open
NAME
open, creat - open and possibly create a file or device
SYNOPSIS
#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> I int open(const char * pathname , int flags ); I int open(const char * pathname , int flags , mode_t mode ); I int creat(const char * pathname , mode_t mode );
DESCRIPTION
Given a
R pathname
for a file,
R open ()
returns a file descriptor, a small, non-negative integer
for use in subsequent 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.
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 file offset is set to the beginning of the file (see
lseek(2)).
A call to
R open ()
creates a new
R 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 parameter
flags
must include one of the following
R access modes :
R 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
R flags .
The
file creation flags
are
R 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:
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
(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,
sockets, and (since Linux 2.6) pipes and FIFOs.
See
fcntl(2)
for further details.
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 filesystem type and mount options,
and the mode of the parent directory, see, for example, the mount options
bsdgroups
and
sysvgroups
of the ext2 filesystem, as described in
mount(8)).
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, that is, at the completion of a
read(2)
or
write(2),
data is guaranteed to have been transferred.
Under Linux 2.4 transfer sizes, and the alignment of user buffer
and 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.
A semantically similar (but deprecated) interface for block devices
is described in
raw(8).
O_DIRECTORY
O_EXCL
When used with
R O_CREAT ,
if the file already exists it is an error and the
R open ()
will fail.
In this context, a symbolic link exists, regardless
of where it points to.
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 file system (e.g., incorporating hostname and pid), 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
R 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
R 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 filesystems.
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.
R O_NONBLOCK or O_NDELAY
When possible, the file is opened in non-blocking mode.
Neither the
R 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
R 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.
R 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
R 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.
The argument
mode
specifies the permissions to use in case a new file is created.
It is
modified by the process's
R umask
in the usual way: the permissions of the created file are
R (mode & ~umask) .
Note that this mode only applies to future accesses of the
newly created file; the
R open ()
call that creates a read-only file may well return a read/write
file descriptor.
The following symbolic constants are provided for
R 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
mode
must be specified when
O_CREAT
is in the
R flags ,
and is ignored otherwise.
R creat ()
is equivalent to
R open ()
with
flags
equal to
R O_CREAT|O_WRONLY|O_TRUNC .
RETURN VALUE
R open ()
and
R creat ()
return the new file descriptor, or -1 if an error 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 directories in the path prefix of
R 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
R O_CREAT and O_EXCL
were used.
EFAULT
R 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.)
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
R pathname ,
or O_NOFOLLOW was specified but
pathname
was a symbolic link.
EMFILE
The process already has the maximum number of files open.
ENAMETOOLONG
R 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
R 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.
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 filesystem 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
R O_NOATIME ,
R O_NOFOLLOW ,
and
O_DIRECTORY
flags are Linux specific.
One may have to define the
_GNU_SOURCE
macro to get their definitions.
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).
The (undefined) effect of
O_RDONLY | O_TRUNC
varies among implementations.
On many systems the file is actually truncated.
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 same name, but without alignment restrictions.
Support was added under Linux in kernel version 2.4.10.
Older Linux kernels simply ignore this flag.
One may have to define the
_GNU_SOURCE
macro to get its definition.
There are many infelicities in the protocol underlying NFS, affecting
amongst others
R 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
R open ()
can open device special files, but
R creat ()
cannot create them; use
mknod(2)
instead.
On NFS file systems with UID mapping enabled,
R open ()
may
return a file descriptor but, for example,
read(2)
requests are denied
with EACCES.
This is because the client performs
R 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
R st_atime ,
R 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.
BUGS
"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
Currently, it is not possible to enable signal-driven
I/O by specifying
O_ASYNC
when calling
R open ();
use
fcntl(2)
to enable this flag.
