aboutsummaryrefslogtreecommitdiff
path: root/linuxthreads/linuxthreads.texi
blob: 5bab1a10d94aa406d5850cb1f92cbfc14ae3b8ae (plain)
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
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
@node POSIX Threads, , Top, Top
@chapter POSIX Threads
@c %MENU% The standard threads library

@c This chapter needs more work bigtime. -zw

This chapter describes the pthreads (POSIX threads) library.  This
library provides support functions for multithreaded programs: thread
primitives, synchronization objects, and so forth.  It also implements
POSIX 1003.1b semaphores (not to be confused with System V semaphores).

The threads operations (@samp{pthread_*}) do not use @var{errno}.
Instead they return an error code directly.  The semaphore operations do
use @var{errno}.

@menu
* Basic Thread Operations::     Creating, terminating, and waiting for threads.
* Thread Attributes::           Tuning thread scheduling.
* Cancellation::                Stopping a thread before it's done.
* Cleanup Handlers::            Deallocating resources when a thread is
                                  cancelled.
* Mutexes::                     One way to synchronize threads.
* Condition Variables::         Another way.
* POSIX Semaphores::            And a third way.
* Thread-Specific Data::        Variables with different values in
                                  different threads.
* Threads and Signal Handling:: Why you should avoid mixing the two, and
                                  how to do it if you must.
* Miscellaneous Thread Functions:: A grab bag of utility routines.
@end menu

@node Basic Thread Operations
@section Basic Thread Operations

These functions are the thread equivalents of @code{fork}, @code{exit},
and @code{wait}.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_create (pthread_t * @var{thread}, pthread_attr_t * @var{attr}, void * (*@var{start_routine})(void *), void * @var{arg})
@code{pthread_create} creates a new thread of control that executes
concurrently with the calling thread. The new thread calls the
function @var{start_routine}, passing it @var{arg} as first argument. The
new thread terminates either explicitly, by calling @code{pthread_exit},
or implicitly, by returning from the @var{start_routine} function. The
latter case is equivalent to calling @code{pthread_exit} with the result
returned by @var{start_routine} as exit code.

The @var{attr} argument specifies thread attributes to be applied to the
new thread. @xref{Thread Attributes}, for details. The @var{attr}
argument can also be @code{NULL}, in which case default attributes are
used: the created thread is joinable (not detached) and has an ordinary
(not realtime) scheduling policy.

On success, the identifier of the newly created thread is stored in the
location pointed by the @var{thread} argument, and a 0 is returned. On
error, a non-zero error code is returned.

This function may return the following errors:
@table @code
@item EAGAIN
Not enough system resources to create a process for the new thread,
or more than @code{PTHREAD_THREADS_MAX} threads are already active.
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_exit (void *@var{retval})
@code{pthread_exit} terminates the execution of the calling thread.  All
cleanup handlers (@pxref{Cleanup Handlers}) that have been set for the
calling thread with @code{pthread_cleanup_push} are executed in reverse
order (the most recently pushed handler is executed first). Finalization
functions for thread-specific data are then called for all keys that
have non-@code{NULL} values associated with them in the calling thread
(@pxref{Thread-Specific Data}).  Finally, execution of the calling
thread is stopped.

The @var{retval} argument is the return value of the thread. It can be
retrieved from another thread using @code{pthread_join}.

The @code{pthread_exit} function never returns.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cancel (pthread_t @var{thread})

@code{pthread_cancel} sends a cancellation request to the thread denoted
by the @var{thread} argument.  If there is no such thread,
@code{pthread_cancel} fails and returns @code{ESRCH}.  Otherwise it
returns 0. @xref{Cancellation}, for details.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_join (pthread_t @var{th}, void **thread_@var{return})
@code{pthread_join} suspends the execution of the calling thread until
the thread identified by @var{th} terminates, either by calling
@code{pthread_exit} or by being cancelled.

If @var{thread_return} is not @code{NULL}, the return value of @var{th}
is stored in the location pointed to by @var{thread_return}.  The return
value of @var{th} is either the argument it gave to @code{pthread_exit},
or @code{PTHREAD_CANCELED} if @var{th} was cancelled.

