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OPEN
2
2008-12-03
Linux
Linux Programmer's Manual
- NAME
- open, creat - open and possibly create a file or device
- SYNOPSIS
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> int open(const char * pathname , int flags ); int open(const char * pathname , int flags , mode_t mode );
int creat(const char * pathname , mode_t mode );
- Given a
- 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) .
O_APPEND may lead to corrupted files on NFS file systems 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
by default, but this can be changed via
fcntl(2) )
when input or output becomes possible on this file descriptor.
This feature is only available for terminals, pseudo-terminals,
sockets, and (since Linux 2.6) pipes and FIFOs.
See
fcntl(2)
for further details.
- 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.
Additionally,
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 conditions
where one thread opens a file descriptor at the same
time as another thread does a
fork(2)
plus
execve(2) .
- O_CREAT
- 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 directory (depending on file system type and mount options,
and the mode of the parent directory, see the mount options
bsdgroups
and
sysvgroups
described in
mount(8) ).
- 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
- 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 I/O is synchronous, that is, at the completion of a
read(2)
or
write(2) ,
data is guaranteed to have been transferred.
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 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, but should not be used outside of the
implementation of
opendir(3) .
- O_EXCL
- Ensure that this call creates the file:
if this flag is specified in conjunction with
O_CREAT ,
and
pathname
already exists, then
open ()
will fail.
The behavior of
O_EXCL is undefined if
O_CREAT is not specified.
When these two flags are specified, symbolic links are not followed:
if
pathname
is a symbolic link, then
open ()
fails regardless of where the symbolic link points to.
O_EXCL is only supported on NFS when using NFSv3 or later on kernel 2.6 or later.
In environments where NFS
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 file system (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 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 obtaining
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 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 file systems.
One example is NFS, where the server maintains the access time.
- O_NOCTTY
- If
pathname
refers to a terminal device -- see
tty(4)
-- it will not become the process's controlling terminal even if the
process does not have one.
- O_NOFOLLOW
- If pathname is a symbolic link, then the open fails.
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.
- O_NONBLOCK " or " O_NDELAY
- When possible, the file is opened in non-blocking 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_SYNC
- The file is opened for synchronous I/O.
Any
write(2) s
on the resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
"But see NOTES below" .
- O_TRUNC
- If the file already exists and is a regular file and the open 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.
Some of these optional flags can be altered using
fcntl(2)
after the file has been opened.
creat ()
is equivalent to
open ()
with
flags
equal to
O_CREAT|O_WRONLY|O_TRUNC .
pathname
for a file,
open ()
returns a file descriptor, a small, non-negative integer
for use in subsequent system calls
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 Linux-specific
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 operation).
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 initially not shared
with any other process,
but sharing may arise via
fork(2) .
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_CREAT ", " O_EXCL ", " O_NOCTTY ", 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 status flags can be retrieved and (in some cases)
modified using
fcntl(2) .
The full list of file creation flags and file status flags is as follows:
mode
specifies the permissions to use in case a new file is created.
This argument must be supplied when
O_CREAT is specified in
flags ;
if
O_CREAT is not specified, then
mode
is ignored.
The effective permissions are modified by
the process's
umask
in the usual way: The permissions of the created file are
"(mode & ~umask)" .
Note that this mode only applies 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 :
- open ()
and
creat ()
return the new file descriptor, or -1 if an error occurred
(in which case,
errno
is set appropriately).
- 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) .)
- 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) .
- EISDIR
- pathname
refers to a directory and the access requested involved writing
(that is,
O_WRONLY or
O_RDWR is set).
- ELOOP
- Too many symbolic links were encountered in resolving
pathname ,
or O_NOFOLLOW was specified but
pathname
was a symbolic link.
- EMFILE
- The process already has the maximum number of files open.
- ENAMETOOLONG
- pathname
was too long.
- ENFILE
- The system limit on the total number of open files has been reached.
- ENODEV
- pathname
refers to a device special file and no corresponding device exists.
(This is a Linux kernel bug; in this situation
ENXIO must be returned.)
- ENOENT
- O_CREAT is not set and the named file does not exist.
Or, a directory component in
pathname
does not exist or is a dangling symbolic link.
- ENOMEM
- Insufficient kernel memory was available.
- ENOSPC
- pathname
was to be created but the device containing
pathname
has no room for the new file.
- ENOTDIR
- A component used as a directory in
pathname
is not, in fact, a directory, or O_DIRECTORY was specified and
pathname
was not a directory.
