1OPEN(2)                    Linux Programmer's Manual                   OPEN(2)


6       open, openat, creat - open and possibly create a file


9       #include <sys/types.h>
10       #include <sys/stat.h>
11       #include <fcntl.h>
13       int open(const char *pathname, int flags);
14       int open(const char *pathname, int flags, mode_t mode);
16       int creat(const char *pathname, mode_t mode);
18       int openat(int dirfd, const char *pathname, int flags);
19       int openat(int dirfd, const char *pathname, int flags, mode_t mode);
21   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
23       openat():
24           Since glibc 2.10:
25               _POSIX_C_SOURCE >= 200809L
26           Before glibc 2.10:
27               _ATFILE_SOURCE


30       The  open()  system  call opens the file specified by pathname.  If the
31       specified file does not exist, it may optionally (if O_CREAT is  speci‐
32       fied in flags) be created by open().
34       The  return  value of open() is a file descriptor, a small, nonnegative
35       integer that is used in subsequent  system  calls  (read(2),  write(2),
36       lseek(2), fcntl(2), etc.) to refer to the open file.  The file descrip‐
37       tor returned by a successful call  will  be  the  lowest-numbered  file
38       descriptor not currently open for the process.
40       By  default,  the  new  file descriptor is set to remain open across an
41       execve(2) (i.e., the  FD_CLOEXEC  file  descriptor  flag  described  in
42       fcntl(2)  is  initially disabled); the O_CLOEXEC flag, described below,
43       can be used to change this default.  The file  offset  is  set  to  the
44       beginning of the file (see lseek(2)).
46       A  call  to open() creates a new open file description, an entry in the
47       system-wide table of open files.  The open file description records the
48       file  offset  and the file status flags (see below).  A file descriptor
49       is a reference to an open file description;  this  reference  is  unaf‐
50       fected  if  pathname  is subsequently removed or modified to refer to a
51       different file.  For further details on  open  file  descriptions,  see
52       NOTES.
54       The  argument  flags  must  include  one of the following access modes:
55       O_RDONLY, O_WRONLY, or O_RDWR.  These request opening  the  file  read-
56       only, write-only, or read/write, respectively.
58       In addition, zero or more file creation flags and file status flags can
59       be bitwise-or'd in flags.   The  file  creation  flags  are  O_CLOEXEC,
61       O_TRUNC.  The file status flags are all of the remaining  flags  listed
62       below.   The  distinction between these two groups of flags is that the
63       file creation flags affect the semantics of the open operation  itself,
64       while  the  file  status  flags  affect the semantics of subsequent I/O
65       operations.  The file status flags can be retrieved and (in some cases)
66       modified; see fcntl(2) for details.
68       The  full  list of file creation flags and file status flags is as fol‐
69       lows:
71       O_APPEND
72              The file is opened in append mode.  Before  each  write(2),  the
73              file  offset  is  positioned  at the end of the file, as if with
74              lseek(2).  The modification of the file  offset  and  the  write
75              operation are performed as a single atomic step.
77              O_APPEND  may lead to corrupted files on NFS filesystems if more
78              than one process appends data  to  a  file  at  once.   This  is
79              because  NFS does not support appending to a file, so the client
80              kernel has to simulate it, which can't be done  without  a  race
81              condition.
83       O_ASYNC
84              Enable  signal-driven  I/O: generate a signal (SIGIO by default,
85              but this can be changed  via  fcntl(2))  when  input  or  output
86              becomes  possible  on  this  file  descriptor.   This feature is
87              available only  for  terminals,  pseudoterminals,  sockets,  and
88              (since  Linux  2.6)  pipes  and FIFOs.  See fcntl(2) for further
89              details.  See also BUGS, below.
91       O_CLOEXEC (since Linux 2.6.23)
92              Enable the close-on-exec  flag  for  the  new  file  descriptor.
93              Specifying  this  flag  permits  a  program  to avoid additional
94              fcntl(2) F_SETFD operations to set the FD_CLOEXEC flag.
96              Note that the use of this  flag  is  essential  in  some  multi‐
97              threaded  programs,  because  using  a separate fcntl(2) F_SETFD
98              operation to set the FD_CLOEXEC flag does not suffice  to  avoid
99              race  conditions  where  one  thread opens a file descriptor and
100              attempts to set its close-on-exec flag  using  fcntl(2)  at  the
101              same  time  as  another  thread  does  a fork(2) plus execve(2).
102              Depending on the order of execution, the race may  lead  to  the
103              file  descriptor returned by open() being unintentionally leaked
104              to the program executed by the child process created by fork(2).
105              (This  kind of race is in principle possible for any system call
106              that creates a file descriptor whose close-on-exec  flag  should
107              be  set, and various other Linux system calls provide an equiva‐
108              lent of the O_CLOEXEC flag to deal with this problem.)
