1 .\" Hey Emacs! This file is -*- nroff -*- source.
3 .\" This manpage is Copyright (C) 1992 Drew Eckhardt;
4 .\" 1993 Michael Haardt, Ian Jackson.
7 .\" Permission is granted to make and distribute verbatim copies of this
8 .\" manual provided the copyright notice and this permission notice are
9 .\" preserved on all copies.
11 .\" Permission is granted to copy and distribute modified versions of this
12 .\" manual under the conditions for verbatim copying, provided that the
13 .\" entire resulting derived work is distributed under the terms of a
14 .\" permission notice identical to this one.
16 .\" Since the Linux kernel and libraries are constantly changing, this
17 .\" manual page may be incorrect or out-of-date. The author(s) assume no
18 .\" responsibility for errors or omissions, or for damages resulting from
19 .\" the use of the information contained herein. The author(s) may not
20 .\" have taken the same level of care in the production of this manual,
21 .\" which is licensed free of charge, as they might when working
24 .\" Formatted or processed versions of this manual, if unaccompanied by
25 .\" the source, must acknowledge the copyright and authors of this work.
27 .\" Modified 1993-07-21 by Rik Faith <faith@cs.unc.edu>
28 .\" Modified 1994-08-21 by Michael Haardt
29 .\" Modified 1996-04-13 by Andries Brouwer <aeb@cwi.nl>
30 .\" Modified 1996-05-13 by Thomas Koenig
31 .\" Modified 1996-12-20 by Michael Haardt
32 .\" Modified 1999-02-19 by Andries Brouwer <aeb@cwi.nl>
33 .\" Modified 1998-11-28 by Joseph S. Myers <jsm28@hermes.cam.ac.uk>
34 .\" Modified 1999-06-03 by Michael Haardt
35 .\" Modified 2002-05-07 by Michael Kerrisk <mtk.manpages@gmail.com>
36 .\" Modified 2004-06-23 by Michael Kerrisk <mtk.manpages@gmail.com>
37 .\" 2004-12-08, mtk, reordered flags list alphabetically
38 .\" 2004-12-08, Martin Pool <mbp@sourcefrog.net> (& mtk), added O_NOATIME
39 .\" 2007-09-18, mtk, Added description of O_CLOEXEC + other minor edits
40 .\" 2008-01-03, mtk, with input from Trond Myklebust
41 .\" <trond.myklebust@fys.uio.no> and Timo Sirainen <tss@iki.fi>
42 .\" Rewrite description of O_EXCL.
43 .\" 2008-01-11, Greg Banks <gnb@melbourne.sgi.com>: add more detail
45 .\" 2008-02-26, Michael Haardt: Reorganized text for O_CREAT and mode
47 .\" FIXME . Apr 08: The next POSIX revision has O_EXEC, O_SEARCH, and
48 .\" O_TTYINIT. Eventually these may need to be documented. --mtk
49 .\" FIXME Linux 2.6.33 has O_DSYNC, and a hidden __O_SYNC.
51 .TH OPEN 2 2010-09-10 "Linux" "Linux Programmer's Manual"
53 open, creat \- open and possibly create a file or device
56 .B #include <sys/types.h>
57 .B #include <sys/stat.h>
60 .BI "int open(const char *" pathname ", int " flags );
61 .BI "int open(const char *" pathname ", int " flags ", mode_t " mode );
63 .BI "int creat(const char *" pathname ", mode_t " mode );
70 returns a file descriptor, a small, nonnegative integer
71 for use in subsequent system calls
72 .RB ( read "(2), " write "(2), " lseek "(2), " fcntl "(2), etc.)."
73 The file descriptor returned by a successful call will be
74 the lowest-numbered file descriptor not currently open for the process.
76 By default, the new file descriptor is set to remain open across an
80 file descriptor flag described in
82 is initially disabled; the Linux-specific
84 flag, described below, can be used to change this default).
85 The file offset is set to the beginning of the file (see
91 .IR "open file description" ,
92 an entry in the system-wide table of open files.
93 This entry records the file offset and the file status flags
98 A file descriptor is a reference to one of these entries;
99 this reference is unaffected if
101 is subsequently removed or modified to refer to a different file.
102 The new open file description is initially not shared
103 with any other process,
104 but sharing may arise via
109 must include one of the following
111 .BR O_RDONLY ", " O_WRONLY ", or " O_RDWR .
112 These request opening the file read-only, write-only, or read/write,
115 In addition, zero or more file creation flags and file status flags
121 .I file creation flags
123 .BR O_CREAT ", " O_EXCL ", " O_NOCTTY ", and " O_TRUNC .
126 are all of the remaining flags listed below.
