1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
|
.\" Copyright (c) 1983, 1993
.\" The Regents of the University of California. All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by the University of
.\" California, Berkeley and its contributors.
.\" 4. Neither the name of the University nor the names of its contributors
.\" may be used to endorse or promote products derived from this software
.\" without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
.\" $FreeBSD$
.\"
.Dd January 12, 1994
.Dt FCNTL 2
.Os
.Sh NAME
.Nm fcntl
.Nd file control
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In fcntl.h
.Ft int
.Fn fcntl "int fd" "int cmd" "..."
.Sh DESCRIPTION
.Fn Fcntl
provides for control over descriptors.
The argument
.Fa fd
is a descriptor to be operated on by
.Fa cmd
as described below. Depending on the value of
.Fa cmd ,
.Nm
can take an additional third argument
.Fa "int arg" .
.Bl -tag -width F_GETOWNX
.It Dv F_DUPFD
Return a new descriptor as follows:
.Pp
.Bl -bullet -compact -offset 4n
.It
Lowest numbered available descriptor greater than or equal to
.Fa arg .
.It
Same object references as the original descriptor.
.It
New descriptor shares the same file offset if the object
was a file.
.It
Same access mode (read, write or read/write).
.It
Same file status flags (i.e., both file descriptors
share the same file status flags).
.It
The close-on-exec flag associated with the new file descriptor
is set to remain open across
.Xr execve 2
system calls.
.El
.It Dv F_GETFD
Get the close-on-exec flag associated with the file descriptor
.Fa fd .
If the low-order bit of the returned value is 0,
the file will remain open across
.Fn exec ,
otherwise the file will be closed upon execution of
.Fn exec
.Fa ( arg
is ignored).
.It Dv F_SETFD
Set the close-on-exec flag associated with
.Fa fd
to the low order bit of
.Fa arg
(0 or 1 as above).
.It Dv F_GETFL
Get descriptor status flags, as described below
.Fa ( arg
is ignored).
.It Dv F_SETFL
Set descriptor status flags to
.Fa arg .
.It Dv F_GETOWN
Get the process ID or process group
currently receiving
.Dv SIGIO
and
.Dv SIGURG
signals; process groups are returned
as negative values
.Fa ( arg
is ignored).
.It Dv F_SETOWN
Set the process or process group
to receive
.Dv SIGIO
and
.Dv SIGURG
signals;
process groups are specified by supplying
.Fa arg
as negative, otherwise
.Fa arg
is interpreted as a process ID.
.El
.Pp
The flags for the
.Dv F_GETFL
and
.Dv F_SETFL
flags are as follows:
.Bl -tag -width O_NONBLOCKX
.It Dv O_NONBLOCK
Non-blocking I/O; if no data is available to a
.Xr read 2
call, or if a
.Xr write 2
operation would block,
the read or write call returns -1 with the error
.Er EAGAIN .
.It Dv O_APPEND
Force each write to append at the end of file;
corresponds to the
.Dv O_APPEND
flag of
.Xr open 2 .
.It Dv O_DIRECT
Minimize or eliminate the cache effects of reading and writing. The system
will attempt to avoid caching the data you read or write. If it cannot
avoid caching the data, it will minimize the impact the data has on the cache.
Use of this flag can drastically reduce performance if not used with care.
.It Dv O_ASYNC
Enable the
.Dv SIGIO
signal to be sent to the process group
when I/O is possible, e.g.,
upon availability of data to be read.
.El
.Pp
Several commands are available for doing advisory file locking;
they all operate on the following structure:
.Bd -literal
struct flock {
off_t l_start; /* starting offset */
off_t l_len; /* len = 0 means until end of file */
pid_t l_pid; /* lock owner */
short l_type; /* lock type: read/write, etc. */
short l_whence; /* type of l_start */
};
.Ed
The commands available for advisory record locking are as follows:
.Bl -tag -width F_SETLKWX
.It Dv F_GETLK
Get the first lock that blocks the lock description pointed to by the
third argument,
.Fa arg ,
taken as a pointer to a
.Fa "struct flock"
(see above).
The information retrieved overwrites the information passed to
.Fn fcntl
in the
.Fa flock
structure.
If no lock is found that would prevent this lock from being created,
the structure is left unchanged by this function call except for the
lock type which is set to
.Dv F_UNLCK .