110       O_CREAT
111              If pathname does not exist, create it as a regular file.
113              The owner (user ID) of the new file is set to the effective user
114              ID of the process.
116              The  group ownership (group ID) of the new file is set either to
117              the effective group ID of the process (System V semantics) or to
118              the group ID of the parent directory (BSD semantics).  On Linux,
119              the behavior depends on whether the set-group-ID mode bit is set
120              on  the parent directory: if that bit is set, then BSD semantics
121              apply; otherwise, System V semantics apply.  For  some  filesys‐
122              tems,  the behavior also depends on the bsdgroups and sysvgroups
123              mount options described in mount(8)).
125              The mode argument specifies the file mode bits be applied when a
126              new  file  is  created.   This  argument  must  be supplied when
127              O_CREAT or O_TMPFILE is specified in flags; if  neither  O_CREAT
128              nor O_TMPFILE is specified, then mode is ignored.  The effective
129              mode is modified by the process's umask in the usual way: in the
130              absence  of  a  default  ACL,  the  mode  of the created file is
131              (mode & ~umask).  Note that this mode  applies  only  to  future
132              accesses of the newly created file; the open() call that creates
133              a read-only file may well return a read/write file descriptor.
135              The following symbolic constants are provided for mode:
137              S_IRWXU  00700 user (file owner) has read,  write,  and  execute
138                       permission
140              S_IRUSR  00400 user has read permission
142              S_IWUSR  00200 user has write permission
144              S_IXUSR  00100 user has execute permission
146              S_IRWXG  00070 group has read, write, and execute permission
148              S_IRGRP  00040 group has read permission
150              S_IWGRP  00020 group has write permission
152              S_IXGRP  00010 group has execute permission
154              S_IRWXO  00007 others have read, write, and execute permission
156              S_IROTH  00004 others have read permission
158              S_IWOTH  00002 others have write permission
160              S_IXOTH  00001 others have execute permission
162              According  to  POSIX, the effect when other bits are set in mode
163              is unspecified.  On Linux, the following bits are  also  honored
164              in mode:
166              S_ISUID  0004000 set-user-ID bit
168              S_ISGID  0002000 set-group-ID bit (see inode(7)).
170              S_ISVTX  0001000 sticky bit (see inode(7)).
172       O_DIRECT (since Linux 2.4.10)
173              Try  to minimize cache effects of the I/O to and from this file.
174              In general this will degrade performance, but it  is  useful  in
175              special  situations,  such  as  when  applications  do their own
176              caching.  File I/O is done directly to/from user-space  buffers.
177              The  O_DIRECT  flag  on its own makes an effort to transfer data
178              synchronously, but does not give the guarantees  of  the  O_SYNC
179              flag that data and necessary metadata are transferred.  To guar‐
180              antee synchronous I/O,  O_SYNC  must  be  used  in  addition  to
181              O_DIRECT.  See NOTES below for further discussion.
183              A  semantically  similar  (but  deprecated)  interface for block
184              devices is described in raw(8).
186       O_DIRECTORY
187              If pathname is not a directory, cause the open  to  fail.   This
188              flag  was  added  in kernel version 2.1.126, to avoid denial-of-
189              service problems if opendir(3) is  called  on  a  FIFO  or  tape
190              device.
192       O_DSYNC
193              Write  operations  on  the  file  will complete according to the
194              requirements of synchronized I/O data integrity completion.
196              By the time write(2) (and similar) return, the output  data  has
197              been transferred to the underlying hardware, along with any file
198              metadata that would be required to retrieve that data (i.e.,  as
199              though  each  write(2)  was followed by a call to fdatasync(2)).
200              See NOTES below.
202       O_EXCL Ensure that this call creates the file: if this flag  is  speci‐
203              fied  in  conjunction with O_CREAT, and pathname already exists,
204              then open() fails with the error EEXIST.
206              When these two flags are specified, symbolic links are not  fol‐
207              lowed: if pathname is a symbolic link, then open() fails regard‐
208              less of where the symbolic link points.
210              In general, the behavior of O_EXCL is undefined if  it  is  used
211              without  O_CREAT.   There  is  one  exception:  on Linux 2.6 and
212              later, O_EXCL can be used without O_CREAT if pathname refers  to
213              a  block  device.   If  the block device is in use by the system
214              (e.g., mounted), open() fails with the error EBUSY.
216              On NFS, O_EXCL is supported only when using NFSv3  or  later  on
217              kernel  2.6  or later.  In NFS environments where O_EXCL support
218              is not provided, programs that rely on it for performing locking
219              tasks  will  contain  a  race condition.  Portable programs that
220              want to perform atomic file locking using a lockfile,  and  need
221              to avoid reliance on NFS support for O_EXCL, can create a unique
222              file on the same filesystem (e.g.,  incorporating  hostname  and
223              PID),  and  use  link(2)  to  make  a  link to the lockfile.  If
224              link(2) returns 0,  the  lock  is  successful.   Otherwise,  use
225              stat(2)  on  the  unique  file  to  check  if its link count has
226              increased to 2, in which case the lock is also successful.
228       O_LARGEFILE
229              (LFS) Allow files whose sizes cannot be represented in an  off_t
230              (but  can  be  represented  in  an  off64_t)  to be opened.  The
231              _LARGEFILE64_SOURCE macro must be defined (before including  any
232              header  files)  in order to obtain this definition.  Setting the
233              _FILE_OFFSET_BITS feature test macro to 64  (rather  than  using
234              O_LARGEFILE) is the preferred method of accessing large files on
235              32-bit systems (see feature_test_macros(7)).
