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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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open(2) System Calls Manual open(2)
open, openat, creat - open and possibly create a file
Standard C library (libc, -lc)
#include <fcntl.h>
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, ...
/* 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)):
openat():
Since glibc 2.10:
_POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
_ATFILE_SOURCE
The open() system call opens the file specified by pathname. If
the specified file does not exist, it may optionally (if O_CREAT
is specified in flags) be created by open().
The return value of open() is a file descriptor, a small,
nonnegative integer that is an index to an entry in the process's
table of open file descriptors. The file descriptor 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 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 ORed 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 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 bsdgroups and sysvgroups
mount options described in mount(8).
The 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 flags, then mode is ignored
(and can thus be specified as 0, or simply omitted). The
mode argument must be supplied if O_CREAT or O_TMPFILE is
specified in 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 is (mode & ~umask).
Note that 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 Linux 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() fails with the error EEXIST.
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.
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 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 in Linux
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 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 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 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.
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), ioctl(2), mmap(2)) fail with the
error EBADF.
The following operations can be performed on the resulting
file descriptor:
• close(2).
• 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, 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.
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.
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.
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 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).
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, 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, "", 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() 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 tmpfs 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.
The dirfd argument is used in conjunction with the pathname
argument as follows:
• If the pathname given in pathname is absolute, then dirfd is
ignored.
• If the pathname given in 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 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). In this case, dirfd must be a directory
that was opened for reading (O_RDONLY) or using the O_PATH
flag.
If the pathname given in pathname is relative, and dirfd is not a
valid file descriptor, an error (EBADF) results. (Specifying an
invalid file descriptor number in dirfd can be used as a means to
ensure that pathname is absolute.)
openat2(2)
The openat2(2) system call is an extension of openat(), and
provides a superset of the features of openat(). It is
documented separately, in openat2(2).
On success, open(), openat(), and creat() return the new file
descriptor (a nonnegative integer). On error, -1 is returned and
errno is set to indicate the error.
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).)
EACCES Where O_CREAT is specified, the protected_fifos or
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_fifos and
/proc/sys/fs/protected_regular in proc_sys_fs(5).
EBADF (openat()) pathname is relative but dirfd is neither
AT_FDCWD nor a valid file descriptor.
EBUSY O_EXCL was specified in flags and pathname refers to a
block device that is in use by the system (e.g., it is
mounted).
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.
EINVAL O_CREAT was specified in flags and the final component
("basename") of the new file's pathname is invalid (e.g.,
it contains characters not permitted by the underlying
filesystem).
EINVAL The final component ("basename") of 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 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.
ENOENT 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.
ENOTDIR
(openat()) pathname is a relative pathname and dirfd is a
file descriptor referring to a file other than 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.
ENXIO The file is a UNIX domain socket.
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; before Linux 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.
ETXTBSY
pathname refers to a file that is currently in use as a
swap file, and the O_TRUNC flag was specified.
ETXTBSY
pathname refers to a file that is currently being read by
the kernel (e.g., for module/firmware loading), 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 (undefined) effect of O_RDONLY | O_TRUNC varies among
implementations. On many systems the file is actually truncated.
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_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 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 before Linux 2.6.33.
C library/kernel differences
Since glibc 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 before glibc
2.26.
open()
creat()
openat()
POSIX.1-2008.
openat2(2) Linux.
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.
open()
creat()
SVr4, 4.3BSD, POSIX.1-2001.
openat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
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).
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
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.
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.
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),
fspick(2), fstatat(2), futimesat(2), linkat(2), mkdirat(2),
mknodat(2), mount_setattr(2), move_mount(2),
name_to_handle_at(2), open_tree(2), openat2(2), readlinkat(2),
renameat(2), renameat2(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,
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 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.)
The 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.
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. The handling of misaligned
O_DIRECT I/Os also varies; they can either fail with EINVAL or
fall back to buffered I/O.
Since Linux 6.1, O_DIRECT support and alignment restrictions for
a file can be queried using statx(2), using the STATX_DIOALIGN
flag. Support for STATX_DIOALIGN varies by filesystem; see
statx(2).
Some filesystems provide their own interfaces for querying
O_DIRECT alignment restrictions, for example the XFS_IOC_DIOINFO
operation in xfsctl(3). STATX_DIOALIGN should be used instead
when it is available.
If none of the above is available, then direct I/O support and
alignment restrictions can only be assumed from known
characteristics of the filesystem, the individual file, the
underlying storage device(s), and the kernel version. In Linux
2.4, most filesystems based on block devices require that the
file offset and the length and memory address of all I/O segments
be multiples of the filesystem block size (typically 4096 bytes).
In Linux 2.6.0, this was relaxed to the logical block size of the
block device (typically 512 bytes). A block device's 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 in Linux 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.
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),
openat2(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 the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
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