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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.
184 Add seals to the file as described below, if the underlying filesystem supports
187 Get seals associated with the file, if the underlying filesystem supports seals.
194 flags are as follows:
195 .Bl -tag -width O_NONBLOCKX
197 Non-blocking I/O; if no data is available to a
201 operation would block,
202 the read or write call returns -1 with the error
205 Force each write to append at the end of file;
211 Minimize or eliminate the cache effects of reading and writing.
213 will attempt to avoid caching the data you read or write.
215 avoid caching the data, it will minimize the impact the data has on the cache.
216 Use of this flag can drastically reduce performance if not used with care.
220 signal to be sent to the process group
221 when I/O is possible, e.g.,
222 upon availability of data to be read.
225 The seals that may be applied with
228 .Bl -tag -width F_SEAL_SHRINK
230 Prevent any further seals from being applied to the file.
232 Prevent the file from being shrunk with
235 Prevent the file from being enlarged with
241 Any writes in progress will finish before
244 If any writeable mappings exist, F_ADD_SEALS will fail and return
248 Seals are on a per-inode basis and require support by the underlying filesystem.
249 If the underlying filesystem does not support seals,
256 Several commands are available for doing advisory file locking;
257 they all operate on the following structure:
260 off_t l_start; /* starting offset */
261 off_t l_len; /* len = 0 means until end of file */
262 pid_t l_pid; /* lock owner */
263 short l_type; /* lock type: read/write, etc. */
264 short l_whence; /* type of l_start */
265 int l_sysid; /* remote system id or zero for local */
268 The commands available for advisory record locking are as follows:
269 .Bl -tag -width F_SETLKWX
271 Get the first lock that blocks the lock description pointed to by the
274 taken as a pointer to a
277 The information retrieved overwrites the information passed to
282 If no lock is found that would prevent this lock from being created,
283 the structure is left unchanged by this system call except for the
284 lock type which is set to
287 Set or clear a file segment lock according to the lock description
288 pointed to by the third argument,
290 taken as a pointer to a
294 is used to establish shared (or read) locks
296 or exclusive (or write) locks,
298 as well as remove either type of lock
300 If a shared or exclusive lock cannot be set,
302 returns immediately with
305 This command is the same as
307 except that if a shared or exclusive lock is blocked by other locks,
308 the process waits until the request can be satisfied.
309 If a signal that is to be caught is received while
311 is waiting for a region, the
313 will be interrupted if the signal handler has not specified the
319 When a shared lock has been set on a segment of a file,
320 other processes can set shared locks on that segment
322 A shared lock prevents any other process from setting an exclusive
323 lock on any portion of the protected area.
324 A request for a shared lock fails if the file descriptor was not
325 opened with read access.
327 An exclusive lock prevents any other process from setting a shared lock or
328 an exclusive lock on any portion of the protected area.
329 A request for an exclusive lock fails if the file was not
330 opened with write access.
339 to indicate that the relative offset,
341 bytes, will be measured from the start of the file,
342 current position, or end of the file, respectively.
345 is the number of consecutive bytes to be locked.
350 means end edge of the region.
355 fields are only used with
357 to return the process ID of the process holding a blocking lock and
358 the system ID of the system that owns that process.
359 Locks created by the local system will have a system ID of zero.
362 request, the value of
367 Locks may start and extend beyond the current end of a file,
368 but may not start or extend before the beginning of the file.
369 A lock is set to extend to the largest possible value of the
370 file offset for that file if
377 point to the beginning of the file, and
379 is zero, the entire file is locked.
380 If an application wishes only to do entire file locking, the
382 system call is much more efficient.
384 There is at most one type of lock set for each byte in the file.
385 Before a successful return from an
389 request when the calling process has previously existing locks
390 on bytes in the region specified by the request,
391 the previous lock type for each byte in the specified
392 region is replaced by the new lock type.
