mirror of https://github.com/mkerrisk/man-pages
561 lines
16 KiB
Groff
561 lines
16 KiB
Groff
.\" Hey Emacs! This file is -*- nroff -*- source.
|
|
.\"
|
|
.\" This manpage is Copyright (C) 1992 Drew Eckhardt;
|
|
.\" 1993 Michael Haardt, Ian Jackson.
|
|
.\"
|
|
.\" Permission is granted to make and distribute verbatim copies of this
|
|
.\" manual provided the copyright notice and this permission notice are
|
|
.\" preserved on all copies.
|
|
.\"
|
|
.\" Permission is granted to copy and distribute modified versions of this
|
|
.\" manual under the conditions for verbatim copying, provided that the
|
|
.\" entire resulting derived work is distributed under the terms of a
|
|
.\" permission notice identical to this one.
|
|
.\"
|
|
.\" Since the Linux kernel and libraries are constantly changing, this
|
|
.\" manual page may be incorrect or out-of-date. The author(s) assume no
|
|
.\" responsibility for errors or omissions, or for damages resulting from
|
|
.\" the use of the information contained herein. The author(s) may not
|
|
.\" have taken the same level of care in the production of this manual,
|
|
.\" which is licensed free of charge, as they might when working
|
|
.\" professionally.
|
|
.\"
|
|
.\" Formatted or processed versions of this manual, if unaccompanied by
|
|
.\" the source, must acknowledge the copyright and authors of this work.
|
|
.\"
|
|
.\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
|
|
.\" Modified 1994-08-21 by Michael Haardt
|
|
.\" Modified 1996-04-13 by Andries Brouwer <aeb@cwi.nl>
|
|
.\" Modified 1996-05-13 by Thomas Koenig
|
|
.\" Modified 1996-12-20 by Michael Haardt
|
|
.\" Modified 1999-02-19 by Andries Brouwer <aeb@cwi.nl>
|
|
.\" Modified 1998-11-28 by Joseph S. Myers <jsm28@hermes.cam.ac.uk>
|
|
.\" Modified 1999-06-03 by Michael Haardt
|
|
.\" Modified 2002-05-07 by Michael Kerrisk <mtk-manpages@gmx.net>
|
|
.\" Modified 2004-06-23 by Michael Kerrisk <mtk-manpages@gmx.net>
|
|
.\" 2004-12-08, mtk, reordered flags list alphabetically
|
|
.\" 2004-12-08, Martin Pool <mbp@sourcefrog.net> (& mtk), added O_NOATIME
|
|
.\"
|
|
.TH OPEN 2 2005-06-22 "Linux 2.6.12" "Linux Programmer's Manual"
|
|
.SH NAME
|
|
open, creat \- open and possibly create a file or device
|
|
.SH SYNOPSIS
|
|
.nf
|
|
.B #include <sys/types.h>
|
|
.B #include <sys/stat.h>
|
|
.B #include <fcntl.h>
|
|
.sp
|
|
.BI "int open(const char *" pathname ", int " flags );
|
|
.BI "int open(const char *" pathname ", int " flags ", mode_t " mode );
|
|
.BI "int creat(const char *" pathname ", mode_t " mode );
|
|
.fi
|
|
.SH DESCRIPTION
|
|
Given a
|
|
.IR pathname
|
|
for a file,
|
|
.BR open ()
|
|
returns a file descriptor, a small, non-negative integer
|
|
for use in subsequent system calls
|
|
.RB ( 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.
|
|
.PP
|
|
The new file descriptor is set to remain open across an
|
|
.BR execve (2)
|
|
(i.e., the
|
|
.B FD_CLOEXEC
|
|
file descriptor flag described in
|
|
.BR fcntl (2)
|
|
is initially disabled).
|
|
The file offset is set to the beginning of the file (see
|
|
.BR lseek (2)).
|
|
.PP
|
|
A call to
|
|
.BR open ()
|
|
creates a new
|
|
.IR "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
|
|
.BR fcntl ()
|
|
.B F_SETFL
|
|
operation).
|
|
A file descriptor is a reference to one of these entries;
|
|
this reference is unaffected if
|
|
.I 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
|
|
.BR fork (2).
|
|
.PP
|
|
The parameter
|
|
.I flags
|
|
must include one of the following
|
|
.IR "access modes" :
|
|
.BR 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
|
|
.RI bitwise- or 'd
|
|
in
|
|
.IR flags .
|
|
The
|
|
.I file creation flags
|
|
are
|
|
.BR O_CREAT ", " O_EXCL ", " O_NOCTTY ", and " O_TRUNC .