127 .\" FIXME . Actually is it true that the "file status flags" are all of the
128 .\" remaining flags listed below? SUSv4 divides the flags into:
132 .\" * Other (O_CLOEXEC, O_DIRECTORY, O_NOFOLLOW)
133 .\" though it's not clear what the difference between "other" and
134 .\" "File creation" flags is. (I've raised an Aardvark to see if this
135 .\" can be clarified in SUSv4; 10 Oct 2008.)
136 The distinction between these two groups of flags is that
137 the file status flags can be retrieved and (in some cases)
140 The full list of file creation flags and file status flags is as follows:
143 The file is opened in append mode.
146 the file offset is positioned at the end of the file,
150 may lead to corrupted files on NFS file systems if more than one process
151 appends data to a file at once.
152 .\" For more background, see
153 .\" http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=453946
154 .\" http://nfs.sourceforge.net/
155 This is because NFS does not support
156 appending to a file, so the client kernel has to simulate it, which
157 can't be done without a race condition.
160 Enable signal-driven I/O:
163 by default, but this can be changed via
165 when input or output becomes possible on this file descriptor.
166 This feature is only available for terminals, pseudoterminals,
167 sockets, and (since Linux 2.6) pipes and FIFOs.
172 .BR O_CLOEXEC " (Since Linux 2.6.23)"
173 Enable the close-on-exec flag for the new file descriptor.
174 Specifying this flag permits a program to avoid additional
177 operations to set the
181 use of this flag is essential in some multithreaded programs
182 since using a separate
187 flag does not suffice to avoid race conditions
188 where one thread opens a file descriptor at the same
189 time as another thread does a
193 .\" This flag fixes only one form of the race condition;
194 .\" The race can also occur with, for example, descriptors
195 .\" returned by accept(), pipe(), etc.
198 If the file does not exist it will be created.
199 The owner (user ID) of the file is set to the effective user ID
201 The group ownership (group ID) is set either to
202 the effective group ID of the process or to the group ID of the
203 parent directory (depending on file system type and mount options,
204 and the mode of the parent directory, see the mount options
210 .\" As at 2.6.25, bsdgroups is supported by ext2, ext3, ext4, and
211 .\" XFS (since 2.6.14).
215 specifies the permissions to use in case a new file is created.
216 This argument must be supplied when
222 is not specified, then
225 The effective permissions are modified by
228 in the usual way: The permissions of the created file are
229 .IR "(mode\ &\ ~umask)" .
230 Note that this mode only applies to future accesses of the
231 newly created file; the
233 call that creates a read-only file may well return a read/write
236 The following symbolic constants are provided for
240 00700 user (file owner) has read, write and execute permission
243 00400 user has read permission
246 00200 user has write permission
249 00100 user has execute permission
252 00070 group has read, write and execute permission
255 00040 group has read permission
258 00020 group has write permission
261 00010 group has execute permission
264 00007 others have read, write and execute permission
267 00004 others have read permission
270 00002 others have write permission
273 00001 others have execute permission
276 .BR O_DIRECT " (Since Linux 2.4.10)"
277 Try to minimize cache effects of the I/O to and from this file.
278 In general this will degrade performance, but it is useful in
279 special situations, such as when applications do their own caching.
280 File I/O is done directly to/from user space buffers.
283 flag on its own makes at an effort to transfer data synchronously,
284 but does not give the guarantees of the
286 that data and necessary metadata are transferred.
287 To guarantee synchronous I/O the
289 must be used in addition to
293 below for further discussion.
295 A semantically similar (but deprecated) interface for block devices
300 If \fIpathname\fP is not a directory, cause the open to fail.
301 .\" But see the following and its replies:
302 .\" http://marc.theaimsgroup.com/?t=112748702800001&r=1&w=2
303 .\" [PATCH] open: O_DIRECTORY and O_CREAT together should fail
304 .\" O_DIRECTORY | O_CREAT causes O_DIRECTORY to be ignored.
305 This flag is Linux-specific, and was added in kernel version 2.1.126, to
306 avoid denial-of-service problems if
309 FIFO or tape device, but should not be used outside of the
314 Ensure that this call creates the file:
315 if this flag is specified in conjunction with
328 When these two flags are specified, symbolic links are not followed:
329 .\" POSIX.1-2001 explicitly requires this behavior.
332 is a symbolic link, then
334 fails regardless of where the symbolic link points to.
338 is only supported when using NFSv3 or later on kernel 2.6 or later.
339 In NFS environments where
341 support is not provided, programs that rely on it
342 for performing locking tasks will contain a race condition.
343 Portable programs that want to perform atomic file locking using a lockfile,
344 and need to avoid reliance on NFS support for
346 can create a unique file on
347 the same file system (e.g., incorporating hostname and PID), and use
349 to make a link to the lockfile.
352 returns 0, the lock is successful.
355 on the unique file to check if its link count has increased to 2,
356 in which case the lock is also successful.
