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31.\" @(#)open.2 8.2 (Berkeley) 11/16/93
32.\"
33.Dd $Mdocdate: August 4 2025 $
34.Dt OPEN 2
35.Os
36.Sh NAME
37.Nm open ,
38.Nm openat
39.Nd open or create a file for reading or writing
40.Sh SYNOPSIS
41.In fcntl.h
42.Ft int
43.Fn open "const char *path" "int flags" ...
44.Ft int
45.Fn openat "int fd" "const char *path" "int flags" ...
46.Sh DESCRIPTION
47The file name specified by
48.Fa path
49is opened
50for reading and/or writing as specified by the
51argument
52.Fa flags
53and the file descriptor returned to the calling process.
54The
55.Fa flags
56argument may indicate the file is to be
57created if it does not exist (by specifying the
58.Dv O_CREAT
59flag), in which case the file is created with a mode
60specified by an additional argument of type
61.Vt mode_t
62as described in
63.Xr chmod 2
64and modified by the process' umask value (see
65.Xr umask 2 ) .
66.Pp
67The
68.Fa flags
69specified are a bitwise OR of the following values.
70Exactly one of the first three values (file access modes) must be specified:
71.Pp
72.Bl -tag -width O_DIRECTORY -offset indent -compact
73.It Dv O_RDONLY
74Open for reading only.
75.It Dv O_WRONLY
76Open for writing only.
77.It Dv O_RDWR
78Open for reading and writing.
79.El
80.Pp
81Any combination of the following flags may additionally be used:
82.Pp
83.Bl -tag -width O_DIRECTORY -offset indent -compact
84.It Dv O_NONBLOCK
85Do not block on open or for data to become available.
86.It Dv O_APPEND
87Append on each write.
88.It Dv O_CREAT
89Create file if it does not exist.
90An additional argument of type
91.Vt mode_t
92must be supplied to the call.
93.It Dv O_TRUNC
94Truncate size to 0.
95.It Dv O_EXCL
96Error if
97.Dv O_CREAT
98is set and file exists.
99.It Dv O_SYNC
100Perform synchronous I/O operations.
101.It Dv O_SHLOCK
102Atomically obtain a shared lock.
103.It Dv O_EXLOCK
104Atomically obtain an exclusive lock.
105.It Dv O_NOFOLLOW
106If last path element is a symlink, don't follow it.
107.It Dv O_CLOEXEC
108Set
109.Dv FD_CLOEXEC
110(the close-on-exec flag)
111on the new file descriptor.
112.It Dv O_CLOFORK
113Set
114.Dv FD_CLOFORK
115(the close-on-fork flag)
116on the new file descriptor.
117.It Dv O_DIRECTORY
118Error if
119.Fa path
120does not name a directory.
121.El
122.Pp
123Opening a file with
124.Dv O_APPEND
125set causes each write on the file
126to be appended to the end.
127If
128.Dv O_TRUNC
129and a writing mode are specified and the
130file exists, the file is truncated to zero length.
131If
132.Dv O_EXCL
133is set with
134.Dv O_CREAT
135and the file already
136exists,
137.Fn open
138returns an error.
139This may be used to implement a simple exclusive access locking mechanism.
140If either of
141.Dv O_EXCL
142or
143.Dv O_NOFOLLOW
144are set and the last component of the pathname is
145a symbolic link,
146.Fn open
147will fail even if the symbolic
148link points to a non-existent name.
149If the
150.Dv O_NONBLOCK
151flag is specified, do not wait for the device or file to be ready or
152available.
153If the
154.Fn open
155call would result
156in the process being blocked for some reason (e.g., waiting for
157carrier on a dialup line),
158.Fn open
159returns immediately.
160This flag also has the effect of making all subsequent I/O on the open file
161non-blocking.
162If the
163.Dv O_SYNC
164flag is set, all I/O operations on the file will be done synchronously.
165.Pp
166A FIFO should either be opened with
167.Dv O_RDONLY
168or with
169.Dv O_WRONLY .
170The behavior for opening a FIFO with
171.Dv O_RDWR
172is undefined.