.It Dv F_SETLK
Set or clear a file segment lock according to the lock description
pointed to by the third argument,
.Fa arg ,
taken as a pointer to a
.Fa "struct flock"
(see above).
.Dv F_SETLK
is used to establish shared (or read) locks
.Pq Dv F_RDLCK
or exclusive (or write) locks,
.Pq Dv F_WRLCK ,
as well as remove either type of lock
.Pq Dv F_UNLCK .
If a shared or exclusive lock cannot be set,
.Fn fcntl
returns immediately with
.Er EAGAIN .
.It Dv F_SETLKW
This command is the same as
.Dv F_SETLK
except that if a shared or exclusive lock is blocked by other locks,
the process waits until the request can be satisfied.
If a signal that is to be caught is received while
.Fn fcntl
is waiting for a region, the
.Fn fcntl
will be interrupted if the signal handler has not specified the
.Dv SA_RESTART
(see
.Xr sigaction 2 ) .
.El
.Pp
When a shared lock has been set on a segment of a file,
other processes can set shared locks on that segment
or a portion of it.
A shared lock prevents any other process from setting an exclusive
lock on any portion of the protected area.
A request for a shared lock fails if the file descriptor was not
opened with read access.
.Pp
An exclusive lock prevents any other process from setting a shared lock or
an exclusive lock on any portion of the protected area.
A request for an exclusive lock fails if the file was not
opened with write access.
.Pp
The value of
.Fa l_whence
is
.Dv SEEK_SET ,
.Dv SEEK_CUR ,
or
.Dv SEEK_END
to indicate that the relative offset,
.Fa l_start
bytes, will be measured from the start of the file,
current position, or end of the file, respectively.
The value of
.Fa l_len
is the number of consecutive bytes to be locked.
If
.Fa l_len
is negative,
.Fa l_start
means end edge of the region.
The
.Fa l_pid
field is only used with
.Dv F_GETLK
to return the process ID of the process holding a blocking lock.
After a successful
.Dv F_GETLK
request, the value of
.Fa l_whence
is
.Dv SEEK_SET .
.Pp
Locks may start and extend beyond the current end of a file,
but may not start or extend before the beginning of the file.
A lock is set to extend to the largest possible value of the
file offset for that file if
.Fa l_len
is set to zero.
If
.Fa l_whence
and
.Fa l_start
point to the beginning of the file, and
.Fa l_len
is zero, the entire file is locked.
If an application wishes only to do entire file locking, the
.Xr flock 2
system call is much more efficient.
.Pp
There is at most one type of lock set for each byte in the file.
Before a successful return from an
.Dv F_SETLK
or an
.Dv F_SETLKW
request when the calling process has previously existing locks
on bytes in the region specified by the request,
the previous lock type for each byte in the specified
region is replaced by the new lock type.
As specified above under the descriptions
of shared locks and exclusive locks, an
.Dv F_SETLK
or an
.Dv F_SETLKW
request fails or blocks respectively when another process has existing
locks on bytes in the specified region and the type of any of those
locks conflicts with the type specified in the request.
.Pp
This interface follows the completely stupid semantics of System V and
.St -p1003.1-88
that require that all locks associated with a file for a given process are
removed when
.Em any
file descriptor for that file is closed by that process.
This semantic means that applications must be aware of any files that
a subroutine library may access.
For example if an application for updating the password file locks the
password file database while making the update, and then calls
.Xr getpwnam 3
to retrieve a record,
the lock will be lost because
.Xr getpwnam 3
opens, reads, and closes the password database.
The database close will release all locks that the process has
associated with the database, even if the library routine never
requested a lock on the database.
Another minor semantic problem with this interface is that
locks are not inherited by a child process created using the
.Xr fork 2
function.
The
.Xr flock 2
interface has much more rational last close semantics and
allows locks to be inherited by child processes.
.Xr Flock 2
is recommended for applications that want to ensure the integrity
of their locks when using library routines or wish to pass locks
to their children.
Note that
.Xr flock 2
and
.Xr fcntl 2
locks may be safely used concurrently but
.Fn fcntl F_GETLK
returns \-1 in
.Fa l_pid
if the process holding a blocking lock previously locked the
file descriptor by
.Xr flock 2 .
.Pp
All locks associated with a file for a given process are
removed when the process terminates.
.Pp
All locks obtained before a call to
.Xr execve 2
remain in effect until the new program releases them.
If the new program does not know about the locks, they will not be
released until the program exits.