237       O_NOATIME (since Linux 2.6.8)
238              Do not update the file last access time (st_atime in the  inode)
239              when the file is read(2).
241              This  flag  can  be employed only if one of the following condi‐
242              tions is true:
244              *  The effective UID of the process matches the owner UID of the
245                 file.
247              *  The calling process has the CAP_FOWNER capability in its user
248                 namespace and the owner UID of the file has a mapping in  the
249                 namespace.
251              This  flag  is  intended for use by indexing or backup programs,
252              where its use can significantly reduce the amount of disk activ‐
253              ity.   This  flag  may not be effective on all filesystems.  One
254              example is NFS, where the server maintains the access time.
256       O_NOCTTY
257              If pathname refers to a terminal device—see tty(4)—it  will  not
258              become  the  process's  controlling terminal even if the process
259              does not have one.
261       O_NOFOLLOW
262              If pathname is a symbolic link, then the open  fails,  with  the
263              error  ELOOP.  Symbolic links in earlier components of the path‐
264              name will still be followed.  (Note that the  ELOOP  error  that
265              can  occur in this case is indistinguishable from the case where
266              an open fails because there are too many  symbolic  links  found
267              while resolving components in the prefix part of the pathname.)
269              This  flag  is  a FreeBSD extension, which was added to Linux in
270              version 2.1.126,  and  has  subsequently  been  standardized  in
271              POSIX.1-2008.
273              See also O_PATH below.
275       O_NONBLOCK or O_NDELAY
276              When  possible, the file is opened in nonblocking mode.  Neither
277              the open()  nor  any  subsequent  I/O  operations  on  the  file
278              descriptor  which  is returned will cause the calling process to
279              wait.
281              Note that the setting of this flag has no effect on  the  opera‐
282              tion  of  poll(2), select(2), epoll(7), and similar, since those
283              interfaces  merely  inform  the  caller  about  whether  a  file
284              descriptor  is  "ready", meaning that an I/O operation performed
285              on the file descriptor with the O_NONBLOCK flag clear would  not
286              block.
288              Note  that  this  flag has no effect for regular files and block
289              devices; that is,  I/O  operations  will  (briefly)  block  when
290              device activity is required, regardless of whether O_NONBLOCK is
291              set.  Since O_NONBLOCK  semantics  might  eventually  be  imple‐
292              mented,  applications  should  not depend upon blocking behavior
293              when specifying this flag for regular files and block devices.
295              For the handling of FIFOs (named pipes), see also fifo(7).   For
296              a  discussion  of  the  effect of O_NONBLOCK in conjunction with
297              mandatory file locks and with file leases, see fcntl(2).
299       O_PATH (since Linux 2.6.39)
300              Obtain a file descriptor that can be used for two  purposes:  to
301              indicate a location in the filesystem tree and to perform opera‐
302              tions that act purely at the file descriptor  level.   The  file
303              itself  is not opened, and other file operations (e.g., read(2),
304              write(2), fchmod(2), fchown(2), fgetxattr(2), ioctl(2), mmap(2))
305              fail with the error EBADF.
307              The  following operations can be performed on the resulting file
308              descriptor:
310              *  close(2).
312              *  fchdir(2), if the  file  descriptor  refers  to  a  directory
313                 (since Linux 3.5).
315              *  fstat(2) (since Linux 3.6).
317              *  fstatfs(2) (since Linux 3.12).
319              *  Duplicating  the  file  descriptor (dup(2), fcntl(2) F_DUPFD,
320                 etc.).
322              *  Getting and setting file descriptor flags  (fcntl(2)  F_GETFD
323                 and F_SETFD).
325              *  Retrieving  open file status flags using the fcntl(2) F_GETFL
326                 operation: the returned flags will include the bit O_PATH.
328              *  Passing the file descriptor as the dirfd argument of openat()
329                 and  the other "*at()" system calls.  This includes linkat(2)
330                 with AT_EMPTY_PATH (or via  procfs  using  AT_SYMLINK_FOLLOW)
331                 even if the file is not a directory.
333              *  Passing  the  file  descriptor  to another process via a UNIX
334                 domain socket (see SCM_RIGHTS in unix(7)).
336              When  O_PATH  is  specified  in  flags,  flag  bits  other  than
337              O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW are ignored.
339              Opening  a  file  or  directory with the O_PATH flag requires no
340              permissions on the object itself (but does require execute  per‐
341              mission  on  the  directories in the path prefix).  Depending on
342              the subsequent operation, a check for suitable file  permissions
343              may be performed (e.g., fchdir(2) requires execute permission on
344              the directory referred to by its file descriptor argument).   By
345              contrast,  obtaining a reference to a filesystem object by open‐
346              ing it with the O_RDONLY flag requires that the caller have read
347              permission  on  the  object,  even when the subsequent operation
348              (e.g., fchdir(2), fstat(2)) does not require read permission  on
349              the object.