|
|
The
|
|
.I 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
|
|
.BR fcntl (2).
|
|
The full list of file creation flags and file status flags is as follows:
|
|
.TP
|
|
.B O_APPEND
|
|
The file is opened in append mode. Before each
|
|
.BR write (),
|
|
the file offset is positioned at the end of the file,
|
|
as if with
|
|
.BR lseek ().
|
|
.B 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.
|
|
.TP
|
|
.B O_ASYNC
|
|
Enable signal-driven I/O:
|
|
generate a signal (SIGIO by default, but this can be changed via
|
|
.BR 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
|
|
.BR fcntl (2)
|
|
for further details.
|
|
.TP
|
|
.B 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, e.g., the mount options
|
|
.I bsdgroups
|
|
and
|
|
.I sysvgroups
|
|
of the ext2 filesystem, as described in
|
|
.BR mount (8)).
|
|
.TP
|
|
.B 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 a
|
|
.BR read (2)
|
|
or
|
|
.BR 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.
|
|
.\" Alignment should satisfy requirements for the underlying device
|
|
.\" There may be coherency problems.
|
|
.sp
|
|
A semantically similar (but deprecated) interface for block devices
|
|
is described in
|
|
.BR raw (8).
|
|
.TP
|
|
.B O_DIRECTORY
|
|
If \fIpathname\fR is not a directory, cause the open to fail.
|
|
.\" But see the following and its replies:
|
|
.\" http://marc.theaimsgroup.com/?t=112748702800001&r=1&w=2
|
|
.\" [PATCH] open: O_DIRECTORY and O_CREAT together should fail
|
|
.\" O_DIRECTORY | O_CREAT causes O_DIRECTORY to be ignored.
|
|
This flag is Linux-specific, and was added in kernel version 2.1.126, to
|
|
avoid denial-of-service problems if \fBopendir\fR(3) is called on a
|
|
FIFO or tape device, but should not be used outside of the
|
|
implementation of \fBopendir\fR.
|
|
.TP
|
|
.B O_EXCL
|
|
When used with
|
|
.BR O_CREAT ,
|
|
if the file already exists it is an error and the
|
|
.BR open ()
|
|
will fail. In this context, a symbolic link exists, regardless
|
|
of where it points to.
|
|
.B 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
|
|
.BR link (2)
|
|
to make a link to the lockfile. If \fBlink\fP() returns 0, the lock is
|
|
successful. Otherwise, use
|
|
.BR stat (2)
|
|
on the unique file to check if its link count has increased to 2,
|
|
in which case the lock is also successful.
|
|
.TP
|
|
.B O_LARGEFILE
|
|
(LFS)
|
|
Allow files whose sizes cannot be represented in an
|
|
.I off_t
|
|
(but can be represented in an
|
|
.IR off64_t )
|
|
to be opened.
|
|
.TP
|
|
.B O_NOATIME
|
|
(Since Linux 2.6.8)
|
|
Do not update the file last access time (st_atime in the inode)
|
|
when the file is
|
|
.BR 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.
|
|
.\" FIXME? The O_NOATIME flag also affects the treatment of st_atime
|
|
.\" by mmap() and readdir(2), MTK, Dec 04.
|
|
.TP
|
|
.B O_NOCTTY
|
|
If
|
|
.I pathname
|
|
refers to a terminal device \(em see
|
|
.BR tty (4)
|
|
\(em it will not become the process's controlling terminal even if the
|
|
process does not have one.
|
|
.TP
|
|
.B O_NOFOLLOW
|
|
If \fIpathname\fR 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; \fIkernels before 2.1.126 will ignore it if
|
|
.\" used\fR.
|
|
.TP
|
|
.BR O_NONBLOCK " or " O_NDELAY
|
|
When possible, the file is opened in non-blocking mode. Neither the
|
|
.BR 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
|
|
.BR fifo (7).
|
|
For a discussion of the effect of
|
|
.BR O_NONBLOCK
|
|
in conjunction with mandatory file locks and with file leases, see
|
|
.BR fcntl (2).
|
|
|
|
.TP
|
|
.B O_SYNC
|
|
The file is opened for synchronous I/O. Any
|
|
.BR write ()s
|
|
on the resulting file descriptor will block the calling process until
|
|
the data has been physically written to the underlying hardware.
|
|
.IR "But see RESTRICTIONS below" .
|
|
.TP
|
|
.B 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.