The joined thread @code{th} must be in the joinable state: it must not
have been detached using @code{pthread_detach} or the
@code{PTHREAD_CREATE_DETACHED} attribute to @code{pthread_create}.

When a joinable thread terminates, its memory resources (thread
descriptor and stack) are not deallocated until another thread performs
@code{pthread_join} on it. Therefore, @code{pthread_join} must be called
once for each joinable thread created to avoid memory leaks.

At most one thread can wait for the termination of a given
thread. Calling @code{pthread_join} on a thread @var{th} on which
another thread is already waiting for termination returns an error.

@code{pthread_join} is a cancellation point. If a thread is canceled
while suspended in @code{pthread_join}, the thread execution resumes
immediately and the cancellation is executed without waiting for the
@var{th} thread to terminate. If cancellation occurs during
@code{pthread_join}, the @var{th} thread remains not joined.

On success, the return value of @var{th} is stored in the location
pointed to by @var{thread_return}, and 0 is returned. On error, one of
the following values is returned:
@table @code
@item ESRCH
No thread could be found corresponding to that specified by @var{th}.
@item EINVAL
The @var{th} thread has been detached, or another thread is already
waiting on termination of @var{th}.
@item EDEADLK
The @var{th} argument refers to the calling thread.
@end table
@end deftypefun

@node Thread Attributes
@section Thread Attributes

@comment pthread.h
@comment POSIX

Threads have a number of attributes that may be set at creation time.
This is done by filling a thread attribute object @var{attr} of type
@code{pthread_attr_t}, then passing it as second argument to
@code{pthread_create}. Passing @code{NULL} is equivalent to passing a
thread attribute object with all attributes set to their default values.

Attribute objects are consulted only when creating a new thread.  The
same attribute object can be used for creating several threads.
Modifying an attribute object after a call to @code{pthread_create} does
not change the attributes of the thread previously created.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_init (pthread_attr_t *@var{attr})
@code{pthread_attr_init} initializes the thread attribute object
@var{attr} and fills it with default values for the attributes. (The
default values are listed below for each attribute.)

Each attribute @var{attrname} (see below for a list of all attributes)
can be individually set using the function
@code{pthread_attr_set@var{attrname}} and retrieved using the function
@code{pthread_attr_get@var{attrname}}.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_destroy (pthread_attr_t *@var{attr})
@code{pthread_attr_destroy} destroys the attribute object pointed to by
@var{attr} releasing any resources associated with it.  @var{attr} is
left in an undefined state, and you must not use it again in a call to
any pthreads function until it has been reinitialized.
@end deftypefun

@findex pthread_attr_setinheritsched
@findex pthread_attr_setschedparam
@findex pthread_attr_setschedpolicy
@findex pthread_attr_setscope
@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_set@var{attr} (pthread_attr_t *@var{obj}, int @var{value})
Set attribute @var{attr} to @var{value} in the attribute object pointed
to by @var{obj}.  See below for a list of possible attributes and the
values they can take.

On success, these functions return 0.  If @var{value} is not meaningful
for the @var{attr} being modified, they will return the error code
@code{EINVAL}.  Some of the functions have other failure modes; see
below.
@end deftypefun

@findex pthread_attr_getinheritsched
@findex pthread_attr_getschedparam
@findex pthread_attr_getschedpolicy
@findex pthread_attr_getscope
@comment pthread.h
@comment POSIX
@deftypefun int pthread_attr_get@var{attr} (const pthread_attr_t *@var{obj}, int *@var{value})
Store the current setting of @var{attr} in @var{obj} into the variable
pointed to by @var{value}.

These functions always return 0.
@end deftypefun

The following thread attributes are supported:
@table @samp
@item detachstate
Choose whether the thread is created in the joinable state (value
@code{PTHREAD_CREATE_JOINABLE}) or in the detached state
(@code{PTHREAD_CREATE_DETACHED}).  The default is
@code{PTHREAD_CREATE_JOINABLE}.

