open, creat - open and possibly create a file or device
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);
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 pro
cess. 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.
flags is one of O_RDONLY, O_WRONLY or O_RDWR which request
opening the file read-only, write-only or read/write,
flags may also be bitwise-or'd with one or more of the
If the file does not exist it will be created.
O_EXCL When used with O_CREAT, if the file already exists
it is an error and the open will fail. O_EXCL is
broken on NFS file systems, programs which rely on
it for performing locking tasks will contain a race
condition. The solution for performing 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. Oth
erwise, 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.
If pathname refers to a terminal device -- see
tty(4) -- it will not become the process's control
ling terminal even if the process does not have
If the file already exists it will be truncated.
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
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).
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 RESTRIC
TIONS below, though.
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.
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-ser
vice problems if opendir(3) is called on a FIFO or
tape device, but should not be used outside of the
implementation of opendir.
On 32-bit systems that support the Large Files Sys
tem, allow files whose sizes cannot be represented
in 31 bits to be opened. The Linux kernel does not
yet have the support for this (as of 2.1.130), but
the flag definition is there and the userspace LFS
interfaces are present in the glibc 2.1 test
Some of these optional flags can be altered using fcntl
created. It is modified by the process's umask in the
usual way: the permissions of the created file are (mode &
The following symbolic constants are provided for mode:
00700 user (file owner) has read, write and execute
00400 user has read permission
00200 user has write permission
00100 user has execute permission
00070 group has read, write and execute permission
00040 group has read permission
00020 group has write permission
00010 group has execute permission
00007 others have read, write and execute permis
00004 others have read permission
00002 others have write permisson
00001 others have execute permission
mode should always be specified when O_CREAT is in the
flags, and is ignored otherwise.
creat is equivalent to open with flags equal to
open and creat return the new file descriptor, or -1 if an
files, but creat cannot create them - use mknod(2)
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 per
formed by the server upon read and write requests.
EEXIST pathname already exists and O_CREAT and O_EXCL were
EISDIR pathname refers to a directory and the access
requested involved writing.
EACCES The requested access to the file is not allowed, or
one of the directories in pathname did not allow
search (execute) permission, or the file did not
exist yet and write access to the parent directory
is not allowed.
pathname was too long.
ENOENT A directory component in pathname does not exist or
is a dangling symbolic link.
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 corre
sponding device exists.
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
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 executed and write access was
EFAULT pathname points outside your accessible address
ing pathname, or O_NOFOLLOW was specified but path
name was a symbolic link.
ENOSPC pathname was to be created but the device contain
ing pathname has no room for the new file.
ENOMEM Insufficient kernel memory was available.
EMFILE The process already has the maximum number of files
ENFILE The limit on the total number of files open on the
system has been reached.
SVr4, SVID, POSIX, X/OPEN, BSD 4.3
There are many infelicities in the protocol underlying
NFS, affecting amongst others O_SYNC and O_NDELAY.
POSIX provides for three different variants of synchro
nised I/O, corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC. Currently (2.1.130) these are all synonymous
read(2), write(2), fcntl(2), close(2), link(2), mknod(2),
mount(2), stat(2), umask(2), unlink(2), socket(2),