|
|
.PP
|
|
Some of these optional flags can be altered using
|
|
.BR fcntl ()
|
|
after the file has been opened.
|
|
|
|
The argument
|
|
.I mode
|
|
specifies the permissions to use in case a new file is created. It is
|
|
modified by the process's
|
|
.BR umask
|
|
in the usual way: the permissions of the created file are
|
|
.BR "(mode & ~umask)" .
|
|
Note that this mode only applies to future accesses of the
|
|
newly created file; the
|
|
.BR open ()
|
|
call that creates a read-only file may well return a read/write
|
|
file descriptor.
|
|
.PP
|
|
The following symbolic constants are provided for
|
|
.IR mode :
|
|
.TP
|
|
.B S_IRWXU
|
|
00700 user (file owner) has read, write and execute permission
|
|
.TP
|
|
.B S_IRUSR
|
|
00400 user has read permission
|
|
.TP
|
|
.B S_IWUSR
|
|
00200 user has write permission
|
|
.TP
|
|
.B S_IXUSR
|
|
00100 user has execute permission
|
|
.TP
|
|
.B S_IRWXG
|
|
00070 group has read, write and execute permission
|
|
.TP
|
|
.B S_IRGRP
|
|
00040 group has read permission
|
|
.TP
|
|
.B S_IWGRP
|
|
00020 group has write permission
|
|
.TP
|
|
.B S_IXGRP
|
|
00010 group has execute permission
|
|
.TP
|
|
.B S_IRWXO
|
|
00007 others have read, write and execute permission
|
|
.TP
|
|
.B S_IROTH
|
|
00004 others have read permission
|
|
.TP
|
|
.B S_IWOTH
|
|
00002 others have write permission
|
|
.TP
|
|
.B S_IXOTH
|
|
00001 others have execute permission
|
|
.PP
|
|
.I mode
|
|
must be specified when
|
|
.B O_CREAT
|
|
is in the
|
|
.IR flags ,
|
|
and is ignored otherwise.
|
|
|
|
.BR creat ()
|
|
is equivalent to
|
|
.BR open ()
|
|
with
|
|
.I flags
|
|
equal to
|
|
.BR O_CREAT|O_WRONLY|O_TRUNC .
|
|
.SH "RETURN VALUE"
|
|
.BR open ()
|
|
and
|
|
.BR creat ()
|
|
return the new file descriptor, or \-1 if an error occurred
|
|
(in which case,
|
|
.I errno
|
|
is set appropriately).
|
|
.SH NOTES
|
|
Note that
|
|
.BR open ()
|
|
can open device special files, but
|
|
.BR creat ()
|
|
cannot create them; use
|
|
.BR mknod (2)
|
|
instead.
|
|
.LP
|
|
On NFS file systems with UID mapping enabled, \fBopen\fP() may
|
|
return a file descriptor but e.g. \fBread\fP(2) requests are denied
|
|
with \fBEACCES\fP.
|
|
This is because the client performs \fBopen\fP() 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
|
|
.BR 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.
|
|
.SH ERRORS
|
|
.TP
|
|
.B 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
|
|
.IR pathname ,
|
|
or the file did not exist yet and write access to the parent directory
|
|
is not allowed.
|
|
(See also
|
|
.BR path_resolution (2).)
|
|
.TP
|
|
.B EEXIST
|
|
.I pathname
|
|
already exists and
|
|
.BR O_CREAT " and " O_EXCL
|
|
were used.
|
|
.TP
|
|
.B EFAULT
|
|
.IR pathname
|
|
points outside your accessible address space.
|
|
.TP
|
|
.B EISDIR
|
|
.I pathname
|
|
refers to a directory and the access requested involved writing
|
|
(that is,
|
|
.B O_WRONLY
|
|
or
|
|
.B O_RDWR
|
|
is set).
|
|
.TP
|
|
.B ELOOP
|
|
Too many symbolic links were encountered in resolving
|
|
.IR pathname ,
|
|
or \fBO_NOFOLLOW\fR was specified but
|
|
.I pathname
|
|
was a symbolic link.
|
|
.TP
|
|
.B EMFILE
|
|
The process already has the maximum number of files open.
|
|
.TP
|
|
.B ENAMETOOLONG
|
|
.IR pathname
|
|
was too long.
|
|
.TP
|
|
.B ENFILE
|
|
The system limit on the total number of open files has been reached.
|
|
.TP
|
|
.B ENODEV
|
|
.I 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.)
|
|
.TP
|
|
.B ENOENT
|
|
O_CREAT is not set and the named file does not exist.