360 Allow files whose sizes cannot be represented in an
362 (but can be represented in an
366 .B _LARGEFILE64_SOURCE
367 macro must be defined
371 in order to obtain this definition.
374 feature test macro to 64 (rather than using
376 is the preferred method of obtaining
377 method of accessing large files on 32-bit systems (see
378 .BR feature_test_macros (7)).
380 .BR O_NOATIME " (Since Linux 2.6.8)"
381 Do not update the file last access time (st_atime in the inode)
384 This flag is intended for use by indexing or backup programs,
385 where its use can significantly reduce the amount of disk activity.
386 This flag may not be effective on all file systems.
387 One example is NFS, where the server maintains the access time.
388 .\" The O_NOATIME flag also affects the treatment of st_atime
389 .\" by mmap() and readdir(2), MTK, Dec 04.
394 refers to a terminal device \(em see
396 \(em it will not become the process's controlling terminal even if the
397 process does not have one.
400 If \fIpathname\fP is a symbolic link, then the open fails.
401 This is a FreeBSD extension, which was added to Linux in version 2.1.126.
402 Symbolic links in earlier components of the pathname will still be
404 .\" The headers from glibc 2.0.100 and later include a
405 .\" definition of this flag; \fIkernels before 2.1.126 will ignore it if
408 .BR O_NONBLOCK " or " O_NDELAY
409 When possible, the file is opened in nonblocking mode.
412 nor any subsequent operations on the file descriptor which is
413 returned will cause the calling process to wait.
414 For the handling of FIFOs (named pipes), see also
416 For a discussion of the effect of
418 in conjunction with mandatory file locks and with file leases, see
422 The file is opened for synchronous I/O.
425 on the resulting file descriptor will block the calling process until
426 the data has been physically written to the underlying hardware.
427 .IR "But see NOTES below" .
430 If the file already exists and is a regular file and the open mode allows
435 it will be truncated to length 0.
436 If the file is a FIFO or terminal device file, the
439 Otherwise the effect of
443 Some of these optional flags can be altered using
445 after the file has been opened.
453 .BR O_CREAT|O_WRONLY|O_TRUNC .
458 return the new file descriptor, or \-1 if an error occurred
461 is set appropriately).
465 The requested access to the file is not allowed, or search permission
466 is denied for one of the directories in the path prefix of
468 or the file did not exist yet and write access to the parent directory
471 .BR path_resolution (7).)
476 .BR O_CREAT " and " O_EXCL
481 points outside your accessible address space.
488 While blocked waiting to complete an open of a slow device
491 the call was interrupted by a signal handler; see
496 refers to a directory and the access requested involved writing
504 Too many symbolic links were encountered in resolving
506 or \fBO_NOFOLLOW\fP was specified but
511 The process already has the maximum number of files open.
518 The system limit on the total number of open files has been reached.
522 refers to a device special file and no corresponding device exists.
523 (This is a Linux kernel bug; in this situation
529 is not set and the named file does not exist.
530 Or, a directory component in
532 does not exist or is a dangling symbolic link.
535 Insufficient kernel memory was available.
539 was to be created but the device containing
541 has no room for the new file.
544 A component used as a directory in
546 is not, in fact, a directory, or \fBO_DIRECTORY\fP was specified and
551 .BR O_NONBLOCK " | " O_WRONLY
552 is set, the named file is a FIFO and
553 no process has the file open for reading.
554 Or, the file is a device special file and no corresponding device exists.
558 refers to a regular file that is too large to be opened.
559 The usual scenario here is that an application compiled
560 on a 32-bit platform without
561 .I -D_FILE_OFFSET_BITS=64
562 tried to open a file whose size exceeds
568 This is the error specified by POSIX.1-2001;
569 in kernels before 2.6.24, Linux gave the error
572 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=7253
573 .\" "Open of a large file on 32-bit fails with EFBIG, should be EOVERFLOW"
574 .\" Reported 2006-10-03
579 flag was specified, but the effective user ID of the caller
580 .\" Strictly speaking, it's the file system UID... (MTK)
581 did not match the owner of the file and the caller was not privileged
586 refers to a file on a read-only file system and write access was
591 refers to an executable image which is currently being executed and
592 write access was requested.
597 flag was specified, and an incompatible lease was held on the file
601 SVr4, 4.3BSD, POSIX.1-2001.
607 flags are Linux-specific, and one may need to define
612 to obtain their definitions.
616 flag is not specified in POSIX.1-2001,
617 but is specified in POSIX.1-2008.
620 is not specified in POSIX; one has to define
625 to get its definition.
629 flag indicates that one wants to open
630 but does not necessarily have the intention to read or write.
631 This is typically used to open devices in order to get a file descriptor
635 Unlike the other values that can be specified in
640 .BR O_RDONLY ", " O_WRONLY ", and " O_RDWR ,
641 do not specify individual bits.
