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28 .\" @(#)fcntl.2 8.2 (Berkeley) 1/12/94
42 .Fn fcntl "int fd" "int cmd" "..."
46 system call provides for control over descriptors.
49 is a descriptor to be operated on by
52 Depending on the value of
55 can take an additional third argument
57 .Bl -tag -width F_DUP2FD_CLOEXEC
59 Return a new descriptor as follows:
61 .Bl -bullet -compact -offset 4n
63 Lowest numbered available descriptor greater than or equal to
66 Same object references as the original descriptor.
68 New descriptor shares the same file offset if the object
71 Same access mode (read, write or read/write).
73 Same file status flags (i.e., both file descriptors
74 share the same file status flags).
76 The close-on-exec flag
78 associated with the new file descriptor is cleared, so the file descriptor is
83 .It Dv F_DUPFD_CLOEXEC
88 flag associated with the new file descriptor is set, so the file descriptor
93 It is functionally equivalent to
94 .Bd -literal -offset indent
97 .It Dv F_DUP2FD_CLOEXEC
102 flag associated with the new file descriptor is set.
108 constants are not portable, so they should not be used if
109 portability is needed.
115 Get the close-on-exec flag associated with the file descriptor
119 If the returned value ANDed with
122 the file will remain open across
124 otherwise the file will be closed upon execution of
129 Set the close-on-exec flag associated with
139 Get descriptor status flags, as described below
143 Set descriptor status flags to
146 Get the process ID or process group
151 signals; process groups are returned
156 Set the process or process group
162 process groups are specified by supplying
164 as negative, otherwise
166 is interpreted as a process ID.
168 Set or clear the read ahead amount for sequential access to the third
171 which is rounded up to the nearest block size.
174 turns off read ahead, a negative value restores the system default.
176 Equivalent to Darwin counterpart which sets read ahead amount of 128KB
177 when the third argument,
182 turns off read ahead.
189 flags are as follows:
190 .Bl -tag -width O_NONBLOCKX
192 Non-blocking I/O; if no data is available to a
196 operation would block,
197 the read or write call returns -1 with the error
200 Force each write to append at the end of file;
206 Minimize or eliminate the cache effects of reading and writing.
208 will attempt to avoid caching the data you read or write.
210 avoid caching the data, it will minimize the impact the data has on the cache.
211 Use of this flag can drastically reduce performance if not used with care.
215 signal to be sent to the process group
216 when I/O is possible, e.g.,
217 upon availability of data to be read.
220 Several commands are available for doing advisory file locking;
221 they all operate on the following structure:
224 off_t l_start; /* starting offset */
225 off_t l_len; /* len = 0 means until end of file */
226 pid_t l_pid; /* lock owner */
227 short l_type; /* lock type: read/write, etc. */
228 short l_whence; /* type of l_start */
229 int l_sysid; /* remote system id or zero for local */
232 The commands available for advisory record locking are as follows:
233 .Bl -tag -width F_SETLKWX
235 Get the first lock that blocks the lock description pointed to by the
238 taken as a pointer to a
241 The information retrieved overwrites the information passed to
246 If no lock is found that would prevent this lock from being created,
247 the structure is left unchanged by this system call except for the
248 lock type which is set to
251 Set or clear a file segment lock according to the lock description
252 pointed to by the third argument,
254 taken as a pointer to a
258 is used to establish shared (or read) locks
260 or exclusive (or write) locks,
262 as well as remove either type of lock
264 If a shared or exclusive lock cannot be set,
266 returns immediately with
269 This command is the same as
271 except that if a shared or exclusive lock is blocked by other locks,
272 the process waits until the request can be satisfied.
273 If a signal that is to be caught is received while
275 is waiting for a region, the
277 will be interrupted if the signal handler has not specified the
283 When a shared lock has been set on a segment of a file,
284 other processes can set shared locks on that segment
286 A shared lock prevents any other process from setting an exclusive
287 lock on any portion of the protected area.
288 A request for a shared lock fails if the file descriptor was not
289 opened with read access.
291 An exclusive lock prevents any other process from setting a shared lock or
292 an exclusive lock on any portion of the protected area.
293 A request for an exclusive lock fails if the file was not
294 opened with write access.
303 to indicate that the relative offset,
305 bytes, will be measured from the start of the file,
306 current position, or end of the file, respectively.
309 is the number of consecutive bytes to be locked.
314 means end edge of the region.
319 fields are only used with
321 to return the process ID of the process holding a blocking lock and
322 the system ID of the system that owns that process.