- ENXIO
- O_NONBLOCK " | " O_WRONLY
is set, the named file is a FIFO and
no process has the file open for reading.
Or, the file is a device special file and no corresponding device exists.
- 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
(2<<31)-1
bits;
see also
O_LARGEFILE 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
- EROFS
- pathname
refers to a file on a read-only file system and write access was
requested.
- ETXTBSY
- pathname
refers to an executable image which is currently being executed and
write access was requested.
- EWOULDBLOCK
- The
O_NONBLOCK flag was specified, and an incompatible lease was held on the file
(see
fcntl(2) ).
- SVr4, 4.3BSD, POSIX.1-2001.
The
O_DIRECTORY ,
O_NOATIME ,
and
O_NOFOLLOW flags are Linux-specific, and one may need to define
_GNU_SOURCE to obtain their definitions.
The
O_CLOEXEC
flag is not specified in POSIX.1-2001,
but is specified in POSIX.1-2008.
O_DIRECT is not specified in POSIX; one has to define
_GNU_SOURCE to get its definition.
- Under Linux, the
O_NONBLOCK flag indicates that one wants to open
but does not necessarily have the intention to read or write.
This is typically used to open devices in order to get a file descriptor
for use with
ioctl(2) .
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, non-standard 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 non-standard access mode is used by some Linux drivers to return a
descriptor that is only to be used for device-specific
ioctl(2)
operations.
The (undefined) effect of
O_RDONLY | O_TRUNC varies among implementations.
On many systems the file is actually truncated.
There are many infelicities in the protocol underlying NFS, affecting
amongst others
O_SYNC " and " O_NDELAY .
POSIX provides for three different variants of synchronized I/O,
corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC.
Currently (2.1.130) these are all synonymous under Linux.
Note that
open ()
can open device special files, but
creat ()
cannot create them; use
mknod(2)
instead.
On NFS file systems 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.
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.
O_DIRECT
The
O_DIRECT flag may impose alignment restrictions on the length and address
of userspace buffers and the file offset of I/Os.
In Linux alignment
restrictions vary by file system and kernel version and might be
absent entirely.
However there is currently no file system-independent
interface for an application to discover these restrictions for a given
file or file system.
Some file systems provide their own interfaces
for doing so, for example the
XFS_IOC_DIOINFO operation in
xfsctl(3) .
Under Linux 2.4, transfer sizes, and the alignment of the user buffer
and the file offset must all be multiples of the logical block size
of the file system.
Under Linux 2.6, alignment to 512-byte boundaries
suffices.
The
O_DIRECT flag was introduced in SGI IRIX, where it has alignment
restrictions similar to those of Linux 2.4.
IRIX has also a
fcntl(2)
call to query appropriate alignments, and sizes.
FreeBSD 4.x introduced
a flag of the same name, but without alignment restrictions.
O_DIRECT support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.
Some file systems may not implement the flag and
open ()
will fail with
EINVAL if it is used.
Applications should avoid mixing
O_DIRECT and normal I/O to the same file,
and especially to overlapping byte regions in the same file.
Even when the file system 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 file systems.
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 only bypass the page cache on the client; the server may
still cache the I/O.
The client asks the server to make the I/O
synchronous to preserve the synchronous semantics of
O_DIRECT .
Some servers will perform poorly under these circumstances, especially
if the I/O size is small.
Some servers may also be configured to
lie to clients about the I/O having reached stable storage; this
will avoid the performance penalty at some risk to data integrity
in the event of server power failure.
The Linux NFS client places no alignment restrictions on
O_DIRECT I/O.
In summary,
O_DIRECT is a potentially powerful tool that should be used with caution.
It is recommended that applications treat use of
O_DIRECT as a performance option which is disabled by default.
"The thing that has always disturbed me about O_DIRECT is that the whole
interface is just stupid, and was probably designed by a deranged monkey
on some serious mind-controlling substances." -- Linus
interface is just stupid, and was probably designed by a deranged monkey
on some serious mind-controlling substances." -- Linus
- Currently, it is not possible to enable signal-driven
I/O by specifying
O_ASYNC when calling
open ();
use
fcntl(2)
to enable this flag.
- This page is part of release 3.19 of the Linux
man-pages
project.
A description of the project,
and information about reporting bugs,
can be found at
http://www.kernel.org/doc/man-pages/.
Current Users: 54 © 1999-2009 PenguinSoft.
All trademarks and copyrights on this page are owned by their respective companies.
Linux is a trademark of Linus Torvalds.