393 As specified above under the descriptions
394 of shared locks and exclusive locks, an
398 request fails or blocks respectively when another process has existing
399 locks on bytes in the specified region and the type of any of those
400 locks conflicts with the type specified in the request.
404 requests on local files is fair;
405 that is, while the thread is blocked,
406 subsequent requests conflicting with its requests will not be granted,
407 even if these requests do not conflict with existing locks.
409 This interface follows the completely stupid semantics of System V and
411 that require that all locks associated with a file for a given process are
414 file descriptor for that file is closed by that process.
415 This semantic means that applications must be aware of any files that
416 a subroutine library may access.
417 For example if an application for updating the password file locks the
418 password file database while making the update, and then calls
420 to retrieve a record,
421 the lock will be lost because
423 opens, reads, and closes the password database.
424 The database close will release all locks that the process has
425 associated with the database, even if the library routine never
426 requested a lock on the database.
427 Another minor semantic problem with this interface is that
428 locks are not inherited by a child process created using the
433 interface has much more rational last close semantics and
434 allows locks to be inherited by child processes.
437 system call is recommended for applications that want to ensure the integrity
438 of their locks when using library routines or wish to pass locks
446 locks are compatible.
447 Processes using different locking interfaces can cooperate
448 over the same file safely.
449 However, only one of such interfaces should be used within
451 If a file is locked by a process through
453 any record within the file will be seen as locked
454 from the viewpoint of another process using
463 if the process holding a blocking lock previously locked the
467 All locks associated with a file for a given process are
468 removed when the process terminates.
470 All locks obtained before a call to
472 remain in effect until the new program releases them.
473 If the new program does not know about the locks, they will not be
474 released until the program exits.
476 A potential for deadlock occurs if a process controlling a locked region
477 is put to sleep by attempting to lock the locked region of another process.
478 This implementation detects that sleeping until a locked region is unlocked
479 would cause a deadlock and fails with an
483 Upon successful completion, the value returned depends on
486 .Bl -tag -width F_GETOWNX -offset indent
488 A new file descriptor.
490 A file descriptor equal to
493 Value of flag (only the low-order bit is defined).
497 Value of file descriptor owner.
502 Otherwise, a value of -1 is returned and
504 is set to indicate the error.
508 system call will fail if:
521 and the segment of a file to be locked is already
522 exclusive-locked by another process;
523 or the type is an exclusive lock and some portion of the
524 segment of a file to be locked is already shared-locked or
525 exclusive-locked by another process.
530 is not a valid open file descriptor.
538 is not a valid file descriptor.
552 is not a valid file descriptor open for reading.
566 is not a valid file descriptor open for writing.
574 and writeable mappings of the file exist.
580 and a deadlock condition was detected.
586 and the system call was interrupted by a signal.
595 is negative or greater than the maximum allowable number
597 .Xr getdtablesize 2 ) .
606 and the data to which
616 and the underlying filesystem does not support sealing.
626 and the maximum number of file descriptors permitted for the
627 process are already in use,
628 or no file descriptors greater than or equal to
634 argument is not a valid file descriptor for the requested operation.
635 This may be the case if
637 is a device node, or a descriptor returned by
646 and satisfying the lock or unlock request would result in the
647 number of locked regions in the system exceeding a system-imposed limit.
658 refers to a file for which locking is not supported.
669 calculation overflowed.
677 the process ID or process group given as an argument is in a
678 different session than the caller.
687 seal has already been set.
695 the process ID given as argument is not in use.
700 refers to a descriptor open on a terminal device (as opposed to a
701 descriptor open on a socket), a
705 can fail for the same reasons as in
711 for the reasons as stated in
718 .Xr getdtablesize 2 ,
727 constant is non portable.
728 It is provided for compatibility with AIX and Solaris.
736 after any caught signal
737 and should continue waiting during thread suspension such as a stop signal.
738 However, in this implementation a call with
740 is restarted after catching a signal with a
742 handler or a thread suspension such as a stop signal.
746 system call appeared in
751 constant first appeared in