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int open(const char *pathname, int flags);
int open(const char *pathname, int flags, mode_t mode);
int creat(const char *pathname, mode_t mode);
int openat(int dirfd, const char *pathname, int flags);
int openat(int dirfd, const char *pathname, int flags, mode_t mode);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.10:
_XOPEN_SOURCE >= 700 || _POSIX_C_SOURCE >= 200809L
Before glibc 2.10:
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 cur-
rently 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. This entry records the file offset
and the file status flags (modifiable via the fcntl(2) F_SETFL opera-
tion). A file descriptor is a reference to one of these entries; this
reference is unaffected if pathname is subsequently removed or modified
to refer to a different file. The new open file description is ini-
tially not shared with any other process, but sharing may arise via
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, O_TRUNC,
and O_TTY_INIT. The file status flags are all of the remaining flags
listed below. The distinction between these two groups of flags is
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. Addi-
tionally, use of this flag is essential in some multithreaded
programs since using a separate fcntl(2) F_SETFD operation to
set the FD_CLOEXEC flag does not suffice to avoid race condi-
tions where one thread opens a file descriptor at the same time
as another thread does a fork(2) plus execve(2).
If the file does not exist it will be created. The owner (user
ID) of the file is set to the effective user ID of the process.
The group ownership (group ID) is set either to the effective
group ID of the process or to the group ID of the parent direc-
tory (depending on filesystem type and mount options, and the
mode of the parent directory; see the mount options bsdgroups
and sysvgroups described in mount(8)).
mode specifies the permissions to use in case a new file is cre-
ated. This argument must be supplied when O_CREAT or O_TMPFILE
is specified in flags; if neither O_CREAT nor O_TMPFILE is spec-
ified, then mode is ignored. The effective permissions are mod-
ified by the process's umask in the usual way: The permissions
of the created file are (mode & ~umask). Note that this mode
applies only to future accesses of the newly created file; the
open() call that creates a read-only file may well return a
read/write file descriptor.
The following symbolic constants are provided for mode:
S_IRWXU 00700 user (file owner) has read, write and execute
S_IRUSR 00400 user has read permission
S_IWUSR 00200 user has write permission
S_IXUSR 00100 user has execute permission
S_IRWXG 00070 group has read, write and execute permission
S_IRGRP 00040 group has read permission
Try to minimize cache effects of the I/O to and from this file.
In general this will degrade performance, but it is useful in
special situations, such as when applications do their own
caching. File I/O is done directly to/from user-space buffers.
The O_DIRECT flag on its own makes an effort to transfer data
synchronously, but does not give the guarantees of the O_SYNC
flag that data and necessary metadata are transferred. To guar-
antee synchronous I/O, O_SYNC must be used in addition to
O_DIRECT. See NOTES below for further discussion.
A semantically similar (but deprecated) interface for block
devices is described in raw(8).
If pathname is not a directory, cause the open to fail. This
flag is Linux-specific, and 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.
Write operations on the file will complete according to the
requirements of synchronized I/O data integrity completion.
By the time write(2) (and similar) return, the output data has
been transferred to the underlying hardware, along with any file
metadata that would be required to retrieve that data (i.e., as
though each write(2) was followed by a call to fdatasync(2)).
See NOTES below.
O_EXCL Ensure that this call creates the file: if this flag is speci-
fied in conjunction with O_CREAT, and pathname already exists,
then open() will fail.
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 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
when the file is read(2). 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 effec-
tive on all filesystems. One example is NFS, where the server
maintains the access time.
If pathname refers to a terminal device--see tty(4)--it will not
become the process's controlling terminal even if the process
does not have one.
If pathname is a symbolic link, then the open fails. This is a
FreeBSD extension, which was added to Linux in version 2.1.126.
Symbolic links in earlier components of the pathname will still
be followed. 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. For
the handling of FIFOs (named pipes), see also fifo(7). For a
discussion of the effect of O_NONBLOCK in conjunction with
mandatory file locks and with file leases, see fcntl(2).
O_PATH (since Linux 2.6.39)
Obtain a file descriptor that can be used for two purposes: to
indicate a location in the filesystem tree and to perform opera-
tions that act purely at the file descriptor level. The file
itself is not opened, and other file operations (e.g., read(2),
write(2), fchmod(2), fchown(2), fgetxattr(2), mmap(2)) fail with
the error EBADF.
The following operations can be performed on the resulting file
* close(2); fchdir(2) (since Linux 3.5); fstat(2) (since Linux
* Duplicating the file descriptor (dup(2), fcntl(2) F_DUPFD,
* Getting and setting file descriptor flags (fcntl(2) F_GETFD
* Retrieving open file status flags using the fcntl(2) F_GETFL
operation: the returned flags will include the bit O_PATH.
