open(2) System Calls Manual open(2)
NAME
open, openat, creat - open and possibly create a file
LIBRARY
Standard C library (libc, -lc)
SYNOPSIS
#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
DESCRIPTION
The open() system call opens the file specified by pathname. If the
specified file does not exist, it may optionally (if O_CREAT is speci-
fied in flags) be created by open().
The return value of open() is a file descriptor, a small, nonnegative
integer that is 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 ex-
ecve(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 oper-
ations. The file status flags can be retrieved and (in some cases) mod-
ified; see fcntl(2) for details.
The full list of file creation flags and file status flags is as fol-
lows:
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 op-
eration 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 be-
comes possible on this file descriptor. This feature is avail-
able 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. Spec-
ifying 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 de-
scriptor 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 filesys-
tems, the behavior also depends on the bsdgroups and sysvgroups
mount options described in mount(8).
The mode argument specifies the file mode bits to be applied when
a new file is created. If neither O_CREAT nor O_TMPFILE is spec-
ified 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 sup-
plied, 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 cre-
ated file is (mode & ~umask).
Note that mode applies only to future accesses of the newly cre-
ated 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 guar-
antee synchronous I/O, O_SYNC must be used in addition to O_DI-
RECT. See NOTES below for further discussion.
A semantically similar (but deprecated) interface for block de-
vices 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 prob-
lems if opendir(3) is called on a FIFO or tape device.
O_DSYNC
Write operations on the file will complete according to the re-
quirements 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 fol-
lowed: if pathname is a symbolic link, then open() fails regard-
less of where the symbolic link points.
In general, the behavior of O_EXCL is undefined if it is used
without O_CREAT. There is one exception: on Linux 2.6 and later,
O_EXCL can be used without O_CREAT if pathname refers to a block
device. If the block device is in use by the system (e.g.,
mounted), open() fails with the error EBUSY.
On NFS, O_EXCL is supported only when using NFSv3 or later on
kernel 2.6 or later. In NFS environments where O_EXCL support is
not provided, programs that rely on it for performing locking
tasks will contain a race condition. Portable programs that want
to perform atomic file locking using a lockfile, and need to
avoid reliance on NFS support for O_EXCL, can create a unique
file on the same filesystem (e.g., incorporating hostname and
PID), and use link(2) to make a link to the lockfile. If link(2)
returns 0, the lock is successful. Otherwise, use stat(2) on the
unique file to check if its link count has increased to 2, in
which case the lock is also successful.
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 _LARGE-
FILE64_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 activ-
ity. This flag may not be effective on all filesystems. One ex-
ample 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 sym-
bolic link, then the open fails, with the error ELOOP. Symbolic
links in earlier components of the pathname will still be fol-
lowed. (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 descrip-
tor 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 inter-
faces merely inform the caller about whether a file descriptor is
"ready", meaning that an I/O operation performed on the file de-
scriptor with the O_NONBLOCK flag clear would not block.
Note that this flag has no effect for regular files and block de-
vices; 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, ap-
plications should not depend upon blocking behavior when specify-
ing 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 manda-
tory file locks and with file leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two purposes: to
indicate a location in the filesystem tree and to perform opera-
tions that act purely at the file descriptor level. The file it-
self 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 do-
main 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 per-
missions on the object itself (but does require execute permis-
sion 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 con-
trast, obtaining a reference to a filesystem object by opening it
with the O_RDONLY flag requires that the caller have read permis-
sion on the object, even when the subsequent operation (e.g.,
fchdir(2), fstat(2)) does not require read permission on the ob-
ject.
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 de-
termine if it is, in fact, an untriggered automount point
(.f_type == AUTOFS_SUPER_MAGIC).
One use of O_PATH for regular files is to provide the equivalent
of POSIX.1's O_EXEC functionality. This permits us to open a
file for which we have execute permission but not read permis-
sion, and then execute that file, with steps something like the
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 re-
quirements of synchronized I/O file integrity completion (by con-
trast with the synchronized I/O data integrity completion pro-
vided by O_DSYNC.)
By the time write(2) (or similar) returns, the output data and
associated file metadata have been transferred to the underlying
hardware (i.e., as though each write(2) was followed by a call to
fsync(2)). See NOTES below.
O_TMPFILE (since Linux 3.11)
Create an unnamed temporary regular file. The pathname argument
specifies a directory; an unnamed inode will be created in that
directory's filesystem. Anything written to the resulting file
will be lost when the last file descriptor is closed, unless the
file is given a name.
O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and,
optionally, O_EXCL. If O_EXCL is not specified, then linkat(2)
can be used to link the temporary file into the filesystem, mak-
ing it permanent, using code like the following:
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 per-
mission mode, as with O_CREAT.
Specifying O_EXCL in conjunction with O_TMPFILE prevents a tempo-
rary file from being linked into the filesystem in the above man-
ner. (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 tem-
porary 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 de-
vise unique names.
• Creating a file that is initially invisible, which is then
populated with data and adjusted to have appropriate filesys-
tem attributes (fchown(2), fchmod(2), fsetxattr(2), etc.) be-
fore 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(), ex-
cept 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 spe-
cial 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 path-
name 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).
RETURN VALUE
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.
ERRORS
open(), openat(), and creat() can fail with the following errors:
EACCES The requested access to the file is not allowed, or search per-
mission is denied for one of the directories in the path prefix
of pathname, or the file did not exist yet and write access to
the parent directory is not allowed. (See also path_resolu-
tion(7).)
EACCES Where O_CREAT is specified, the protected_fifos or protected_reg-
ular 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 di-
rectory is both world- or group-writable and sticky. For de-
tails, 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 de-
vice 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 han-
dler; see signal(7).