In the joinable state, another thread can synchronize on the thread
termination and recover its termination code using @code{pthread_join},
but some of the thread resources are kept allocated after the thread
terminates, and reclaimed only when another thread performs
@code{pthread_join} on that thread.

In the detached state, the thread resources are immediately freed when
it terminates, but @code{pthread_join} cannot be used to synchronize on
the thread termination.

A thread created in the joinable state can later be put in the detached
thread using @code{pthread_detach}.

@item schedpolicy
Select the scheduling policy for the thread: one of @code{SCHED_OTHER}
(regular, non-realtime scheduling), @code{SCHED_RR} (realtime,
round-robin) or @code{SCHED_FIFO} (realtime, first-in first-out).
The default is @code{SCHED_OTHER}.
@c Not doc'd in our manual: FIXME.
@c See @code{sched_setpolicy} for more information on scheduling policies.

The realtime scheduling policies @code{SCHED_RR} and @code{SCHED_FIFO}
are available only to processes with superuser privileges.
@code{pthread_attr_setschedparam} will fail and return @code{ENOTSUP} if
you try to set a realtime policy when you are unprivileged.

The scheduling policy of a thread can be changed after creation with
@code{pthread_setschedparam}.

@item schedparam
Change the scheduling parameter (the scheduling priority)
for the thread.  The default is 0.

This attribute is not significant if the scheduling policy is
@code{SCHED_OTHER}; it only matters for the realtime policies
@code{SCHED_RR} and @code{SCHED_FIFO}.

The scheduling priority of a thread can be changed after creation with
@code{pthread_setschedparam}.

@item inheritsched
Choose whether the scheduling policy and scheduling parameter for the
newly created thread are determined by the values of the
@var{schedpolicy} and @var{schedparam} attributes (value
@code{PTHREAD_EXPLICIT_SCHED}) or are inherited from the parent thread
(value @code{PTHREAD_INHERIT_SCHED}).  The default is
@code{PTHREAD_EXPLICIT_SCHED}.

@item scope
Choose the scheduling contention scope for the created thread.  The
default is @code{PTHREAD_SCOPE_SYSTEM}, meaning that the threads contend
for CPU time with all processes running on the machine. In particular,
thread priorities are interpreted relative to the priorities of all
other processes on the machine. The other possibility,
@code{PTHREAD_SCOPE_PROCESS}, means that scheduling contention occurs
only between the threads of the running process: thread priorities are
interpreted relative to the priorities of the other threads of the
process, regardless of the priorities of other processes.

@code{PTHREAD_SCOPE_PROCESS} is not supported in LinuxThreads.  If you
try to set the scope to this value @code{pthread_attr_setscope} will
fail and return @code{ENOTSUP}.
@end table

@node Cancellation
@section Cancellation

Cancellation is the mechanism by which a thread can terminate the
execution of another thread. More precisely, a thread can send a
cancellation request to another thread. Depending on its settings, the
target thread can then either ignore the request, honor it immediately,
or defer it till it reaches a cancellation point.  When threads are
first created by @code{pthread_create}, they always defer cancellation
requests.

When a thread eventually honors a cancellation request, it behaves as if
@code{pthread_exit(PTHREAD_CANCELED)} was called.  All cleanup handlers
are executed in reverse order, finalization functions for
thread-specific data are called, and finally the thread stops executing.
If the cancelled thread was joinable, the return value
@code{PTHREAD_CANCELED} is provided to whichever thread calls
@var{pthread_join} on it. See @code{pthread_exit} for more information.