* Passing the file descriptor as the dirfd argument of ope-
nat(2) and the other "*at()" system calls.
requirements of synchronized I/O file integrity completion (by
contrast with 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 speci-
fies a directory; an unnamed inode will be created in that
directory's filesystem. Anything written to the resulting file
will be lost when the last file descriptor is closed, unless the
file is given a name.
O_TMPFILE must be specified with one of O_RDWR or O_WRONLY and,
optionally, O_EXCL. If O_EXCL is not specified, then linkat(2)
can be used to link the temporary file into the filesystem, mak-
ing it permanent, using code like the following:
fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
S_IRUSR | S_IWUSR);
/* File I/O on 'fd'... */
snprintf(path, PATH_MAX, "/proc/self/fd/%d", fd);
linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
In this case, the open() mode argument determines the file per-
mission mode, as with O_CREAT.
Specifying O_EXCL in conjunction with O_TMPFILE prevents a tem-
porary file from being linked into the filesystem in the above
manner. (Note that the meaning of O_EXCL in this case is dif-
ferent from the meaning of O_EXCL otherwise.)
There are two main use cases for O_TMPFILE:
* Improved tmpfile(3) functionality: race-free creation of tem-
porary files that (1) are automatically deleted when closed;
(2) can never be reached via any pathname; (3) are not sub-
ject to symlink attacks; and (4) do not require the caller to
devise unique names.
* Creating a file that is initially invisible, which is then
populated with data and adjusted to have appropriate filesys-
tem attributes (chown(2), chmod(2), fsetxattr(2), etc.)
before being atomically linked into the filesystem in a fully
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 relative to by the file descriptor dirfd
(rather than relative to the current working directory of the calling
process, as is done by open() for a relative pathname).
If pathname is relative and dirfd is the special value AT_FDCWD, then
pathname is interpreted relative to the current working directory of
the calling process (like open()).
If pathname is absolute, then dirfd is ignored.
open(), openat(), and creat() return the new file descriptor, or -1 if
an error occurred (in which case, errno is set appropriately).
open(), openat(), and creat() can fail with the following errors:
EACCES The requested access to the file is not allowed, or search per-
mission is denied for one of the directories in the path prefix
of pathname, or the file did not exist yet and write access to
the parent directory is not allowed. (See also path_resolu-
EDQUOT Where O_CREAT is specified, the file does not exist, and the
user's quota of disk blocks or inodes on the filesystem has been
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EFAULT pathname points outside your accessible address space.
EFBIG See EOVERFLOW.
EINTR While blocked waiting to complete an open of a slow device
(e.g., a FIFO; see fifo(7)), the call was interrupted by a sig-
nal handler; see signal(7).
EINVAL The filesystem does not support the O_DIRECT flag. See NOTES
for more information.
EINVAL Invalid value in flags.
EINVAL O_TMPFILE was specified in flags, but neither O_WRONLY nor
O_RDWR was specified.
pathname was too long.
ENFILE The system limit on the total number of open files has been
ENODEV pathname refers to a device special file and no corresponding
device exists. (This is a Linux kernel bug; in this situation
ENXIO must be returned.)
ENOENT O_CREAT is not set and the named file does not exist. Or, a
directory component in pathname does not exist or is a dangling
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 Insufficient kernel memory was available.
ENOSPC pathname was to be created but the device containing pathname
has no room for the new file.
A component used as a directory in pathname is not, in fact, a
directory, or O_DIRECTORY was specified and pathname was not a
ENXIO O_NONBLOCK | O_WRONLY is set, the named file is a FIFO and no
process has the file open for reading. Or, the file is a device
special file and no corresponding device exists.
The filesystem containing pathname does not support O_TMPFILE.
pathname refers to a regular file that is too large to be
opened. The usual scenario here is that an application compiled
on a 32-bit platform without -D_FILE_OFFSET_BITS=64 tried to
open a file whose size exceeds (2<<31)-1 bits; see also O_LARGE-
FILE above. This is the error specified by POSIX.1-2001; 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 (CAP_FOWNER).
EROFS pathname refers to a file on a read-only filesystem and write
access was requested.
pathname refers to an executable image which is currently being
openat() was added to Linux in kernel 2.6.16; library support was added
to glibc in version 2.4.
open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
The O_DIRECT, O_NOATIME, O_PATH, and O_TMPFILE flags are Linux-spe-
cific. One must define _GNU_SOURCE to obtain their definitions.
The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified in
POSIX.1-2001, but are specified in POSIX.1-2008. Since glibc 2.12, one
can obtain their definitions by defining either _POSIX_C_SOURCE with a
value greater than or equal to 200809L or _XOPEN_SOURCE with a value
greater than or equal to 700. In glibc 2.11 and earlier, one obtains
the definitions by defining _GNU_SOURCE.