351              If  pathname  is a symbolic link and the O_NOFOLLOW flag is also
352              specified, then the call returns a file descriptor referring  to
353              the  symbolic  link.   This  file  descriptor can be used as the
354              dirfd argument in calls to fchownat(2),  fstatat(2),  linkat(2),
355              and readlinkat(2) with an empty pathname to have the calls oper‐
356              ate on the symbolic link.
358              If pathname refers to an automount point that has not  yet  been
359              triggered,  so  no  other  filesystem is mounted on it, then the
360              call returns a file descriptor referring to the automount direc‐
361              tory without triggering a mount.  fstatfs(2) can then be used to
362              determine if it is, in  fact,  an  untriggered  automount  point
363              (.f_type == AUTOFS_SUPER_MAGIC).
365              One use of O_PATH for regular files is to provide the equivalent
366              of POSIX.1's O_EXEC functionality.  This permits us  to  open  a
367              file  for  which we have execute permission but not read permis‐
368              sion, and then execute that file, with steps something like  the
369              following:
371                  char buf[PATH_MAX];
372                  fd = open("some_prog", O_PATH);
373                  snprintf(buf, PATH_MAX, "/proc/self/fd/%d", fd);
374                  execl(buf, "some_prog", (char *) NULL);
376              An  O_PATH file descriptor can also be passed as the argument of
377              fexecve(3).
379       O_SYNC Write operations on the file  will  complete  according  to  the
380              requirements  of  synchronized I/O file integrity completion (by
381              contrast with the synchronized  I/O  data  integrity  completion
382              provided by O_DSYNC.)
384              By  the  time write(2) (or similar) returns, the output data and
385              associated file metadata have been transferred to the underlying
386              hardware  (i.e.,  as though each write(2) was followed by a call
387              to fsync(2)).  See NOTES below.
389       O_TMPFILE (since Linux 3.11)
390              Create an unnamed temporary regular file.  The pathname argument
391              specifies  a directory; an unnamed inode will be created in that
392              directory's filesystem.  Anything written to the resulting  file
393              will be lost when the last file descriptor is closed, unless the
394              file is given a name.
396              O_TMPFILE must be specified with one of O_RDWR or O_WRONLY  and,
397              optionally,  O_EXCL.  If O_EXCL is not specified, then linkat(2)
398              can be used to link the temporary file into the filesystem, mak‐
399              ing it permanent, using code like the following:
401                  char path[PATH_MAX];
402                  fd = open("/path/to/dir", O_TMPFILE | O_RDWR,
403                                          S_IRUSR | S_IWUSR);
405                  /* File I/O on 'fd'... */
407                  snprintf(path, PATH_MAX,  "/proc/self/fd/%d", fd);
408                  linkat(AT_FDCWD, path, AT_FDCWD, "/path/for/file",
409                                          AT_SYMLINK_FOLLOW);
411              In  this case, the open() mode argument determines the file per‐
412              mission mode, as with O_CREAT.
414              Specifying O_EXCL in conjunction with O_TMPFILE prevents a  tem‐
415              porary  file  from being linked into the filesystem in the above
416              manner.  (Note that the meaning of O_EXCL in this case  is  dif‐
417              ferent from the meaning of O_EXCL otherwise.)
419              There are two main use cases for O_TMPFILE:
421              *  Improved tmpfile(3) functionality: race-free creation of tem‐
422                 porary files that (1) are automatically deleted when  closed;
423                 (2)  can  never be reached via any pathname; (3) are not sub‐
424                 ject to symlink attacks; and (4) do not require the caller to
425                 devise unique names.
427              *  Creating  a  file  that is initially invisible, which is then
428                 populated with data and adjusted to have appropriate filesys‐
429                 tem  attributes  (fchown(2),  fchmod(2),  fsetxattr(2), etc.)
430                 before being atomically linked into the filesystem in a fully
431                 formed state (using linkat(2) as described above).
433              O_TMPFILE  requires support by the underlying filesystem; only a
434              subset of Linux filesystems provide that support.  In  the  ini‐
435              tial  implementation,  support  was  provided in the ext2, ext3,
436              ext4, UDF, Minix, and  shmem  filesystems.   Support  for  other
437              filesystems  has  subsequently been added as follows: XFS (Linux
438              3.15); Btrfs (Linux 3.16); F2FS (Linux 3.16); and  ubifs  (Linux
439              4.9)
441       O_TRUNC
442              If  the file already exists and is a regular file and the access
443              mode allows writing (i.e., is O_RDWR or  O_WRONLY)  it  will  be
444              truncated to length 0.  If the file is a FIFO or terminal device
445              file, the O_TRUNC flag is ignored.   Otherwise,  the  effect  of
446              O_TRUNC is unspecified.
448   creat()
449       A  call  to creat() is equivalent to calling open() with flags equal to
452   openat()
453       The openat() system call operates in exactly the same  way  as  open(),
454       except for the differences described here.
456       If  the  pathname given in pathname is relative, then it is interpreted
457       relative to the directory referred to  by  the  file  descriptor  dirfd
458       (rather  than  relative to the current working directory of the calling
459       process, as is done by open() for a relative pathname).