Cancellation points are the points where the thread checks for pending
cancellation requests and performs them.  The POSIX threads functions
@code{pthread_join}, @code{pthread_cond_wait},
@code{pthread_cond_timedwait}, @code{pthread_testcancel},
@code{sem_wait}, and @code{sigwait} are cancellation points.  In
addition, these system calls are cancellation points:

@multitable @columnfractions .33 .33 .33
@item @t{accept}	@tab @t{open}		@tab @t{sendmsg}
@item @t{close}		@tab @t{pause}		@tab @t{sendto}
@item @t{connect}	@tab @t{read}		@tab @t{system}
@item @t{fcntl}		@tab @t{recv}		@tab @t{tcdrain}
@item @t{fsync}		@tab @t{recvfrom}	@tab @t{wait}
@item @t{lseek}		@tab @t{recvmsg}	@tab @t{waitpid}
@item @t{msync}		@tab @t{send}		@tab @t{write}
@item @t{nanosleep}
@end multitable

@noindent
All library functions that call these functions (such as
@code{printf}) are also cancellation points.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcancelstate (int @var{state}, int *@var{oldstate})
@code{pthread_setcancelstate} changes the cancellation state for the
calling thread -- that is, whether cancellation requests are ignored or
not. The @var{state} argument is the new cancellation state: either
@code{PTHREAD_CANCEL_ENABLE} to enable cancellation, or
@code{PTHREAD_CANCEL_DISABLE} to disable cancellation (cancellation
requests are ignored).

If @var{oldstate} is not @code{NULL}, the previous cancellation state is
stored in the location pointed to by @var{oldstate}, and can thus be
restored later by another call to @code{pthread_setcancelstate}.

If the @var{state} argument is not @code{PTHREAD_CANCEL_ENABLE} or
@code{PTHREAD_CANCEL_DISABLE}, @code{pthread_setcancelstate} fails and
returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setcanceltype (int @var{type}, int *@var{oldtype})
@code{pthread_setcanceltype} changes the type of responses to
cancellation requests for the calling thread: asynchronous (immediate)
or deferred.  The @var{type} argument is the new cancellation type:
either @code{PTHREAD_CANCEL_ASYNCHRONOUS} to cancel the calling thread
as soon as the cancellation request is received, or
@code{PTHREAD_CANCEL_DEFERRED} to keep the cancellation request pending
until the next cancellation point. If @var{oldtype} is not @code{NULL},
the previous cancellation state is stored in the location pointed to by
@var{oldtype}, and can thus be restored later by another call to
@code{pthread_setcanceltype}.

If the @var{type} argument is not @code{PTHREAD_CANCEL_DEFERRED} or
@code{PTHREAD_CANCEL_ASYNCHRONOUS}, @code{pthread_setcanceltype} fails
and returns @code{EINVAL}.  Otherwise it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_testcancel (@var{void})
@code{pthread_testcancel} does nothing except testing for pending
cancellation and executing it. Its purpose is to introduce explicit
checks for cancellation in long sequences of code that do not call
cancellation point functions otherwise.
@end deftypefun

@node Cleanup Handlers
@section Cleanup Handlers

Cleanup handlers are functions that get called when a thread terminates,
either by calling @code{pthread_exit} or because of
cancellation. Cleanup handlers are installed and removed following a
stack-like discipline.

The purpose of cleanup handlers is to free the resources that a thread
may hold at the time it terminates. In particular, if a thread exits or
is cancelled while it owns a locked mutex, the mutex will remain locked
forever and prevent other threads from executing normally. The best way
to avoid this is, just before locking the mutex, to install a cleanup
handler whose effect is to unlock the mutex. Cleanup handlers can be
used similarly to free blocks allocated with @code{malloc} or close file
descriptors on thread termination.

Here is how to lock a mutex @var{mut} in such a way that it will be
unlocked if the thread is canceled while @var{mut} is locked:

@smallexample
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_mutex_unlock(&mut);
pthread_cleanup_pop(0);
@end smallexample

Equivalently, the last two lines can be replaced by

@smallexample
pthread_cleanup_pop(1);
@end smallexample

Notice that the code above is safe only in deferred cancellation mode
(see @code{pthread_setcanceltype}). In asynchronous cancellation mode, a
cancellation can occur between @code{pthread_cleanup_push} and
@code{pthread_mutex_lock}, or between @code{pthread_mutex_unlock} and
@code{pthread_cleanup_pop}, resulting in both cases in the thread trying
to unlock a mutex not locked by the current thread. This is the main
reason why asynchronous cancellation is difficult to use.