642 Rather, they define the low order two bits of
644 and are defined respectively as 0, 1, and 2.
645 In other words, the combination
646 .B "O_RDONLY | O_WRONLY"
647 is a logical error, and certainly does not have the same meaning as
649 Linux reserves the special, nonstandard access mode 3 (binary 11) in
652 check for read and write permission on the file and return a descriptor
653 that can't be used for reading or writing.
654 This nonstandard access mode is used by some Linux drivers to return a
655 descriptor that is only to be used for device-specific
658 .\" See for example util-linux's disk-utils/setfdprm.c
659 .\" For some background on access mode 3, see
660 .\" http://thread.gmane.org/gmane.linux.kernel/653123
661 .\" "[RFC] correct flags to f_mode conversion in __dentry_open"
662 .\" LKML, 12 Mar 2008
664 The (undefined) effect of
665 .B O_RDONLY | O_TRUNC
666 varies among implementations.
667 On many systems the file is actually truncated.
668 .\" Linux 2.0, 2.5: truncate
669 .\" Solaris 5.7, 5.8: truncate
670 .\" Irix 6.5: truncate
671 .\" Tru64 5.1B: truncate
672 .\" HP-UX 11.22: truncate
673 .\" FreeBSD 4.7: truncate
675 There are many infelicities in the protocol underlying NFS, affecting
677 .BR O_SYNC " and " O_NDELAY .
679 POSIX provides for three different variants of synchronized I/O,
680 corresponding to the flags
685 Currently (2.6.31), Linux only implements
691 to the same numerical value as
693 Most Linux file systems don't actually implement the POSIX
695 semantics, which require all metadata updates of a write
696 to be on disk on returning to userspace, but only the
698 semantics, which require only actual file data and metadata necessary
699 to retrieve it to be on disk by the time the system call returns.
703 can open device special files, but
705 cannot create them; use
709 On NFS file systems with UID mapping enabled,
712 return a file descriptor but, for example,
716 This is because the client performs
719 permissions, but UID mapping is performed by the server upon
720 read and write requests.
722 If the file is newly created, its
727 (respectively, time of last access, time of last status change, and
728 time of last modification; see
731 to the current time, and so are the
737 Otherwise, if the file is modified because of the
739 flag, its st_ctime and st_mtime fields are set to the current time.
744 flag may impose alignment restrictions on the length and address
745 of userspace buffers and the file offset of I/Os.
747 restrictions vary by file system and kernel version and might be
749 However there is currently no file system\-independent
750 interface for an application to discover these restrictions for a given
752 Some file systems provide their own interfaces
753 for doing so, for example the
758 Under Linux 2.4, transfer sizes, and the alignment of the user buffer
759 and the file offset must all be multiples of the logical block size
761 Under Linux 2.6, alignment to 512-byte boundaries
766 flag was introduced in SGI IRIX, where it has alignment
767 restrictions similar to those of Linux 2.4.
770 call to query appropriate alignments, and sizes.
771 FreeBSD 4.x introduced
772 a flag of the same name, but without alignment restrictions.
775 support was added under Linux in kernel version 2.4.10.
776 Older Linux kernels simply ignore this flag.
777 Some file systems may not implement the flag and
783 Applications should avoid mixing
785 and normal I/O to the same file,
786 and especially to overlapping byte regions in the same file.
787 Even when the file system correctly handles the coherency issues in
788 this situation, overall I/O throughput is likely to be slower than
789 using either mode alone.
790 Likewise, applications should avoid mixing
792 of files with direct I/O to the same files.
796 with NFS will differ from local file systems.
798 kernels configured in certain ways, may not support this combination.
799 The NFS protocol does not support passing the flag to the server, so
801 I/O will only bypass the page cache on the client; the server may
803 The client asks the server to make the I/O
804 synchronous to preserve the synchronous semantics of
806 Some servers will perform poorly under these circumstances, especially
807 if the I/O size is small.
808 Some servers may also be configured to
809 lie to clients about the I/O having reached stable storage; this
810 will avoid the performance penalty at some risk to data integrity
811 in the event of server power failure.
812 The Linux NFS client places no alignment restrictions on
818 is a potentially powerful tool that should be used with caution.
819 It is recommended that applications treat use of
821 as a performance option which is disabled by default.
824 "The thing that has always disturbed me about O_DIRECT is that the whole
825 interface is just stupid, and was probably designed by a deranged monkey
826 on some serious mind-controlling substances." \(em Linus
829 Currently, it is not possible to enable signal-driven
837 .\" FIXME . Check bugzilla report on open(O_ASYNC)
838 .\" See http://bugzilla.kernel.org/show_bug.cgi?id=5993
859 .BR path_resolution (7),
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