461       If pathname is relative and dirfd is the special value  AT_FDCWD,  then
462       pathname  is  interpreted  relative to the current working directory of
463       the calling process (like open()).
465       If pathname is absolute, then dirfd is ignored.


468       open(), openat(), and creat() return the new file descriptor, or -1  if
469       an error occurred (in which case, errno is set appropriately).


472       open(), openat(), and creat() can fail with the following errors:
474       EACCES The  requested access to the file is not allowed, or search per‐
475              mission is denied for one of the directories in the path  prefix
476              of  pathname,  or the file did not exist yet and write access to
477              the parent directory is not  allowed.   (See  also  path_resolu‐
478              tion(7).)
480       EDQUOT Where  O_CREAT  is  specified,  the file does not exist, and the
481              user's quota of disk blocks or inodes on the filesystem has been
482              exhausted.
484       EEXIST pathname already exists and O_CREAT and O_EXCL were used.
486       EFAULT pathname points outside your accessible address space.
488       EFBIG  See EOVERFLOW.
490       EINTR  While  blocked  waiting  to  complete  an  open of a slow device
491              (e.g., a FIFO; see fifo(7)), the call was interrupted by a  sig‐
492              nal handler; see signal(7).
494       EINVAL The  filesystem  does  not support the O_DIRECT flag.  See NOTES
495              for more information.
497       EINVAL Invalid value in flags.
499       EINVAL O_TMPFILE was specified  in  flags,  but  neither  O_WRONLY  nor
500              O_RDWR was specified.
502       EINVAL O_CREAT  was  specified in flags and the final component ("base‐
503              name") of the new file's pathname is invalid (e.g., it  contains
504              characters not permitted by the underlying filesystem).
506       EISDIR pathname refers to a directory and the access requested involved
507              writing (that is, O_WRONLY or O_RDWR is set).
509       EISDIR pathname refers to an existing directory, O_TMPFILE and  one  of
510              O_WRONLY or O_RDWR were specified in flags, but this kernel ver‐
511              sion does not provide the O_TMPFILE functionality.
513       ELOOP  Too many symbolic links were encountered in resolving pathname.
515       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW but
516              not O_PATH.
518       EMFILE The per-process limit on the number of open file descriptors has
519              been reached (see the  description  of  RLIMIT_NOFILE  in  getr‐
520              limit(2)).
523              pathname was too long.
525       ENFILE The system-wide limit on the total number of open files has been
526              reached.
528       ENODEV pathname refers to a device special file  and  no  corresponding
529              device  exists.   (This is a Linux kernel bug; in this situation
530              ENXIO must be returned.)
532       ENOENT O_CREAT is not set and the named file does not exist.
534       ENOENT A directory component in pathname does not exist or  is  a  dan‐
535              gling symbolic link.
537       ENOENT pathname refers to a nonexistent directory, O_TMPFILE and one of
538              O_WRONLY or O_RDWR were specified in flags, but this kernel ver‐
539              sion does not provide the O_TMPFILE functionality.
541       ENOMEM The  named  file is a FIFO, but memory for the FIFO buffer can't
542              be allocated because the per-user hard limit on  memory  alloca‐
543              tion  for  pipes  has  been reached and the caller is not privi‐
544              leged; see pipe(7).
546       ENOMEM Insufficient kernel memory was available.
548       ENOSPC pathname was to be created but the  device  containing  pathname
549              has no room for the new file.
551       ENOTDIR
552              A  component  used as a directory in pathname is not, in fact, a
553              directory, or O_DIRECTORY was specified and pathname was  not  a
554              directory.
556       ENXIO  O_NONBLOCK  |  O_WRONLY is set, the named file is a FIFO, and no
557              process has the FIFO open for reading.
559       ENXIO  The file is a device special file and  no  corresponding  device
560              exists.
562       ENXIO  The file is a UNIX domain socket.
564       EOPNOTSUPP
565              The filesystem containing pathname does not support O_TMPFILE.
567       EOVERFLOW
568              pathname  refers  to  a  regular  file  that  is too large to be
569              opened.  The usual scenario here is that an application compiled
570              on  a  32-bit  platform  without -D_FILE_OFFSET_BITS=64 tried to
571              open a  file  whose  size  exceeds  (1<<31)-1  bytes;  see  also
572              O_LARGEFILE  above.   This is the error specified by POSIX.1; in
573              kernels before 2.6.24, Linux gave the error EFBIG for this case.
575       EPERM  The O_NOATIME flag was specified, but the effective user  ID  of
576              the  caller  did  not match the owner of the file and the caller
577              was not privileged.
579       EPERM  The operation was prevented by a file seal; see fcntl(2).
581       EROFS  pathname refers to a file on a read-only  filesystem  and  write
582              access was requested.
584       ETXTBSY
585              pathname  refers to an executable image which is currently being
586              executed and write access was requested.
588       ETXTBSY
589              pathname refers to a file that is currently in  use  as  a  swap
590              file, and the O_TRUNC flag was specified.
