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28.\" @(#)open.2 8.2 (Berkeley) 11/16/93
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29.\" $FreeBSD: head/lib/libc/sys/open.2 165903 2007-01-09 00:28:16Z imp $
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| 29.\" $FreeBSD: head/lib/libc/sys/open.2 178245 2008-04-16 13:03:12Z kib $ |
30.\"
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31.Dd January 7, 2007
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| 31.Dd April 10, 2008 |
32.Dt OPEN 2
33.Os
34.Sh NAME
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35.Nm open
36.Nd open or create a file for reading or writing
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35.Nm open , openat
36.Nd open or create a file for reading, writing or executing |
37.Sh LIBRARY
38.Lb libc
39.Sh SYNOPSIS
40.In fcntl.h
41.Ft int
42.Fn open "const char *path" "int flags" "..."
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43.Ft int
44.Fn openat "int fd" "const char *path" "int flags" "..." |
45.Sh DESCRIPTION
46The file name specified by
47.Fa path
48is opened
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47for reading and/or writing as specified by the
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| 49for either execution or reading and/or writing as specified by the |
50argument
51.Fa flags
52and the file descriptor returned to the calling process.
53The
54.Fa flags
55argument may indicate the file is to be
56created if it does not exist (by specifying the
57.Dv O_CREAT
58flag).
59In this case
60.Fn open
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59requires a third argument
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61and
62.Fn openat
63require an additional argument |
64.Fa "mode_t mode" ,
65and the file is created with mode
66.Fa mode
67as described in
68.Xr chmod 2
69and modified by the process' umask value (see
70.Xr umask 2 ) .
71.Pp
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72The
73.Fn openat
74function is equivalent to the
75.Fn open
76function except in the case where the
77.Fa path
78specifies a relative path.
79In this case the file to be opened is determined relative to the directory
80associated with the file descriptor
81.Fa fd
82instead of the current working directory.
83The
84.Fa flag
85parameter and the optional fourth parameter correspond exactly to
86the parameters of
87.Fn open .
88If
89.Fn openat
90is passed the special value
91.Dv AT_FDCWD
92in the
93.Fa fd
94parameter, the current working directory is used
95and the behavior is identical to a call to
96.Fn open .
97.Pp |
98The flags specified are formed by
99.Em or Ns 'ing
100the following values
101.Pp
102.Bd -literal -offset indent -compact
103O_RDONLY open for reading only
104O_WRONLY open for writing only
105O_RDWR open for reading and writing
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| 106O_EXEC open for execute only |
107O_NONBLOCK do not block on open
108O_APPEND append on each write
109O_CREAT create file if it does not exist
110O_TRUNC truncate size to 0
111O_EXCL error if create and file exists
112O_SHLOCK atomically obtain a shared lock
113O_EXLOCK atomically obtain an exclusive lock
114O_DIRECT eliminate or reduce cache effects
115O_FSYNC synchronous writes
116O_SYNC synchronous writes
117O_NOFOLLOW do not follow symlinks
118O_NOCTTY don't assign controlling terminal
119.Ed
120.Pp
121Opening a file with
122.Dv O_APPEND
123set causes each write on the file
124to be appended to the end.
125If
126.Dv O_TRUNC
127is specified and the
128file exists, the file is truncated to zero length.
129If
130.Dv O_EXCL
131is set with
132.Dv O_CREAT
133and the file already
134exists,
135.Fn open
136returns an error.
137This may be used to
138implement a simple exclusive access locking mechanism.
139If
140.Dv O_EXCL
141is set and the last component of the pathname is
142a symbolic link,
143.Fn open
144will fail even if the symbolic
145link points to a non-existent name.
146If the
147.Dv O_NONBLOCK
148flag is specified and the
149.Fn open
150system call would result
151in the process being blocked for some reason (e.g., waiting for
152carrier on a dialup line),
153.Fn open
154returns immediately.
155The descriptor remains in non-blocking mode for subsequent operations.
156.Pp
157If
158.Dv O_FSYNC
159is used in the mask, all writes will
160immediately be written to disk,
161the kernel will not cache written data
162and all writes on the descriptor will not return until
163the data to be written completes.
