMPFILE was specified in
neither O_WRONLY nor O_RDWR was
O_CREAT was specified in
flags and the
final component ("basename") of the new file's
invalid (e.g., it contains characters not permitted by the underlying
The final component ("basename") of
pathname is invalid
(e.g., it contains characters not permitted by the underlying
pathname refers to a directory and the access requested
involved writing (that is, O_WRONLY or
O_RDWR is set).
pathname refers to an existing directory,
O_TMPFILE and one of O_WRONLY or
O_RDWR were specified in
flags, but this
kernel version does not provide the O_TMPFILE
Too many symbolic links were encountered in resolving
pathname was a symbolic link, and
O_NOFOLLOW but not O_PATH.
The per-process limit on the number of open file descriptors has been reached (see the description of RLIMIT_NOFILE in getrlimit(2)).
pathname was too long.
The system-wide 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.
A directory component in
pathname does not exist or is a
dangling symbolic link.
pathname refers to a nonexistent directory,
O_TMPFILE and one of O_WRONLY or
O_RDWR were specified in
flags, but this
kernel version does not provide the O_TMPFILE
The named file is a FIFO, but memory for the FIFO buffer can't be allocated because the per-user hard limit on memory allocation for pipes has been reached and the caller is not privileged; see pipe(7).
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 FIFO open for reading.
The file is a device special file and no corresponding device exists.
The file is a UNIX domain socket.
The filesystem containing
pathname does not support
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.
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.
The operation was prevented by a file seal; see fcntl(2).
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():
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.
openat2(2) is Linux-specific.
The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-specific. 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 implementations. On many systems the file is actually truncated.
If the file is newly created, its
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_mtime fields of the parent directory.
Otherwise, if the file is modified because of the
O_TRUNC flag, its
st_mtime fields are set to the current time.
The files in the
/proc/[pid]/fd directory show the open file
descriptors 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 directories.
The Linux header file <asm/fcntl.h> doesn't define O_ASYNC; the (BSD-derived) FASYNC synonym is defined instead.
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 processes: a child process created via fork(2) inherits duplicates of its parent's file descriptors, and those duplicates refer to the same open file descriptions.
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 processes) 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
incorrectly, 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
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 integrity completion.
To understand the difference between the two types of completion,
consider two pieces of file metadata: the file last modification
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 provided the
equivalent of synchronized I/O
data integrity completion (i.e.,
O_SYNC was actually implemented as the equivalent of
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.
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 requests.
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 details.
Unlike the other values that can be specified in
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
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
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), fspick(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2), move_mount(2), mknodat(2), name_to_handle_at(2), open_tree(2), openat2(2), readlinkat(2), renameat(2), statx(2), symlinkat(2), unlinkat(2), utimensat(2), mkfifoat(3), and scandirat(3)) address two problems with the older interfaces 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,
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 other benefits:
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 application. (This functionality can also be obtained
by tricks based on the use of
/proc/self/fd/dirfd, but less
dirfd argument for these APIs can be obtained by using
open() or openat() to open a directory
(with either the O_RDONLY or the
O_PATH flag). Alternatively, such a file descriptor can
be obtained by applying dirfd(3) to a directory stream
created using opendir(3).
When these APIs are given a
dirfd argument of
AT_FDCWD or the specified pathname is absolute, then
they handle their pathname argument in the same way as the corresponding
conventional APIs. However, in this case, several of the APIs have a
flags argument that provides access to functionality that is
not available with the corresponding conventional APIs.
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-independent interface for an application to discover these restrictions for a given file or filesystem. Some filesystems provide their own interfaces for doing so, for example the XFS_IOC_DIOINFO operation in xfsctl(3).
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 created 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 introduced 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 implement the flag, in which case open() fails with the error 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.
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 configured 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.
One must check for two different error codes, EISDIR and ENOENT, when trying to determine whether the kernel supports O_TMPFILE functionality.
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).
This page is part of release 5.10 of the Linux
project. A description of the project, information about reporting bugs,
and the latest version of this page, can be found at