OPEN(2) System calls 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>
intopen(constchar*pathname,int flags);
intopen(constchar*pathname,int flags,mode_t mode);
intcreat(constchar*pathname,mode_t mode);
DESCRIPTION
The open() system call is used to convert a pathname into a file
descriptor (a small, non-negative integer for use in subsequent I/O as
with read, write, etc.). When the call is successful, the file
descriptor returned will be the lowest file descriptor not currently
open for the process. This call creates a new open file, not shared
with any other process. (But shared open files may arise via the
fork(2) system call.) The new file descriptor is set to remain open
across exec functions (see fcntl(2)). The file offset is set to the
beginning of the file.
The parameter flags is one of O_RDONLY, O_WRONLY or O_RDWR which
request opening the file read-only, write-only or read/write, respec-
tively, bitwise-or'd with zero or more of the following:
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 direc-
tory (depending on filesystem type and mount options, and the
mode of the parent directory, see, e.g., the mount options bsd-
groups and sysvgroups of the ext2 filesystem, as described in
mount(8)).
O_EXCL When used with O_CREAT, if the file already exists it is an
error and the open will fail. In this context, a symbolic link
exists, regardless of where its points to. O_EXCL is broken on
NFS file systems, programs which rely on it for performing lock-
ing tasks will contain a race condition. The solution for per-
forming atomic file locking using a lockfile is to create a
unique file on the same fs (e.g., incorporating hostname and
pid), use link(2) to make a link to the lockfile. If link()
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_NOCTTY
If pathname refers to a terminal device -- see tty(4) -- it will
not become the process's controlling terminal even if the pro-
cess does not have one.
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. (On many Linux versions it will be
ignored; on other versions it will return an error.)
O_APPEND
The file is opened in append mode. Before each write, the file
pointer is positioned at the end of the file, as if with lseek.
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_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(4). This
mode need not have any effect on files other than FIFOs.
O_SYNC The file is opened for synchronous I/O. Any writes on the
resulting file descriptor will block the calling process until
the data has been physically written to the underlying hardware.
See RESTRICTIONS below, though.
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. The headers from glibc 2.0.100 and later include a
definition of this flag; kernels before 2.1.126 will ignore it
if used.
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.
O_DIRECT
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, i.e., at the completion of the read(2)
or write(2) system call, data is guaranteed to have been trans-
ferred. Transfer sizes, and the alignment of user buffer and
file offset must all be multiples of the logical block size of
the file system.
This flag is supported on a number of Unix-like systems; support
was added under Linux in kernel version 2.4.10.
A semantically similar interface for block devices is described
in raw(8).
O_ASYNC
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 available for terminals,
pseudo-terminals, and sockets. See fcntl(2) for further details.
O_LARGEFILE
On 32-bit systems that support the Large Files System, allow
files whose sizes cannot be represented in 31 bits to be opened.
Some of these optional flags can be altered using fcntl after the file
has been opened.
The argument mode specifies the permissions to use in case a new file
is created. It is 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(S_IREAD)
00400 user has read permission
S_IWUSR(S_IWRITE)
00200 user has write permission
S_IXUSR(S_IEXEC)
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 permisson
S_IXOTH
00001 others have execute permission
mode must be specified when O_CREAT is in the flags, and is ignored
otherwise.
creat is equivalent to open with flags equal to
O_CREAT|O_WRONLY|O_TRUNC.
RETURNVALUE
open and creat return the new file descriptor, or -1 if an error
occurred (in which case, errno is set appropriately). 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 e.g. 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 atime, ctime, mtime fields are set to
the current time, and so are the ctime and mtime fields of the parent
directory. Otherwise, if the file is modified because of the O_TRUNC
flag, its ctime and mtime fields are set to the current time.
ERRORS
EEXIST pathname already exists and O_CREAT and O_EXCL were used.
EISDIR pathname refers to a directory and the access requested involved
writing (that is, O_WRONLY or O_RDWR is set).
EACCES The requested access to the file is not allowed, or one of the
directories in pathname did not allow search (execute) permis-
sion, or the file did not exist yet and write access to the par-
ent directory is not allowed.
ENAMETOOLONG
pathname was too long.
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.
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.
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.)
EROFS pathname refers to a file on a read-only filesystem and write
access was requested.
ETXTBSY
pathname refers to an executable image which is currently being
executed and write access was requested.
EFAULT pathname points outside your accessible address space.
ELOOP Too many symbolic links were encountered in resolving pathname,
or O_NOFOLLOW was specified but pathname was a symbolic link.
ENOSPC pathname was to be created but the device containing pathname
has no room for the new file.
ENOMEM Insufficient kernel memory was available.
EMFILE The process already has the maximum number of files open.
ENFILE The limit on the total number of files open on the system has
been reached.
CONFORMINGTO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3 The O_NOFOLLOW and O_DIRECTORY flags
are Linux-specific. One may have to define the _GNU_SOURCE macro to
get their definitions.
RESTRICTIONS
There are many infelicities in the protocol underlying NFS, affecting
amongst others O_SYNC and O_NDELAY.
POSIX provides for three different variants of synchronised I/O, corre-
sponding to the flags O_SYNC, O_DSYNC and O_RSYNC. Currently (2.1.130)
these are all synonymous under Linux.
SEEALSO
read(2), write(2), fcntl(2), close(2), link(2), mknod(2), mount(2),
stat(2), umask(2), unlink(2), socket(2), fopen(3), fifo(4)
Linux 1999-06-03 OPEN(2)
Take our 