|
|
Or, a directory component in
|
|
.I pathname
|
|
does not exist or is a dangling symbolic link.
|
|
.TP
|
|
.B ENOMEM
|
|
Insufficient kernel memory was available.
|
|
.TP
|
|
.B ENOSPC
|
|
.I pathname
|
|
was to be created but the device containing
|
|
.I pathname
|
|
has no room for the new file.
|
|
.TP
|
|
.B ENOTDIR
|
|
A component used as a directory in
|
|
.I pathname
|
|
is not, in fact, a directory, or \fBO_DIRECTORY\fR was specified and
|
|
.I pathname
|
|
was not a directory.
|
|
.TP
|
|
.B 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.
|
|
.TP
|
|
.B EOVERFLOW
|
|
.I pathname
|
|
refers to a regular file, too large to be opened; see O_LARGEFILE above.
|
|
.TP
|
|
.B EPERM
|
|
The
|
|
.B O_NOATIME
|
|
flag was specified, but the effective user ID of the caller
|
|
.\" Strictly speaking, it's the file system UID... (MTK)
|
|
did not match the owner of the file and the caller was not privileged
|
|
.RB ( CAP_FOWNER ).
|
|
.TP
|
|
.B EROFS
|
|
.I pathname
|
|
refers to a file on a read-only filesystem and write access was
|
|
requested.
|
|
.TP
|
|
.B ETXTBSY
|
|
.I pathname
|
|
refers to an executable image which is currently being executed and
|
|
write access was requested.
|
|
.TP
|
|
.B EWOULDBLOCK
|
|
The
|
|
.B O_NONBLOCK
|
|
flag was specified, and an incompatible lease was held on the file
|
|
(see
|
|
.BR fcntl (2)).
|
|
.SH NOTE
|
|
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
|
|
.BR ioctl (2).
|
|
.SH "CONFORMING TO"
|
|
SVr4, 4.3BSD, POSIX.1-2001.
|
|
The
|
|
.BR O_NOATIME ,
|
|
.BR O_NOFOLLOW ,
|
|
and
|
|
.B O_DIRECTORY
|
|
flags are Linux-specific.
|
|
One may have to define the
|
|
.B _GNU_SOURCE
|
|
macro to get their definitions.
|
|
.LP
|
|
The (undefined) effect of
|
|
.B O_RDONLY | O_TRUNC
|
|
varies among implementations. On many systems the file is actually
|
|
truncated.
|
|
.\" Linux 2.0, 2.5: truncate
|
|
.\" Solaris 5.7, 5.8: truncate
|
|
.\" Irix 6.5: truncate
|
|
.\" Tru64 5.1B: truncate
|
|
.\" HP-UX 11.22: truncate
|
|
.\" FreeBSD 4.7: truncate
|
|
.LP
|
|
The
|
|
.B 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
|
|
.B _GNU_SOURCE
|
|
macro to get its definition.
|
|
.SH 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." \(em Linus
|
|
|
|
Currently, it is not possible to enable signal-driven
|
|
I/O by specifying
|
|
.B O_ASYNC
|
|
when calling
|
|
.BR open ();
|
|
use
|
|
.BR fcntl (2)
|
|
to enable this flag.
|
|
.\" FIXME Check bugzilla report on open(O_ASYNC)
|
|
.\" See http://bugzilla.kernel.org/show_bug.cgi?id=5993
|
|
.SH RESTRICTIONS
|
|
There are many infelicities in the protocol underlying NFS, affecting
|
|
amongst others
|
|
.BR O_SYNC " and " O_NDELAY .
|
|
|
|
POSIX provides for three different variants of synchronised I/O,
|
|
corresponding to the flags \fBO_SYNC\fR, \fBO_DSYNC\fR and
|
|
\fBO_RSYNC\fR. Currently (2.1.130) these are all synonymous under Linux.
|
|
.SH "SEE ALSO"
|
|
.BR close (2),
|
|
.BR dup (2),
|
|
.BR fcntl (2),
|
|
.BR link (2),
|
|
.BR lseek (2),
|
|
.BR mknod (2),
|
|
.BR mount (2),
|
|
.BR mmap (2),
|
|
.BR openat (2),
|
|
.BR path_resolution (2),
|
|
.BR read (2),
|
|
.BR socket (2),
|
|
.BR stat (2),
|
|
.BR umask (2),
|
|
.BR unlink (2),
|
|
.BR write (2),
|
|
.BR fopen (3),
|
|
.BR fifo (7),
|
|
.BR feature_test_macros (7)
|