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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_GETOWNX
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 associated with the new file descriptor
77 is set to remain open across
82 It is functionally equivalent to
83 .Bd -literal -offset indent
89 constant is not portable, so it should not be used if portability is needed.
94 Get the close-on-exec flag associated with the file descriptor
98 If the returned value ANDed with
101 the file will remain open across
103 otherwise the file will be closed upon execution of
108 Set the close-on-exec flag associated with
118 Get descriptor status flags, as described below
122 Set descriptor status flags to
125 Get the process ID or process group
130 signals; process groups are returned
135 Set the process or process group
141 process groups are specified by supplying
143 as negative, otherwise
145 is interpreted as a process ID.
152 flags are as follows:
153 .Bl -tag -width O_NONBLOCKX
155 Non-blocking I/O; if no data is available to a
159 operation would block,
160 the read or write call returns -1 with the error
163 Force each write to append at the end of file;
169 Minimize or eliminate the cache effects of reading and writing.
171 will attempt to avoid caching the data you read or write.
173 avoid caching the data, it will minimize the impact the data has on the cache.
174 Use of this flag can drastically reduce performance if not used with care.
178 signal to be sent to the process group
179 when I/O is possible, e.g.,
180 upon availability of data to be read.
183 Several commands are available for doing advisory file locking;
184 they all operate on the following structure:
187 off_t l_start; /* starting offset */
188 off_t l_len; /* len = 0 means until end of file */
189 pid_t l_pid; /* lock owner */
190 short l_type; /* lock type: read/write, etc. */
191 short l_whence; /* type of l_start */
192 int l_sysid; /* remote system id or zero for local */
195 The commands available for advisory record locking are as follows:
196 .Bl -tag -width F_SETLKWX
198 Get the first lock that blocks the lock description pointed to by the
201 taken as a pointer to a
204 The information retrieved overwrites the information passed to
209 If no lock is found that would prevent this lock from being created,
210 the structure is left unchanged by this system call except for the
211 lock type which is set to
214 Set or clear a file segment lock according to the lock description
215 pointed to by the third argument,
217 taken as a pointer to a
221 is used to establish shared (or read) locks
223 or exclusive (or write) locks,
225 as well as remove either type of lock
227 If a shared or exclusive lock cannot be set,
229 returns immediately with
232 This command is the same as
234 except that if a shared or exclusive lock is blocked by other locks,
235 the process waits until the request can be satisfied.
236 If a signal that is to be caught is received while
238 is waiting for a region, the
240 will be interrupted if the signal handler has not specified the
246 When a shared lock has been set on a segment of a file,
247 other processes can set shared locks on that segment
249 A shared lock prevents any other process from setting an exclusive
250 lock on any portion of the protected area.
251 A request for a shared lock fails if the file descriptor was not
252 opened with read access.
254 An exclusive lock prevents any other process from setting a shared lock or
255 an exclusive lock on any portion of the protected area.
256 A request for an exclusive lock fails if the file was not
257 opened with write access.
266 to indicate that the relative offset,
268 bytes, will be measured from the start of the file,
269 current position, or end of the file, respectively.
272 is the number of consecutive bytes to be locked.
277 means end edge of the region.
282 fields are only used with
284 to return the process ID of the process holding a blocking lock and
285 the system ID of the system that owns that process.
286 Locks created by the local system will have a system ID of zero.
289 request, the value of
294 Locks may start and extend beyond the current end of a file,
295 but may not start or extend before the beginning of the file.
296 A lock is set to extend to the largest possible value of the
297 file offset for that file if
304 point to the beginning of the file, and
306 is zero, the entire file is locked.
307 If an application wishes only to do entire file locking, the
309 system call is much more efficient.
311 There is at most one type of lock set for each byte in the file.
312 Before a successful return from an
316 request when the calling process has previously existing locks
317 on bytes in the region specified by the request,
318 the previous lock type for each byte in the specified
319 region is replaced by the new lock type.
320 As specified above under the descriptions
321 of shared locks and exclusive locks, an
325 request fails or blocks respectively when another process has existing
326 locks on bytes in the specified region and the type of any of those
327 locks conflicts with the type specified in the request.
329 This interface follows the completely stupid semantics of System V and
331 that require that all locks associated with a file for a given process are
334 file descriptor for that file is closed by that process.
335 This semantic means that applications must be aware of any files that
336 a subroutine library may access.
337 For example if an application for updating the password file locks the
338 password file database while making the update, and then calls
340 to retrieve a record,
341 the lock will be lost because
343 opens, reads, and closes the password database.
344 The database close will release all locks that the process has
345 associated with the database, even if the library routine never
346 requested a lock on the database.
347 Another minor semantic problem with this interface is that
348 locks are not inherited by a child process created using the
353 interface has much more rational last close semantics and
354 allows locks to be inherited by child processes.
357 system call is recommended for applications that want to ensure the integrity
358 of their locks when using library routines or wish to pass locks
366 locks are compatible.
367 Processes using different locking interfaces can cooperate
368 over the same file safely.
369 However, only one of such interfaces should be used within
371 If a file is locked by a process through
373 any record within the file will be seen as locked
374 from the viewpoint of another process using
383 if the process holding a blocking lock previously locked the
387 All locks associated with a file for a given process are
388 removed when the process terminates.
390 All locks obtained before a call to
392 remain in effect until the new program releases them.
393 If the new program does not know about the locks, they will not be
394 released until the program exits.
396 A potential for deadlock occurs if a process controlling a locked region
397 is put to sleep by attempting to lock the locked region of another process.
398 This implementation detects that sleeping until a locked region is unlocked
399 would cause a deadlock and fails with an
403 Upon successful completion, the value returned depends on
406 .Bl -tag -width F_GETOWNX -offset indent
408 A new file descriptor.
410 A file descriptor equal to
413 Value of flag (only the low-order bit is defined).
417 Value of file descriptor owner.
422 Otherwise, a value of -1 is returned and
424 is set to indicate the error.
428 system call will fail if:
441 and the segment of a file to be locked is already
442 exclusive-locked by another process;
443 or the type is an exclusive lock and some portion of the
444 segment of a file to be locked is already shared-locked or
445 exclusive-locked by another process.
450 is not a valid open file descriptor.
458 is not a valid file descriptor.
472 is not a valid file descriptor open for reading.
486 is not a valid file descriptor open for writing.
492 and a deadlock condition was detected.
498 and the system call was interrupted by a signal.
507 is negative or greater than the maximum allowable number
509 .Xr getdtablesize 2 ) .
518 and the data to which
528 and the maximum number of file descriptors permitted for the
529 process are already in use,
530 or no file descriptors greater than or equal to
540 and satisfying the lock or unlock request would result in the
541 number of locked regions in the system exceeding a system-imposed limit.
552 refers to a file for which locking is not supported.
563 calculation overflowed.
571 the process ID or process group given as an argument is in a
572 different session than the caller.
580 the process ID given as argument is not in use.
585 refers to a descriptor open on a terminal device (as opposed to a
586 descriptor open on a socket), a
590 can fail for the same reasons as in
596 for the reasons as stated in
603 .Xr getdtablesize 2 ,
612 constant is non portable.
613 It is provided for compatibility with AIX and Solaris.
617 system call appeared in
622 constant first appeared in