As noted in feature_test_macros(7), feature test macros such as
_POSIX_C_SOURCE, _XOPEN_SOURCE, and _GNU_SOURCE must be defined before
including any header files.
Under Linux, the O_NONBLOCK flag indicates that one wants to open but
does not necessarily have the intention to read or write. This is typ-
ically used to open devices in order to get a file descriptor for use
The (undefined) effect of O_RDONLY | O_TRUNC varies among implementa-
tions. On many systems the file is actually truncated.
Note that open() can open device special files, but creat() cannot cre-
ate them; use mknod(2) instead.
If the file is newly created, its st_atime, st_ctime, st_mtime fields
(respectively, time of last access, time of last status change, and
time of last modification; see stat(2)) are set to the current time,
and so are the st_ctime and st_mtime fields of the parent directory.
Otherwise, if the file is modified because of the O_TRUNC flag, its
st_ctime and st_mtime fields are set to the current time.
The POSIX.1-2008 "synchronized I/O" option specifies different variants
of synchronized I/O, and specifies the open() flags O_SYNC, O_DSYNC,
and O_RSYNC for controlling the behavior. Regardless of whether an
implementation supports this option, it must at least support the use
of O_SYNC for regular files.
Linux implements O_SYNC and O_DSYNC, but not O_RSYNC. (Somewhat incor-
rectly, glibc defines O_RSYNC to have the same value as O_SYNC.)
the last file modification timestamp, but only writes that add data to
the end of the file will change the file length. The last modification
timestamp is not needed to ensure that a read completes successfully,
but the file length is. Thus, O_DSYNC would only guarantee to flush
updates to the file length metadata (whereas O_SYNC would also always
flush the last modification timestamp metadata).
Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
However, when that flag was specified, most filesystems actually pro-
vided the equivalent of synchronized I/O data integrity completion
(i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).
Since Linux 2.6.33, proper O_SYNC support is provided. However, to
ensure backward binary compatibility, O_DSYNC was defined with the same
value as the historical O_SYNC, and O_SYNC was defined as a new (two-
bit) flag value that includes the O_DSYNC flag value. This ensures
that applications compiled against new headers get at least O_DSYNC
semantics on pre-2.6.33 kernels.
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
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 descriptor that can't be used for reading or writing. This
nonstandard access mode is used by some Linux drivers to return a
descriptor that is to be used only for device-specific ioctl(2) opera-
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., faccessat(2), fan-
otify_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), symlinkat(2), unlinkat(2), utimensat(2) mkfifoat(3), and
scandirat(3)) are supported for two reasons. Here, the explanation is
in terms of the openat() call, but the rationale is analogous for the
The O_DIRECT flag may impose alignment restrictions on the length and
address of user-space buffers and the file offset of I/Os. In Linux
alignment restrictions vary by filesystem and kernel version and might
be absent entirely. However there is currently no filesystem-indepen-
dent interface for an application to discover these restrictions for a
given file or filesystem. Some filesystems provide their own inter-
faces for doing so, for example the XFS_IOC_DIOINFO operation in
Under Linux 2.4, transfer sizes, and the alignment of the user buffer
and the file offset must all be multiples of the logical block size of
the filesystem. Under Linux 2.6, alignment to 512-byte boundaries suf-
O_DIRECT I/Os should never be run concurrently with the fork(2) system
call, if the memory buffer is a private mapping (i.e., any mapping cre-
ated with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
on the heap and statically allocated buffers). Any such I/Os, whether
submitted via an asynchronous I/O interface or from another thread in
the process, should be completed before fork(2) is called. Failure to
do so can result in data corruption and undefined behavior in parent
and child processes. This restriction does not apply when the memory
buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with
the MAP_SHARED flag. Nor does this restriction apply when the memory
buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that
it will not be available to the child after fork(2).
The O_DIRECT flag was introduced in SGI IRIX, where it has alignment
restrictions similar to those of Linux 2.4. IRIX has also a fcntl(2)
call to query appropriate alignments, and sizes. FreeBSD 4.x intro-
duced a flag of the same name, but without alignment restrictions.
O_DIRECT support was added under Linux in kernel version 2.4.10. Older
Linux kernels simply ignore this flag. Some filesystems may not imple-
ment the flag 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 behaviour 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,
Currently, it is not possible to enable signal-driven I/O by specifying
O_ASYNC when calling open(); use fcntl(2) to enable this flag.
One must check for two different error codes, EISDIR and ENOENT, when
trying to determine whether the kernel supports O_TMPFILE functional-
chmod(2), chown(2), close(2), dup(2), fcntl(2), link(2), lseek(2),
mknod(2), mmap(2), mount(2), read(2), socket(2), stat(2), umask(2),
unlink(2), write(2), fopen(3), fifo(7), path_resolution(7), symlink(7)
Linux 2014-03-16 OPEN(2)