OPEN
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
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.
flags
is one of
O_RDONLY, O_WRONLY or O_RDWR
which request opening the file read-only, write-only or read/write,
respectively.
flags
may also be
bitwise-or'd
with one or more of the following:
- O_CREAT
-
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. 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
process does not have one.
- O_TRUNC
-
If the file already exists it will be truncated.
- 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
-
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 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_LARGEFILE
-
On 32-bit systems that support the Large Files System, 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 releases.
Some of these optional flags can be altered using
fcntl
after the file has been opened.
mode
specifies the permissions to use if 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).
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
should always 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.
RETURN VALUE
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.
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.
- 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.
- ENAMETOOLONG
-
pathname was too long.
- ENOENT
-
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.
CONFORMING TO
SVr4, SVID, POSIX, X/OPEN, BSD 4.3
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,
corresponding to the flags O_SYNC, O_DSYNC and
O_RSYNC. Currently (2.1.130) these are all synonymous under Linux.
SEE ALSO
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)