592       ETXTBSY
593              pathname  refers  to  a file that is currently being read by the
594              kernel (e.g. for module/firmware loading), and write access  was
595              requested.
598              The O_NONBLOCK flag was specified, and an incompatible lease was
599              held on the file (see fcntl(2)).
601       The following additional errors can occur for openat():
603       EBADF  dirfd is not a valid file descriptor.
605       ENOTDIR
606              pathname is a relative pathname and dirfd is a  file  descriptor
607              referring to a file other than a directory.


610       openat() was added to Linux in kernel 2.6.16; library support was added
611       to glibc in version 2.4.


614       open(), creat() SVr4, 4.3BSD, POSIX.1-2001, POSIX.1-2008.
616       openat(): POSIX.1-2008.
618       The O_DIRECT, O_NOATIME, O_PATH, and  O_TMPFILE  flags  are  Linux-spe‐
619       cific.  One must define _GNU_SOURCE to obtain their definitions.
621       The  O_CLOEXEC,  O_DIRECTORY, and O_NOFOLLOW flags are not specified in
622       POSIX.1-2001, but are specified in POSIX.1-2008.  Since glibc 2.12, one
623       can  obtain their definitions by defining either _POSIX_C_SOURCE with a
624       value greater than or equal to 200809L or _XOPEN_SOURCE  with  a  value
625       greater  than  or equal to 700.  In glibc 2.11 and earlier, one obtains
626       the definitions by defining _GNU_SOURCE.
628       As  noted  in  feature_test_macros(7),  feature  test  macros  such  as
629       _POSIX_C_SOURCE,  _XOPEN_SOURCE, and _GNU_SOURCE must be defined before
630       including any header files.


633       Under Linux, the O_NONBLOCK flag is sometimes used in cases  where  one
634       wants  to  open  but does not necessarily have the intention to read or
635       write.  For example, this may be used to open a device in order to  get
636       a file descriptor for use with ioctl(2).
638       The  (undefined)  effect of O_RDONLY | O_TRUNC varies among implementa‐
639       tions.  On many systems the file is actually truncated.
641       Note that open() can open device special files, but creat() cannot cre‐
642       ate them; use mknod(2) instead.
644       If  the  file is newly created, its st_atime, st_ctime, st_mtime fields
645       (respectively, time of last access, time of  last  status  change,  and
646       time  of  last  modification; see stat(2)) are set to the current time,
647       and so are the st_ctime and st_mtime fields of  the  parent  directory.
648       Otherwise,  if  the  file  is modified because of the O_TRUNC flag, its
649       st_ctime and st_mtime fields are set to the current time.
651       The files in the /proc/[pid]/fd directory show the open  file  descrip‐
652       tors   of   the   process   with   the  PID  pid.   The  files  in  the
653       /proc/[pid]/fdinfo directory show even  more  information  about  these
654       file  descriptors.   See  proc(5)  for further details of both of these
655       directories.
657       The Linux header file <asm/fcntl.h> doesn't define O_ASYNC;  the  (BSD-
658       derived) FASYNC synonym is defined instead.
660   Open file descriptions
661       The term open file description is the one used by POSIX to refer to the
662       entries in the system-wide table of open  files.   In  other  contexts,
663       this  object  is  variously  also called an "open file object", a "file
664       handle", an "open file table entry", or—in kernel-developer  parlance—a
665       struct file.
667       When  a  file  descriptor  is duplicated (using dup(2) or similar), the
668       duplicate refers to the same open file description as the original file
669       descriptor,  and  the  two file descriptors consequently share the file
670       offset and file status flags.  Such sharing can also occur between pro‐
671       cesses:  a child process created via fork(2) inherits duplicates of its
672       parent's file descriptors, and those duplicates refer to the same  open
673       file descriptions.
675       Each  open() of a file creates a new open file description; thus, there
676       may be multiple open file descriptions corresponding to a file inode.
678       On Linux, one can use the kcmp(2) KCMP_FILE operation to  test  whether
679       two  file  descriptors  (in  the  same process or in two different pro‐
680       cesses) refer to the same open file description.
682   Synchronized I/O
683       The POSIX.1-2008 "synchronized I/O" option specifies different variants
684       of  synchronized  I/O,  and specifies the open() flags O_SYNC, O_DSYNC,
685       and O_RSYNC for controlling the behavior.   Regardless  of  whether  an
686       implementation  supports  this option, it must at least support the use
687       of O_SYNC for regular files.
689       Linux implements O_SYNC and O_DSYNC, but not O_RSYNC.  Somewhat  incor‐
690       rectly,  glibc  defines  O_RSYNC  to  have  the  same  value as O_SYNC.
691       (O_RSYNC is defined in the Linux header file <asm/fcntl.h>  on  HP  PA-
692       RISC, but it is not used.)
694       O_SYNC  provides  synchronized  I/O  file integrity completion, meaning
695       write operations will flush data and all  associated  metadata  to  the
696       underlying  hardware.  O_DSYNC provides synchronized I/O data integrity
697       completion, meaning write operations will flush data to the  underlying
698       hardware,  but  will  only  flush metadata updates that are required to
699       allow a subsequent  read  operation  to  complete  successfully.   Data
700       integrity  completion can reduce the number of disk operations that are
701       required for applications  that  don't  need  the  guarantees  of  file
702       integrity completion.