323 Locks created by the local system will have a system ID of zero.
326 request, the value of
331 Locks may start and extend beyond the current end of a file,
332 but may not start or extend before the beginning of the file.
333 A lock is set to extend to the largest possible value of the
334 file offset for that file if
341 point to the beginning of the file, and
343 is zero, the entire file is locked.
344 If an application wishes only to do entire file locking, the
346 system call is much more efficient.
348 There is at most one type of lock set for each byte in the file.
349 Before a successful return from an
353 request when the calling process has previously existing locks
354 on bytes in the region specified by the request,
355 the previous lock type for each byte in the specified
356 region is replaced by the new lock type.
357 As specified above under the descriptions
358 of shared locks and exclusive locks, an
362 request fails or blocks respectively when another process has existing
363 locks on bytes in the specified region and the type of any of those
364 locks conflicts with the type specified in the request.
368 requests on local files is fair;
369 that is, while the thread is blocked,
370 subsequent requests conflicting with its requests will not be granted,
371 even if these requests do not conflict with existing locks.
373 This interface follows the completely stupid semantics of System V and
375 that require that all locks associated with a file for a given process are
378 file descriptor for that file is closed by that process.
379 This semantic means that applications must be aware of any files that
380 a subroutine library may access.
381 For example if an application for updating the password file locks the
382 password file database while making the update, and then calls
384 to retrieve a record,
385 the lock will be lost because
387 opens, reads, and closes the password database.
388 The database close will release all locks that the process has
389 associated with the database, even if the library routine never
390 requested a lock on the database.
391 Another minor semantic problem with this interface is that
392 locks are not inherited by a child process created using the
397 interface has much more rational last close semantics and
398 allows locks to be inherited by child processes.
401 system call is recommended for applications that want to ensure the integrity
402 of their locks when using library routines or wish to pass locks
410 locks are compatible.
411 Processes using different locking interfaces can cooperate
412 over the same file safely.
413 However, only one of such interfaces should be used within
415 If a file is locked by a process through
417 any record within the file will be seen as locked
418 from the viewpoint of another process using
427 if the process holding a blocking lock previously locked the
431 All locks associated with a file for a given process are
432 removed when the process terminates.
434 All locks obtained before a call to
436 remain in effect until the new program releases them.
437 If the new program does not know about the locks, they will not be
438 released until the program exits.
440 A potential for deadlock occurs if a process controlling a locked region
441 is put to sleep by attempting to lock the locked region of another process.
442 This implementation detects that sleeping until a locked region is unlocked
443 would cause a deadlock and fails with an
447 Upon successful completion, the value returned depends on
450 .Bl -tag -width F_GETOWNX -offset indent
452 A new file descriptor.
454 A file descriptor equal to
457 Value of flag (only the low-order bit is defined).
461 Value of file descriptor owner.
466 Otherwise, a value of -1 is returned and
468 is set to indicate the error.
472 system call will fail if:
485 and the segment of a file to be locked is already
486 exclusive-locked by another process;
487 or the type is an exclusive lock and some portion of the
488 segment of a file to be locked is already shared-locked or
489 exclusive-locked by another process.
494 is not a valid open file descriptor.
502 is not a valid file descriptor.
516 is not a valid file descriptor open for reading.
530 is not a valid file descriptor open for writing.
536 and a deadlock condition was detected.
542 and the system call was interrupted by a signal.
551 is negative or greater than the maximum allowable number
553 .Xr getdtablesize 2 ) .
562 and the data to which
570 and the maximum number of file descriptors permitted for the
571 process are already in use,
572 or no file descriptors greater than or equal to
582 and satisfying the lock or unlock request would result in the
583 number of locked regions in the system exceeding a system-imposed limit.
594 refers to a file for which locking is not supported.
605 calculation overflowed.
613 the process ID or process group given as an argument is in a
614 different session than the caller.
622 the process ID given as argument is not in use.
627 refers to a descriptor open on a terminal device (as opposed to a
628 descriptor open on a socket), a
632 can fail for the same reasons as in
638 for the reasons as stated in
645 .Xr getdtablesize 2 ,
654 constant is non portable.
655 It is provided for compatibility with AIX and Solaris.
663 after any caught signal
664 and should continue waiting during thread suspension such as a stop signal.
665 However, in this implementation a call with
667 is restarted after catching a signal with a
669 handler or a thread suspension such as a stop signal.
673 system call appeared in
678 constant first appeared in