If the code above must also work in asynchronous cancellation mode,
then it must switch to deferred mode for locking and unlocking the
mutex:

@smallexample
pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
pthread_cleanup_push(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop(1);
pthread_setcanceltype(oldtype, NULL);
@end smallexample

The code above can be rewritten in a more compact and efficient way,
using the non-portable functions @code{pthread_cleanup_push_defer_np}
and @code{pthread_cleanup_pop_restore_np}:

@smallexample
pthread_cleanup_push_defer_np(pthread_mutex_unlock, (void *) &mut);
pthread_mutex_lock(&mut);
/* do some work */
pthread_cleanup_pop_restore_np(1);
@end smallexample

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_push (void (*@var{routine}) (void *), void *@var{arg})

@code{pthread_cleanup_push} installs the @var{routine} function with
argument @var{arg} as a cleanup handler. From this point on to the
matching @code{pthread_cleanup_pop}, the function @var{routine} will be
called with arguments @var{arg} when the thread terminates, either
through @code{pthread_exit} or by cancellation. If several cleanup
handlers are active at that point, they are called in LIFO order: the
most recently installed handler is called first.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun void pthread_cleanup_pop (int @var{execute})
@code{pthread_cleanup_pop} removes the most recently installed cleanup
handler. If the @var{execute} argument is not 0, it also executes the
handler, by calling the @var{routine} function with arguments
@var{arg}. If the @var{execute} argument is 0, the handler is only
removed but not executed.
@end deftypefun

Matching pairs of @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} must occur in the same function, at the same
level of block nesting.  Actually, @code{pthread_cleanup_push} and
@code{pthread_cleanup_pop} are macros, and the expansion of
@code{pthread_cleanup_push} introduces an open brace @code{@{} with the
matching closing brace @code{@}} being introduced by the expansion of the
matching @code{pthread_cleanup_pop}.

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_push_defer_np (void (*@var{routine}) (void *), void *@var{arg})
@code{pthread_cleanup_push_defer_np} is a non-portable extension that
combines @code{pthread_cleanup_push} and @code{pthread_setcanceltype}.
It pushes a cleanup handler just as @code{pthread_cleanup_push} does,
but also saves the current cancellation type and sets it to deferred
cancellation. This ensures that the cleanup mechanism is effective even
if the thread was initially in asynchronous cancellation mode.
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun void pthread_cleanup_pop_restore_np (int @var{execute})
@code{pthread_cleanup_pop_restore_np} pops a cleanup handler introduced
by @code{pthread_cleanup_push_defer_np}, and restores the cancellation
type to its value at the time @code{pthread_cleanup_push_defer_np} was
called.
@end deftypefun

@code{pthread_cleanup_push_defer_np} and
@code{pthread_cleanup_pop_restore_np} must occur in matching pairs, at
the same level of block nesting.

The sequence

@smallexample
pthread_cleanup_push_defer_np(routine, arg);
...
pthread_cleanup_pop_defer_np(execute);
@end smallexample

@noindent
is functionally equivalent to (but more compact and efficient than)

@smallexample
@{
  int oldtype;
  pthread_setcanceltype(PTHREAD_CANCEL_DEFERRED, &oldtype);
  pthread_cleanup_push(routine, arg);
  ...
  pthread_cleanup_pop(execute);
  pthread_setcanceltype(oldtype, NULL);
@}
@end smallexample


@node Mutexes
@section Mutexes

A mutex is a MUTual EXclusion device, and is useful for protecting
shared data structures from concurrent modifications, and implementing
critical sections and monitors.

A mutex has two possible states: unlocked (not owned by any thread),
and locked (owned by one thread). A mutex can never be owned by two
different threads simultaneously. A thread attempting to lock a mutex
that is already locked by another thread is suspended until the owning
thread unlocks the mutex first.

None of the mutex functions is a cancellation point, not even
@code{pthread_mutex_lock}, in spite of the fact that it can suspend a
thread for arbitrary durations. This way, the status of mutexes at
cancellation points is predictable, allowing cancellation handlers to
unlock precisely those mutexes that need to be unlocked before the
thread stops executing. Consequently, threads using deferred
cancellation should never hold a mutex for extended periods of time.