164.Pp
165.Dv O_SYNC
166is a synonym for
167.Dv O_FSYNC
168required by
169.Tn POSIX .
170.Pp
171If
172.Dv O_NOFOLLOW
173is used in the mask and the target file passed to
174.Fn open
175is a symbolic link then the
176.Fn open
177will fail.
178.Pp
179When opening a file, a lock with
180.Xr flock 2
181semantics can be obtained by setting
182.Dv O_SHLOCK
183for a shared lock, or
184.Dv O_EXLOCK
185for an exclusive lock.
186If creating a file with
187.Dv O_CREAT ,
188the request for the lock will never fail
189(provided that the underlying file system supports locking).
190.Pp
191.Dv O_DIRECT
192may be used to minimize or eliminate the cache effects of reading and writing.
193The system will attempt to avoid caching the data you read or write.
194If it cannot avoid caching the data,
195it will minimize the impact the data has on the cache.
196Use of this flag can drastically reduce performance if not used with care.
197.Pp
198.Dv O_NOCTTY
199may be used to ensure the OS does not assign this file as the
200controlling terminal when it opens a tty device.
201This is the default on
202.Fx ,
203but is present for
204.Tn POSIX
205compatibility.
206The
207.Fn open
208system call will not assign controlling terminals on
209.Fx .
210.Pp
211If successful,
212.Fn open
213returns a non-negative integer, termed a file descriptor.
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183It returns -1 on failure.
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| 214It returns \-1 on failure. |
215The file pointer used to mark the current position within the
216file is set to the beginning of the file.
217.Pp
218When a new file is created it is given the group of the directory
219which contains it.
220.Pp
221The new descriptor is set to remain open across
222.Xr execve 2
223system calls; see
224.Xr close 2
225and
226.Xr fcntl 2 .
227.Pp
228The system imposes a limit on the number of file descriptors
229open simultaneously by one process.
230The
231.Xr getdtablesize 2
232system call returns the current system limit.
233.Sh RETURN VALUES
234If successful,
235.Fn open
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205returns a non-negative integer, termed a file descriptor.
206It returns -1 on failure, and sets
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236and
237.Fn openat
238return a non-negative integer, termed a file descriptor.
239They return \-1 on failure, and set |
240.Va errno
241to indicate the error.
242.Sh ERRORS
243The named file is opened unless:
244.Bl -tag -width Er
245.It Bq Er ENOTDIR
246A component of the path prefix is not a directory.
247.It Bq Er ENAMETOOLONG
248A component of a pathname exceeded 255 characters,
249or an entire path name exceeded 1023 characters.
250.It Bq Er ENOENT
251.Dv O_CREAT
252is not set and the named file does not exist.
253.It Bq Er ENOENT
254A component of the path name that must exist does not exist.
255.It Bq Er EACCES
256Search permission is denied for a component of the path prefix.
257.It Bq Er EACCES
258The required permissions (for reading and/or writing)
259are denied for the given flags.
260.It Bq Er EACCES
261.Dv O_TRUNC
262is specified and write permission is denied.
263.It Bq Er EACCES
264.Dv O_CREAT
265is specified,
266the file does not exist,
267and the directory in which it is to be created
268does not permit writing.
269.It Bq Er EPERM
270.Dv O_CREAT
271is specified, the file does not exist, and the directory in which it is to be
272created has its immutable flag set, see the
273.Xr chflags 2
274manual page for more information.
275.It Bq Er EPERM
276.Dv The named file has its immutable flag set and the file is to be modified.
277.It Bq Er EPERM
278.Dv The named file has its append-only flag set, the file is to be modified, and
279.Dv O_TRUNC
280is specified or
281.Dv O_APPEND
282is not specified.
283.It Bq Er ELOOP
284Too many symbolic links were encountered in translating the pathname.
285.It Bq Er EISDIR
286The named file is a directory, and the arguments specify
287it is to be modified.
288.It Bq Er EROFS
289The named file resides on a read-only file system,
290and the file is to be modified.
291.It Bq Er EROFS
292.Dv O_CREAT
293is specified and the named file would reside on a read-only file system.
294.It Bq Er EMFILE
295The process has already reached its limit for open file descriptors.
296.It Bq Er ENFILE
297The system file table is full.