.Pp
A potential for deadlock occurs if a process controlling a locked region
is put to sleep by attempting to lock the locked region of another process.
This implementation detects that sleeping until a locked region is unlocked
would cause a deadlock and fails with an
.Er EDEADLK
error.
.Sh RETURN VALUES
Upon successful completion, the value returned depends on
.Fa cmd
as follows:
.Bl -tag -width F_GETOWNX -offset indent
.It Dv F_DUPFD
A new file descriptor.
.It Dv F_GETFD
Value of flag (only the low-order bit is defined).
.It Dv F_GETFL
Value of flags.
.It Dv F_GETOWN
Value of file descriptor owner.
.It other
Value other than -1.
.El
.Pp
Otherwise, a value of -1 is returned and
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Fcntl
will fail if:
.Bl -tag -width Er
.It Bq Er EAGAIN
The argument
.Fa cmd
is
.Dv F_SETLK ,
the type of lock
.Pq Fa l_type
is a shared lock
.Pq Dv F_RDLCK
or exclusive lock
.Pq Dv F_WRLCK ,
and the segment of a file to be locked is already
exclusive-locked by another process;
or the type is an exclusive lock and some portion of the
segment of a file to be locked is already shared-locked or
exclusive-locked by another process.
.It Bq Er EBADF
.Fa \&Fd
is not a valid open file descriptor.
.Pp
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
the type of lock
.Pq Fa l_type
is a shared lock
.Pq Dv F_RDLCK ,
and
.Fa fd
is not a valid file descriptor open for reading.
.Pp
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
the type of lock
.Pq Fa l_type
is an exclusive lock
.Pq Dv F_WRLCK ,
and
.Fa fd
is not a valid file descriptor open for writing.
.It Bq Er EDEADLK
The argument
.Fa cmd
is
.Dv F_SETLKW ,
and a deadlock condition was detected.
.It Bq Er EINTR
The argument
.Fa cmd
is
.Dv F_SETLKW ,
and the function was interrupted by a signal.
.It Bq Er EINVAL
.Fa Cmd
is
.Dv F_DUPFD
and
.Fa arg
is negative or greater than the maximum allowable number
(see
.Xr getdtablesize 2 ) .
.Pp
The argument
.Fa cmd
is
.Dv F_GETLK ,
.Dv F_SETLK
or
.Dv F_SETLKW
and the data to which
.Fa arg
points is not valid.
.It Bq Er EMFILE
The argument
.Fa cmd
is
.Dv F_DUPFD
and the maximum number of file descriptors permitted for the
process are already in use,
or no file descriptors greater than or equal to
.Fa arg
are available.
.It Bq Er ENOLCK
The argument
.Fa cmd
is
.Dv F_SETLK
or
.Dv F_SETLKW ,
and satisfying the lock or unlock request would result in the
number of locked regions in the system exceeding a system-imposed limit.
.It Bq Er EOPNOTSUPP
The argument
.Fa cmd
is
.Dv F_GETLK ,
.Dv F_SETLK
or
.Dv F_SETLKW
and
.Fa fd
refers to a file for which locking is not supported.
.It Bq Er EOVERFLOW
The argument
.Fa cmd
is
.Dv F_GETLK ,
.Dv F_SETLK
or
.Dv F_SETLKW
and an
.Fa off_t
calculation overflowed.
.It Bq Er EPERM
.Fa Cmd
is
.Dv F_SETOWN
and
the process ID or process group given as an argument is in a
different session than the caller.
.It Bq Er ESRCH
.Fa Cmd
is
.Dv F_SETOWN
and
the process ID given as argument is not in use.
.El
.Pp
In addition, if
.Fa fd
refers to a descriptor open on a terminal device (as opposed to a
descriptor open on a socket), a
.Fa cmd
of
.Dv F_SETOWN
can fail for the same reasons as in
.Xr tcsetpgrp 3 ,
and a
.Fa cmd
of
.Dv F_GETOWN
for the reasons as stated in
.Xr tcgetpgrp 3 .
.Sh SEE ALSO
.Xr close 2 ,
.Xr execve 2 ,
.Xr flock 2 ,
.Xr getdtablesize 2 ,
.Xr open 2 ,
.Xr sigvec 2 ,
.Xr tcgetpgrp 3 ,
.Xr tcsetpgrp 3
.Sh HISTORY
The
.Fn fcntl
function call appeared in
.Bx 4.2 .
|