EINVAL The filesystem does not support the O_DIRECT flag. See NOTES for
more information.
EINVAL Invalid value in flags.
EINVAL O_TMPFILE was specified in flags, but neither O_WRONLY nor O_RDWR
was specified.
EINVAL O_CREAT was specified in flags and the final component ("base-
name") of the new file's pathname is invalid (e.g., it contains
characters not permitted by the underlying filesystem).
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 ver-
sion does not provide the O_TMPFILE functionality.
ELOOP Too many symbolic links were encountered in resolving pathname.
ELOOP pathname was a symbolic link, and flags specified O_NOFOLLOW but
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 getr-
limit(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 de-
vice 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 ver-
sion does not provide the O_TMPFILE functionality.
ENOMEM The named file is a FIFO, but memory for the FIFO buffer can't be
allocated because the per-user hard limit on memory 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 ex-
ists.
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 ac-
cess 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)).
VERSIONS
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementa-
tions. 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 imple-
mentation supports this option, it must at least support the use of
O_SYNC for regular files.
Linux implements O_SYNC and O_DSYNC, but not O_RSYNC. Somewhat incor-
rectly, glibc defines O_RSYNC to have the same value as O_SYNC.
(O_RSYNC is defined in the Linux header file <asm/fcntl.h> on HP PA-
RISC, but it is not used.)
O_SYNC provides synchronized I/O file integrity completion, meaning
write operations will flush data and all associated metadata to the un-
derlying 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 al-
low a subsequent read operation to complete successfully. Data in-
tegrity completion can reduce the number of disk operations that are re-
quired for applications that don't need the guarantees of file integrity
completion.
To understand the difference between the two types of completion, con-
sider two pieces of file metadata: the file last modification timestamp
(st_mtime) and the file length. All write operations will update the
last file modification timestamp, but only writes that add data to the
end of the file will change the file length. The last modification
timestamp is not needed to ensure that a read completes successfully,
but the file length is. Thus, O_DSYNC would only guarantee to flush up-
dates to the file length metadata (whereas O_SYNC would also always
flush the last modification timestamp metadata).
Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
However, when that flag was specified, most filesystems actually pro-
vided the equivalent of synchronized I/O data integrity completion
(i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).
Since Linux 2.6.33, proper O_SYNC support is provided. However, to en-
sure 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 ope-
nat() system call, rather than the kernel's open() system call. For
certain architectures, this is also true before glibc 2.26.
STANDARDS
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.
HISTORY
open()
creat()
SVr4, 4.3BSD, POSIX.1-2001.
openat()
POSIX.1-2008. Linux 2.6.16, glibc 2.4.
NOTES
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 cre-
ate them; use mknod(2) instead.
If the file is newly created, its st_atime, st_ctime, st_mtime fields
(respectively, time of last access, time of last status change, and time
of last modification; see stat(2)) are set to the current time, and so
are the st_ctime and st_mtime fields of the parent directory. Other-
wise, 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 di-
rectory 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-de-
rived) 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 du-
plicate 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 de-
tails.
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 re-
spectively 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 mean-
ing 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 re-
turn 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) opera-
tions.
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), fu-
timesat(2), linkat(2), mkdirat(2), mknodat(2), mount_setattr(2),
move_mount(2), name_to_handle_at(2), open_tree(2), openat2(2), read-
linkat(2), renameat(2), renameat2(2), statx(2), symlinkat(2), un-
linkat(2), utimensat(2), mkfifoat(3), and scandirat(3)) address two
problems with the older interfaces that preceded them. Here, the expla-
nation 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) fs-
tatat(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 be-
ing dismounted, just as when a process has a current working direc-
tory on a filesystem.
Second, openat() allows the implementation of a per-thread "current
working directory", via file descriptor(s) maintained by the applica-
tion. (This functionality can also be obtained by tricks based on the
use of /proc/self/fd/dirfd, but less efficiently.)
The 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 apply-
ing 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 func-
tionality 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 xf-
sctl(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 de-
vices 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 cre-
ated with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
on the heap and statically allocated buffers). Any such I/Os, whether
submitted via an asynchronous I/O interface or from another thread in
the process, should be completed before fork(2) is called. Failure to
do so can result in data corruption and undefined behavior in parent and
child processes. This restriction does not apply when the memory buffer
for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with the
MAP_SHARED flag. Nor does this restriction apply when the memory buffer
has been advised as MADV_DONTFORK with madvise(2), ensuring that it will
not be available to the child after fork(2).
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment re-
strictions 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 situ-
ation, overall I/O throughput is likely to be slower than using either
mode alone. Likewise, applications should avoid mixing mmap(2) of files
with direct I/O to the same files.
The behavior of O_DIRECT with NFS will differ from local filesystems.
Older kernels, or kernels configured in certain ways, may not support
this combination. The NFS protocol does not support passing the flag to
the server, so O_DIRECT I/O will bypass the page cache only on the
client; the server may still cache the I/O. The client asks the server
to make the I/O synchronous to preserve the synchronous semantics of
O_DIRECT. Some servers will perform poorly under these circumstances,
especially if the I/O size is small. Some servers may also be config-
ured to lie to clients about the I/O having reached stable storage; this
will avoid the performance penalty at some risk to data integrity in the
event of server power failure. The Linux NFS client places no alignment
restrictions on O_DIRECT I/O.
In summary, O_DIRECT is a potentially powerful tool that should be used
with caution. It is recommended that applications treat use of O_DIRECT
as a performance option which is disabled by default.
BUGS
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).
SEE ALSO
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)
Linux man-pages 6.9.1 2024-05-02 open(2)
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