298.It Bq Er EMLINK
299.Dv O_NOFOLLOW
300was specified and the target is a symbolic link.
301.It Bq Er ENXIO
302The named file is a character special or block
303special file, and the device associated with this special file
304does not exist.
305.It Bq Er ENXIO
306.Dv O_NONBLOCK
307is set, the named file is a fifo,
308.Dv O_WRONLY
309is set, and no process has the file open for reading.
310.It Bq Er EINTR
311The
312.Fn open
313operation was interrupted by a signal.
314.It Bq Er EOPNOTSUPP
315.Dv O_SHLOCK
316or
317.Dv O_EXLOCK
318is specified but the underlying file system does not support locking.
319.It Bq Er EOPNOTSUPP
320The named file is a special file mounted through a file system that
321does not support access to it (e.g.\& NFS).
322.It Bq Er EWOULDBLOCK
323.Dv O_NONBLOCK
324and one of
325.Dv O_SHLOCK
326or
327.Dv O_EXLOCK
328is specified and the file is locked.
329.It Bq Er ENOSPC
330.Dv O_CREAT
331is specified,
332the file does not exist,
333and the directory in which the entry for the new file is being placed
334cannot be extended because there is no space left on the file
335system containing the directory.
336.It Bq Er ENOSPC
337.Dv O_CREAT
338is specified,
339the file does not exist,
340and there are no free inodes on the file system on which the
341file is being created.
342.It Bq Er EDQUOT
343.Dv O_CREAT
344is specified,
345the file does not exist,
346and the directory in which the entry for the new file
347is being placed cannot be extended because the
348user's quota of disk blocks on the file system
349containing the directory has been exhausted.
350.It Bq Er EDQUOT
351.Dv O_CREAT
352is specified,
353the file does not exist,
354and the user's quota of inodes on the file system on
355which the file is being created has been exhausted.
356.It Bq Er EIO
357An I/O error occurred while making the directory entry or
358allocating the inode for
359.Dv O_CREAT .
360.It Bq Er ETXTBSY
361The file is a pure procedure (shared text) file that is being
362executed and the
363.Fn open
364system call requests write access.
365.It Bq Er EFAULT
366The
367.Fa path
368argument
369points outside the process's allocated address space.
370.It Bq Er EEXIST
371.Dv O_CREAT
372and
373.Dv O_EXCL
374were specified and the file exists.
375.It Bq Er EOPNOTSUPP
376An attempt was made to open a socket (not currently implemented).
377.It Bq Er EINVAL
378An attempt was made to open a descriptor with an illegal combination
379of
380.Dv O_RDONLY ,
381.Dv O_WRONLY ,
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| 382.Dv O_RDWR |
383and
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350.Dv O_RDWR .
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384.Dv O_EXEC.
385.It Bq Eq EBADF
386The
387.Fa path
388argument does not specify an absolute path and the
389.Fa fd
390argument is
391neither
392.Dv AT_FDCWD
393nor a valid file descriptor open for searching.
394.It Bq Eq ENOTDIR
395The
396.Fa path
397argument is not an absolute path and
398.Fa fd
399is neither
400.Dv AT_FDCWD
401nor a file descriptor associated with a directory. |
402.El
403.Sh SEE ALSO
404.Xr chmod 2 ,
405.Xr close 2 ,
406.Xr dup 2 ,
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| 407.Xr fexecve 2 , |
408.Xr fhopen 2 ,
409.Xr getdtablesize 2 ,
410.Xr getfh 2 ,
411.Xr lgetfh 2 ,
412.Xr lseek 2 ,
413.Xr read 2 ,
414.Xr umask 2 ,
415.Xr write 2 ,
416.Xr fopen 3
417.Sh HISTORY
418The
419.Fn open
420function appeared in
421.At v6 .
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422The
423.Fn openat
424function was introduced in
425.Fx 8.0 .
426.Sh BUGS
427The Open Group Extended API Set 2 specification requires that the test
428for whether
429.Fa fd
430is searchable is based on whether
431.Fa fd
432is open for searching, not whether the underlying directory currently
433permits searches.
434The present implementation of the
435.Fa openat
436checks the current permissions of directory instead. |
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