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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON |
OPEN(2) Linux Programmer's Manual OPEN(2)
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);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
openat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
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 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 pathname is subsequently removed or modified to
refer to a different file. For further details on open file
descriptions, see NOTES.
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
follows:
O_APPEND
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.
O_ASYNC
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
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.)
O_CREAT
If the file does not exist, it will be created.
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
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
permission
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
in mode:
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 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).
O_DIRECTORY
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
device.
O_DSYNC
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
specified in conjunction with O_CREAT, and pathname already
exists, then open() will fail.
When these two flags are specified, symbolic links are not
followed: if pathname is a symbolic link, then open() fails
regardless 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.
O_LARGEFILE
(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
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.
O_NOCTTY
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.
O_NOFOLLOW
If 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.
O_NONBLOCK or O_NDELAY
When possible, the file is opened in nonblocking mode.
Neither the open() nor any subsequent operations on the file
descriptor which is returned will cause the calling process to
wait.
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.
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
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),
mmap(2)) fail with the error EBADF.
The following operations can be performed on the resulting
file descriptor:
* close(2); fchdir(2) (since Linux 3.5); fstat(2) (since
Linux 3.6).
* Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,
etc.).
* 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 flags, flag bits other than
O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
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
operate on the symbolic link.
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) (and similar) return, 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 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, 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'... */
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() mode argument determines the file
permission mode, as with O_CREAT.
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)
O_TRUNC
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.
creat()
A call to creat() is equivalent to calling open() with flags equal to
O_CREAT|O_WRONLY|O_TRUNC.
openat()
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
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
path_resolution(7).)
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 exhausted.
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
signal 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.
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
version 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 not O_PATH.
EMFILE The per-process limit on the number of open file descriptors
has been reached (see the description of RLIMIT_NOFILE in
getrlimit(2)).
ENAMETOOLONG
pathname was too long.
ENFILE The system-wide limit on the total number of open files has
been reached.
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 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
version 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
allocation for pipes has been reached and the caller is not
privileged; 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.
ENOTDIR
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.
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
exists.
EOPNOTSUPP
The filesystem containing pathname does not support O_TMPFILE.
EOVERFLOW
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.
ETXTBSY
pathname refers to an executable image which is currently
being executed and write access was requested.
EWOULDBLOCK
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.
ENOTDIR
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.
openat(): POSIX.1-2008.
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 indicates that one wants to open but
does not necessarily have the intention to read or write. This is
typically used to open devices in order to get a file descriptor for
use with ioctl(2).
The (undefined) effect of O_RDONLY | O_TRUNC varies among
implementations. On many systems the file is actually truncated.
Note that open() can open device special files, but creat() cannot
create 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
descriptors of the process with the PID pid. The files in the
/proc/[pid]/fdinfo directory show even more information about these
files descriptors. See proc(5) for further details of both of these
directories.
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 struct file.
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.
Synchronized I/O
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_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 integrity completion.
To understand the difference between the two types of completion,
consider 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 provided 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.
NFS
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.
FIFOs
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.
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) operations.
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), 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 path/to/xxx.dep if the file
path/to/xxx exists. The problem is that between the existence check
and the file creation step, path or to (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 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 efficiently.)
O_DIRECT
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:
blockdev --getss
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 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.
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.
"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.
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).
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)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 OPEN(2)
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