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OPEN(2) Linux Programmer’sManual OPEN(2)
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);
DESCRIPTION
Givenapathname for a file, open() returns a file descriptor,asmall, non-negative integer for use in subse-
quent system calls (read(2), write(2), lseek(2), fcntl(2), etc.). The file descriptor returned by a successful
call will be the lowest-numbered file descriptor not currently open for the process.
By default, the newfile 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)).
Acall to open() creates a new open file description,anentry 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
removedormodified to refer to a different file. The newopen file description is initially not shared with
anyother 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,orread/write, respectively.
In addition, zero or more file creation flags and file status flags can be bitwise-or’d in flags.The file cre-
ation 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 twogroups of flags is that the file status flags
can be retrievedand (in some cases) modified using fcntl(2). The full list of file creation flags and file sta-
tus 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). 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’tbedone without a race condition.
O_ASYNC
Enable signal-drivenI/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 only avail-
able 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-execflag for the newfile descriptor.Specifying this flag permits a program to
avoid an additional fcntl(2) F_SETFD operation 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
Linux 2008-08-21 1
OPEN(2) Linux Programmer’sManual OPEN(2)
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)).
mode specifies the permissions to use in case a newfile is created. This argument must be sup-
plied 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:
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)
Trytominimize cache effects of the I/O to and from this file. In general this will degrade perfor-
mance, 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 belowfor fur-
ther discussion.
Asemantically 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).
Linux 2008-08-21 2
OPEN(2) Linux Programmer’sManual OPEN(2)
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 twoflags 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.Inenviron-
ments where NFS O_EXCL support is not provided, programs that rely on it for performing lock-
ing 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 makealink 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) Allowfiles whose sizes cannot be represented in an off_t (but can be represented in an
off64_t)tobeopened. 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)isthe 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 onall 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’scontrol-
ling terminal evenifthe 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 anysubsequent
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_NON-
BLOCK 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)itwill 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.
Linux 2008-08-21 3
OPEN(2) Linux Programmer’sManual OPEN(2)
RETURN VALUE
open() and creat() return the newfile descriptor,or−1ifanerror occurred (in which case, errno is set
appropriately).
ERRORS
EACCES
The requested access to the file is not allowed, or search permission is denied for one of the direc-
tories in the path prefix of pathname,orthe 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
pathname refers to a regular file, too large to be opened; see O_LARGEFILE above.
(POSIX.1-2001 specifies the error EOVERFLOW for this case.)
EINTR
While blocked waiting to complete an open of a slowdevice (e.g., a FIFO; see fifo(7)), the call
wasinterrupted 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
Toomanysymbolic links were encountered in resolving pathname,or O_NOFOLLOW was
specified but pathname wasasymbolic link.
EMFILE
The process already has the maximum number of files open.
ENAMETOOLONG
pathname wastoo 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 ker-
nel 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 wastobecreated but the device containing pathname has no room for the newfile.
ENOTDIR
Acomponent used as a directory in pathname is not, in fact, a directory,or O_DIRECTORY was
specified and pathname wasnot 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.
Linux 2008-08-21 4
OPEN(2) Linux Programmer’sManual OPEN(2)
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 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)).
CONFORMING TO
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.1-2001.
O_DIRECT is not specified in POSIX; one has to define _GNU_SOURCE to get its definition.
NOTES
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).
Unlikethe other values that can be specified in flags,the access mode values O_RDONLY, O_WRONLY,
and O_RDWR,donot specify individual bits. Rather,theydefine the loworder twobits of flags,and are
defined respectively as 0, 1, and 2. In other words, the combination O_RDONLY|O_WRONLY is a logi-
cal 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’tbeused 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 manysystems the
file is actually truncated.
There are manyinfelicities 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.Howev erthere is currently no file system−independent interface for an application to
Linux 2008-08-21 5
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