173.Pp
174When opening a file, a lock with
175.Xr flock 2
176semantics can be obtained by setting
177.Dv O_SHLOCK
178for a shared lock, or
179.Dv O_EXLOCK
180for an exclusive lock.
181If creating a file with
182.Dv O_CREAT ,
183the request for the lock will never fail
184(provided that the underlying filesystem supports locking).
185.Pp
186If
187.Fn open
188is successful, the file pointer used to mark the current position within
189the file is set to the beginning of the file.
190.Pp
191When a new file is created, it is given the group of the directory
192which contains it.
193.Pp
194The new descriptor is set to remain open across
195.Xr execve 2
196system calls; see
197.Xr close 2
198and
199.Xr fcntl 2 .
200.Pp
201The system imposes a limit on the number of file descriptors
202open simultaneously by one process.
203.Xr getdtablesize 3
204returns the current system limit.
205.Pp
206The
207.Fn openat
208function is equivalent to
209.Fn open
210except that where
211.Fa path
212specifies a relative path,
213the file to be opened is determined relative to
214the directory associated with file descriptor
215.Fa fd
216instead of the current working directory.
217.Pp
218If
219.Fn openat
220is passed the special value
221.Dv AT_FDCWD
222(defined in
223.In fcntl.h )
224in the
225.Fa fd
226parameter, the current working directory is used
227and the behavior is identical to a call to
228.Fn open .
229.Sh RETURN VALUES
230If successful,
231.Fn open
232returns a non-negative integer, termed a file descriptor.
233Otherwise, a value of \-1 is returned and
234.Va errno
235is set to indicate the error.
236.Sh ERRORS
237The
238.Fn open
239and
240.Fn openat
241functions will fail if:
242.Bl -tag -width Er
243.It Bq Er ENOTDIR
244A component of the path prefix is not a directory.
245.It Bq Er ENOTDIR
246.Dv O_DIRECTORY
247is specified and
248.Fa path
249does not name a directory.
250.It Bq Er ENAMETOOLONG
251A component of a pathname exceeded
252.Dv NAME_MAX
253characters, or an entire pathname (including the terminating NUL)
254exceeded
255.Dv PATH_MAX
256bytes.
257.It Bq Er ENOENT
258.Dv O_CREAT
259is not set and the named file does not exist.
260.It Bq Er ENOENT
261A component of the pathname that must exist does not exist.
262.It Bq Er EACCES
263Search permission is denied for a component of the path prefix.
264.It Bq Er EACCES
265The required permissions (for reading and/or writing)
266are denied for the given
267.Fa flags .
268.It Bq Er EACCES
269.Dv O_CREAT
270is specified,
271the file does not exist,
272and the directory in which it is to be created
273does not permit writing.
274.It Bq Er ELOOP
275Too many symbolic links were encountered in translating the pathname,
276or the
277.Dv O_NOFOLLOW
278flag was specified and the target is a symbolic link.
279.It Bq Er EISDIR
280The named file is a directory, and the arguments specify
281it is to be opened for writing.
282.It Bq Er EINVAL
283The
284.Fa flags
285specified for opening the file are not valid.
286.It Bq Er EROFS
287The named file resides on a read-only file system,
288and the file is to be modified.
289.It Bq Er EMFILE
290The process has already reached its limit for open file descriptors.
291.It Bq Er ENFILE
292The system file table is full.
293.It Bq Er ENXIO
294The named file is a character special or block
295special file, and the device associated with this special file
296does not exist.
297.It Bq Er ENXIO
298The named file is a FIFO, the
299.Dv O_NONBLOCK
300and
301.Dv O_WRONLY
302flags are set, and no process has the file open for reading.
303.It Bq Er EINTR
304The
305.Fn open
306operation was interrupted by a signal.
307.It Bq Er EOPNOTSUPP
308.Dv O_SHLOCK
309or
310.Dv O_EXLOCK
311is specified but the underlying filesystem does not support locking.
312.It Bq Er EWOULDBLOCK
313.Dv O_NONBLOCK
314and one of
315.Dv O_SHLOCK
316or
317.Dv O_EXLOCK
318is specified and the file is already locked.