It is not safe to call mutex functions from a signal handler.  In
particular, calling @code{pthread_mutex_lock} or
@code{pthread_mutex_unlock} from a signal handler may deadlock the
calling thread.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_init (pthread_mutex_t *@var{mutex}, const pthread_mutexattr_t *@var{mutexattr})

@code{pthread_mutex_init} initializes the mutex object pointed to by
@var{mutex} according to the mutex attributes specified in @var{mutexattr}.
If @var{mutexattr} is @code{NULL}, default attributes are used instead.

The LinuxThreads implementation supports only one mutex attribute,
the @var{mutex kind}, which is either ``fast'', ``recursive'', or
``error checking''. The kind of a mutex determines whether
it can be locked again by a thread that already owns it.
The default kind is ``fast''.

Variables of type @code{pthread_mutex_t} can also be initialized
statically, using the constants @code{PTHREAD_MUTEX_INITIALIZER} (for
fast mutexes), @code{PTHREAD_RECURSIVE_MUTEX_INITIALIZER_NP} (for
recursive mutexes), and @code{PTHREAD_ERRORCHECK_MUTEX_INITIALIZER_NP}
(for error checking mutexes).

@code{pthread_mutex_init} always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_lock (pthread_mutex_t *mutex))
@code{pthread_mutex_lock} locks the given mutex. If the mutex is
currently unlocked, it becomes locked and owned by the calling thread,
and @code{pthread_mutex_lock} returns immediately. If the mutex is
already locked by another thread, @code{pthread_mutex_lock} suspends the
calling thread until the mutex is unlocked.

If the mutex is already locked by the calling thread, the behavior of
@code{pthread_mutex_lock} depends on the kind of the mutex. If the mutex
is of the ``fast'' kind, the calling thread is suspended.  It will
remain suspended forever, because no other thread can unlock the mutex.
If  the mutex is of the ``error checking'' kind, @code{pthread_mutex_lock}
returns immediately with the error code @code{EDEADLK}.  If the mutex is
of the ``recursive'' kind, @code{pthread_mutex_lock} succeeds and
returns immediately, recording the number of times the calling thread
has locked the mutex. An equal number of @code{pthread_mutex_unlock}
operations must be performed before the mutex returns to the unlocked
state.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_trylock (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_trylock} behaves identically to
@code{pthread_mutex_lock}, except that it does not block the calling
thread if the mutex is already locked by another thread (or by the
calling thread in the case of a ``fast'' mutex). Instead,
@code{pthread_mutex_trylock} returns immediately with the error code
@code{EBUSY}.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_unlock (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_unlock} unlocks the given mutex. The mutex is
assumed to be locked and owned by the calling thread on entrance to
@code{pthread_mutex_unlock}. If the mutex is of the ``fast'' kind,
@code{pthread_mutex_unlock} always returns it to the unlocked state. If
it is of the ``recursive'' kind, it decrements the locking count of the
mutex (number of @code{pthread_mutex_lock} operations performed on it by
the calling thread), and only when this count reaches zero is the mutex
actually unlocked.

On ``error checking'' mutexes, @code{pthread_mutex_unlock} actually
checks at run-time that the mutex is locked on entrance, and that it was
locked by the same thread that is now calling
@code{pthread_mutex_unlock}.  If these conditions are not met,
@code{pthread_mutex_unlock} returns @code{EPERM}, and the mutex remains
unchanged.  ``Fast'' and ``recursive'' mutexes perform no such checks,
thus allowing a locked mutex to be unlocked by a thread other than its
owner. This is non-portable behavior and must not be relied upon.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutex_destroy (pthread_mutex_t *@var{mutex})
@code{pthread_mutex_destroy} destroys a mutex object, freeing the
resources it might hold. The mutex must be unlocked on entrance. In the
LinuxThreads implementation, no resources are associated with mutex
objects, thus @code{pthread_mutex_destroy} actually does nothing except
checking that the mutex is unlocked.