704       To  understand the difference between the two types of completion, con‐
705       sider two pieces of file metadata: the file last modification timestamp
706       (st_mtime)  and  the file length.  All write operations will update the
707       last file modification timestamp, but only writes that add data to  the
708       end  of  the  file  will change the file length.  The last modification
709       timestamp is not needed to ensure that a read  completes  successfully,
710       but  the  file  length is.  Thus, O_DSYNC would only guarantee to flush
711       updates to the file length metadata (whereas O_SYNC would  also  always
712       flush the last modification timestamp metadata).
714       Before Linux 2.6.33, Linux implemented only the O_SYNC flag for open().
715       However, when that flag was specified, most filesystems  actually  pro‐
716       vided  the  equivalent  of  synchronized  I/O data integrity completion
717       (i.e., O_SYNC was actually implemented as the equivalent of O_DSYNC).
719       Since Linux 2.6.33, proper O_SYNC support  is  provided.   However,  to
720       ensure backward binary compatibility, O_DSYNC was defined with the same
721       value as the historical O_SYNC, and O_SYNC was defined as a  new  (two-
722       bit)  flag  value  that  includes the O_DSYNC flag value.  This ensures
723       that applications compiled against new headers  get  at  least  O_DSYNC
724       semantics on pre-2.6.33 kernels.
726   C library/kernel differences
727       Since  version  2.26, the glibc wrapper function for open() employs the
728       openat() system call, rather than the kernel's open() system call.  For
729       certain architectures, this is also true in glibc versions before 2.26.
731   NFS
732       There  are  many infelicities in the protocol underlying NFS, affecting
733       amongst others O_SYNC and O_NDELAY.
735       On NFS filesystems with UID mapping enabled, open() may return  a  file
736       descriptor  but,  for example, read(2) requests are denied with EACCES.
737       This is because the client performs open() by checking the permissions,
738       but  UID  mapping  is  performed  by  the  server  upon  read and write
739       requests.
741   FIFOs
742       Opening the read or write end of a FIFO blocks until the other  end  is
743       also  opened  (by  another process or thread).  See fifo(7) for further
744       details.
746   File access mode
747       Unlike the other values that can be specified in flags, the access mode
748       values  O_RDONLY,  O_WRONLY, and O_RDWR do not specify individual bits.
749       Rather, they define the low order two bits of flags,  and  are  defined
750       respectively  as 0, 1, and 2.  In other words, the combination O_RDONLY
751       | O_WRONLY is a logical error, and certainly does  not  have  the  same
752       meaning as O_RDWR.
754       Linux  reserves  the  special, nonstandard access mode 3 (binary 11) in
755       flags to mean: check for read and write  permission  on  the  file  and
756       return  a  file  descriptor  that can't be used for reading or writing.
757       This nonstandard access mode is used by some Linux drivers to return  a
758       file  descriptor  that  is to be used only for device-specific ioctl(2)
759       operations.
761   Rationale for openat() and other directory file descriptor APIs
762       openat() and the other system calls and library functions that  take  a
763       directory  file  descriptor  argument (i.e., execveat(2), faccessat(2),
764       fanotify_mark(2), fchmodat(2), fchownat(2),  fstatat(2),  futimesat(2),
765       linkat(2), mkdirat(2), mknodat(2), name_to_handle_at(2), readlinkat(2),
766       renameat(2), statx(2), symlinkat(2), unlinkat(2),  utimensat(2),  mkfi‐
767       foat(3),  and  scandirat(3)) address two problems with the older inter‐
768       faces that preceded them.  Here, the explanation is  in  terms  of  the
769       openat() call, but the rationale is analogous for the other interfaces.
771       First,  openat()  allows  an  application to avoid race conditions that
772       could occur when using open() to open files in directories  other  than
773       the  current  working directory.  These race conditions result from the
774       fact that some component of the directory prefix given to open()  could
775       be  changed in parallel with the call to open().  Suppose, for example,
776       that  we  wish  to  create  the  file  dir1/dir2/xxx.dep  if  the  file
777       dir1/dir2/xxx  exists.  The problem is that between the existence check
778       and the file-creation step, dir1  or  dir2  (which  might  be  symbolic
779       links)  could be modified to point to a different location.  Such races
780       can be avoided by opening a file descriptor for the  target  directory,
781       and then specifying that file descriptor as the dirfd argument of (say)
782       fstatat(2) and openat().  The use of the dirfd file descriptor also has
783       other benefits:
785       *  the  file descriptor is a stable reference to the directory, even if
786          the directory is renamed; and
788       *  the open file descriptor prevents  the  underlying  filesystem  from
789          being  dismounted,  just  as  when  a  process has a current working
790          directory on a filesystem.
792       Second, openat() allows the implementation  of  a  per-thread  "current
793       working  directory",  via file descriptor(s) maintained by the applica‐
794       tion.  (This functionality can also be obtained by tricks based on  the
795       use of /proc/self/fd/dirfd, but less efficiently.)