319.It Bq Er ENOSPC
320.Dv O_CREAT
321is specified,
322the file does not exist,
323and the directory in which the entry for the new file is being placed
324cannot be extended because there is no space left on the file
325system containing the directory.
326.It Bq Er ENOSPC
327.Dv O_CREAT
328is specified,
329the file does not exist,
330and there are no free inodes on the file system on which the
331file is being created.
332.It Bq Er EDQUOT
333.Dv O_CREAT
334is specified,
335the file does not exist,
336and the directory in which the entry for the new file
337is being placed cannot be extended because the
338user's quota of disk blocks on the file system
339containing the directory has been exhausted.
340.It Bq Er EDQUOT
341.Dv O_CREAT
342is specified,
343the file does not exist,
344and the user's quota of inodes on the file system on
345which the file is being created has been exhausted.
346.It Bq Er EIO
347An I/O error occurred while making the directory entry or
348allocating the inode for
349.Dv O_CREAT .
350.It Bq Er ETXTBSY
351The file is a pure procedure (shared text) file that is being
352executed and the
353.Fn open
354call requests write access.
355.It Bq Er EFAULT
356.Fa path
357points outside the process's allocated address space.
358.It Bq Er EEXIST
359.Dv O_CREAT
360and
361.Dv O_EXCL
362were specified and the file exists.
363.It Bq Er EPERM
364The file named by
365.Fa path
366is flagged append-only but
367.Dv O_APPEND
368was not specified in
369.Fa flags .
370.It Bq Er EOPNOTSUPP
371An attempt was made to open a socket (not currently implemented).
372.It Bq Er EBUSY
373An attempt was made to open a terminal device that requires exclusive
374access and the specified device has already be opened.
375.El
376.Pp
377Additionally, the
378.Fn openat
379function will fail if:
380.Bl -tag -width Er
381.It Bq Er EBADF
382The
383.Fa path
384argument specifies a relative path and the
385.Fa fd
386argument is neither
387.Dv AT_FDCWD
388nor a valid file descriptor.
389.It Bq Er ENOTDIR
390The
391.Fa path
392argument specifies a relative path and the
393.Fa fd
394argument is a valid file descriptor but it does not reference a directory.
395.It Bq Er EACCES
396The
397.Fa path
398argument specifies a relative path but search permission is denied
399for the directory which the
400.Fa fd
401file descriptor references.
402.El
403.Sh SEE ALSO
404.Xr chflags 2 ,
405.Xr chmod 2 ,
406.Xr close 2 ,
407.Xr dup 2 ,
408.Xr flock 2 ,
409.Xr lseek 2 ,
410.Xr read 2 ,
411.Xr umask 2 ,
412.Xr write 2 ,
413.Xr getdtablesize 3
414.Sh STANDARDS
415The
416.Fn open
417and
418.Fn openat
419functions conform to
420.St -p1003.1-2008 .
421.Pp
422.Dv POSIX
423specifies three different flavors for synchronous I/O:
424.Dv O_SYNC ,
425.Dv O_DSYNC ,
426and
427.Dv O_RSYNC .
428In
429.Ox ,
430these are all equivalent.
431.Pp
432The
433.Dv O_SHLOCK
434and
435.Dv O_EXLOCK
436flags are non-standard extensions and should not be used if portability
437is of concern.
438.Sh HISTORY
439An
440.Fn open
441system call first appeared in
442.At v1 .
443The
444.Fa flags
445argument has been supported since
446.Bx 4.2 .
447Before that, a dedicated
448.Fn creat
449system call had to be used to create new files;
450it appeared in
451.At v1 ,
452was deprecated in
453.Bx 4.3 Reno ,
454and removed in
455.Ox 5.0 .
456.Pp
457The
458.Fn openat
459system call has been available since
460.Ox 5.0 .
461.Sh CAVEATS
462The
463.Dv O_TRUNC
464flag requires that one of
465.Dv O_RDWR
466or
467.Dv O_WRONLY
468also be specified, else
469.Er EINVAL
470is returned.