If the mutex is locked by some thread, @code{pthread_mutex_destroy}
returns @code{EBUSY}.  Otherwise it returns 0.
@end deftypefun

If any of the above functions (except @code{pthread_mutex_init})
is applied to an uninitialized mutex, they will simply return
@code{EINVAL} and do nothing.

A shared global variable @var{x} can be protected by a mutex as follows:

@smallexample
int x;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
@end smallexample

All accesses and modifications to @var{x} should be bracketed by calls to
@code{pthread_mutex_lock} and @code{pthread_mutex_unlock} as follows:

@smallexample
pthread_mutex_lock(&mut);
/* operate on x */
pthread_mutex_unlock(&mut);
@end smallexample

Mutex attributes can be specified at mutex creation time, by passing a
mutex attribute object as second argument to @code{pthread_mutex_init}.
Passing @code{NULL} is equivalent to passing a mutex attribute object
with all attributes set to their default values.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutexattr_init (pthread_mutexattr_t *@var{attr})
@code{pthread_mutexattr_init} initializes the mutex attribute object
@var{attr} and fills it with default values for the attributes.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_mutexattr_destroy (pthread_mutexattr_t *@var{attr})
@code{pthread_mutexattr_destroy} destroys a mutex attribute object,
which must not be reused until it is
reinitialized. @code{pthread_mutexattr_destroy} does nothing in the
LinuxThreads implementation.

This function always returns 0.
@end deftypefun

LinuxThreads supports only one mutex attribute: the mutex kind, which is
either @code{PTHREAD_MUTEX_FAST_NP} for ``fast'' mutexes,
@code{PTHREAD_MUTEX_RECURSIVE_NP} for ``recursive'' mutexes, or
@code{PTHREAD_MUTEX_ERRORCHECK_NP} for ``error checking'' mutexes.  As
the @code{NP} suffix indicates, this is a non-portable extension to the
POSIX standard and should not be employed in portable programs.

The mutex kind determines what happens if a thread attempts to lock a
mutex it already owns with @code{pthread_mutex_lock}. If the mutex is of
the ``fast'' kind, @code{pthread_mutex_lock} simply suspends the calling
thread forever.  If the mutex is of the ``error checking'' kind,
@code{pthread_mutex_lock} returns immediately with the error code
@code{EDEADLK}.  If the mutex is of the ``recursive'' kind, the call to
@code{pthread_mutex_lock} returns immediately with a success return
code. The number of times the thread owning the mutex has locked it is
recorded in the mutex. The owning thread must call
@code{pthread_mutex_unlock} the same number of times before the mutex
returns to the unlocked state.

The default mutex kind is ``fast'', that is, @code{PTHREAD_MUTEX_FAST_NP}.

@comment pthread.h
@comment GNU
@deftypefun int pthread_mutexattr_setkind_np (pthread_mutexattr_t *@var{attr}, int @var{kind})
@code{pthread_mutexattr_setkind_np} sets the mutex kind attribute in
@var{attr} to the value specified by @var{kind}.

If @var{kind} is not @code{PTHREAD_MUTEX_FAST_NP},
@code{PTHREAD_MUTEX_RECURSIVE_NP}, or
@code{PTHREAD_MUTEX_ERRORCHECK_NP}, this function will return
@code{EINVAL} and leave @var{attr} unchanged.
@end deftypefun

@comment pthread.h
@comment GNU
@deftypefun int pthread_mutexattr_getkind_np (const pthread_mutexattr_t *@var{attr}, int *@var{kind})
@code{pthread_mutexattr_getkind_np} retrieves the current value of the
mutex kind attribute in @var{attr} and stores it in the location pointed
to by @var{kind}.

This function always returns 0.
@end deftypefun

@node Condition Variables
@section Condition Variables

A condition (short for ``condition variable'') is a synchronization
device that allows threads to suspend execution until some predicate on
shared data is satisfied. The basic operations on conditions are: signal
the condition (when the predicate becomes true), and wait for the
condition, suspending the thread execution until another thread signals
the condition.