797   O_DIRECT
798       The  O_DIRECT  flag may impose alignment restrictions on the length and
799       address of user-space buffers and the file offset of  I/Os.   In  Linux
800       alignment  restrictions vary by filesystem and kernel version and might
801       be absent entirely.  However there is currently no  filesystem-indepen‐
802       dent  interface for an application to discover these restrictions for a
803       given file or filesystem.  Some filesystems provide  their  own  inter‐
804       faces  for  doing  so,  for  example  the  XFS_IOC_DIOINFO operation in
805       xfsctl(3).
807       Under Linux 2.4, transfer sizes, and the alignment of the  user  buffer
808       and  the file offset must all be multiples of the logical block size of
809       the filesystem.  Since Linux 2.6.0, alignment to the logical block size
810       of  the underlying storage (typically 512 bytes) suffices.  The logical
811       block size can be determined using the ioctl(2) BLKSSZGET operation  or
812       from the shell using the command:
814           blockdev --getss
816       O_DIRECT  I/Os should never be run concurrently with the fork(2) system
817       call, if the memory buffer is a private mapping (i.e., any mapping cre‐
818       ated  with the mmap(2) MAP_PRIVATE flag; this includes memory allocated
819       on the heap and statically allocated buffers).  Any such I/Os,  whether
820       submitted  via  an asynchronous I/O interface or from another thread in
821       the process, should be completed before fork(2) is called.  Failure  to
822       do  so  can  result in data corruption and undefined behavior in parent
823       and child processes.  This restriction does not apply when  the  memory
824       buffer for the O_DIRECT I/Os was created using shmat(2) or mmap(2) with
825       the MAP_SHARED flag.  Nor does this restriction apply when  the  memory
826       buffer has been advised as MADV_DONTFORK with madvise(2), ensuring that
827       it will not be available to the child after fork(2).
829       The O_DIRECT flag was introduced in SGI IRIX, where  it  has  alignment
830       restrictions  similar  to those of Linux 2.4.  IRIX has also a fcntl(2)
831       call to query appropriate alignments, and sizes.   FreeBSD  4.x  intro‐
832       duced a flag of the same name, but without alignment restrictions.
834       O_DIRECT support was added under Linux in kernel version 2.4.10.  Older
835       Linux kernels simply ignore this flag.  Some filesystems may not imple‐
836       ment  the  flag, in which case open() fails with the error EINVAL if it
837       is used.
839       Applications should avoid mixing O_DIRECT and normal I/O  to  the  same
840       file,  and  especially  to  overlapping  byte regions in the same file.
841       Even when the filesystem correctly handles the coherency issues in this
842       situation,  overall  I/O  throughput  is likely to be slower than using
843       either mode alone.  Likewise, applications should avoid mixing  mmap(2)
844       of files with direct I/O to the same files.
846       The  behavior  of O_DIRECT with NFS will differ from local filesystems.
847       Older kernels, or kernels configured in certain ways, may  not  support
848       this  combination.   The NFS protocol does not support passing the flag
849       to the server, so O_DIRECT I/O will bypass the page cache only  on  the
850       client; the server may still cache the I/O.  The client asks the server
851       to make the I/O synchronous to preserve the  synchronous  semantics  of
852       O_DIRECT.   Some servers will perform poorly under these circumstances,
853       especially if the I/O size is small.  Some servers may also be  config‐
854       ured  to  lie  to  clients about the I/O having reached stable storage;
855       this will avoid the performance penalty at some risk to data  integrity
856       in  the  event of server power failure.  The Linux NFS client places no
857       alignment restrictions on O_DIRECT I/O.
859       In summary, O_DIRECT is a potentially powerful tool that should be used
860       with  caution.   It  is  recommended  that  applications  treat  use of
861       O_DIRECT as a performance option which is disabled by default.


864       Currently, it is not possible to enable signal-driven I/O by specifying
865       O_ASYNC when calling open(); use fcntl(2) to enable this flag.
867       One  must  check for two different error codes, EISDIR and ENOENT, when
868       trying to determine whether the kernel supports  O_TMPFILE  functional‐
869       ity.
871       When  both  O_CREAT and O_DIRECTORY are specified in flags and the file
872       specified by pathname does not exist, open() will create a regular file
873       (i.e., O_DIRECTORY is ignored).


876       chmod(2),  chown(2),  close(2),  dup(2),  fcntl(2),  link(2), lseek(2),
877       mknod(2), mmap(2), mount(2), open_by_handle_at(2), read(2),  socket(2),
878       stat(2),  umask(2),  unlink(2),  write(2),  fopen(3),  acl(5), fifo(7),
879       inode(7), path_resolution(7), symlink(7)


882       This page is part of release 5.04 of the Linux  man-pages  project.   A
883       description  of  the project, information about reporting bugs, and the
884       latest    version    of    this    page,    can     be     found     at
885       https://www.kernel.org/doc/man-pages/.
889Linux                             2018-04-30                           OPEN(2)