A condition variable must always be associated with a mutex, to avoid
the race condition where a thread prepares to wait on a condition
variable and another thread signals the condition just before the first
thread actually waits on it.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_init (pthread_cond_t *@var{cond}, pthread_condattr_t *cond_@var{attr})

@code{pthread_cond_init} initializes the condition variable @var{cond},
using the condition attributes specified in @var{cond_attr}, or default
attributes if @var{cond_attr} is @code{NULL}. The LinuxThreads
implementation supports no attributes for conditions, hence the
@var{cond_attr} parameter is actually ignored.

Variables of type @code{pthread_cond_t} can also be initialized
statically, using the constant @code{PTHREAD_COND_INITIALIZER}.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_signal (pthread_cond_t *@var{cond})
@code{pthread_cond_signal} restarts one of the threads that are waiting
on the condition variable @var{cond}. If no threads are waiting on
@var{cond}, nothing happens. If several threads are waiting on
@var{cond}, exactly one is restarted, but it is not specified which.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_broadcast (pthread_cond_t *@var{cond})
@code{pthread_cond_broadcast} restarts all the threads that are waiting
on the condition variable @var{cond}. Nothing happens if no threads are
waiting on @var{cond}.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_wait (pthread_cond_t *@var{cond}, pthread_mutex_t *@var{mutex})
@code{pthread_cond_wait} atomically unlocks the @var{mutex} (as per
@code{pthread_unlock_mutex}) and waits for the condition variable
@var{cond} to be signaled. The thread execution is suspended and does
not consume any CPU time until the condition variable is signaled. The
@var{mutex} must be locked by the calling thread on entrance to
@code{pthread_cond_wait}. Before returning to the calling thread,
@code{pthread_cond_wait} re-acquires @var{mutex} (as per
@code{pthread_lock_mutex}).

Unlocking the mutex and suspending on the condition variable is done
atomically. Thus, if all threads always acquire the mutex before
signaling the condition, this guarantees that the condition cannot be
signaled (and thus ignored) between the time a thread locks the mutex
and the time it waits on the condition variable.

This function always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_timedwait (pthread_cond_t *@var{cond}, pthread_mutex_t *@var{mutex}, const struct timespec *@var{abstime})
@code{pthread_cond_timedwait} atomically unlocks @var{mutex} and waits
on @var{cond}, as @code{pthread_cond_wait} does, but it also bounds the
duration of the wait. If @var{cond} has not been signaled before time
@var{abstime}, the mutex @var{mutex} is re-acquired and
@code{pthread_cond_timedwait} returns the error code @code{ETIMEDOUT}.
The wait can also be interrupted by a signal; in that case
@code{pthread_cond_timedwait} returns @code{EINTR}.

The @var{abstime} parameter specifies an absolute time, with the same
origin as @code{time} and @code{gettimeofday}: an @var{abstime} of 0
corresponds to 00:00:00 GMT, January 1, 1970.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_cond_destroy (pthread_cond_t *@var{cond})
@code{pthread_cond_destroy} destroys the condition variable @var{cond},
freeing the resources it might hold.  If any threads are waiting on the
condition variable, @code{pthread_cond_destroy} leaves @var{cond}
untouched and returns @code{EBUSY}.  Otherwise it returns 0, and
@var{cond} must not be used again until it is reinitialized.

In the LinuxThreads implementation, no resources are associated with
condition variables, so @code{pthread_cond_destroy} actually does
nothing.
@end deftypefun

@code{pthread_cond_wait} and @code{pthread_cond_timedwait} are
cancellation points. If a thread is cancelled while suspended in one of
these functions, the thread immediately resumes execution, relocks the
mutex specified by  @var{mutex}, and finally executes the cancellation.
Consequently, cleanup handlers are assured that @var{mutex} is locked
when they are called.

It is not safe to call the condition variable functions from a signal
handler. In particular, calling @code{pthread_cond_signal} or
@code{pthread_cond_broadcast} from a signal handler may deadlock the
calling thread.

Consider two shared variables @var{x} and @var{y}, protected by the
mutex @var{mut}, and a condition variable @var{cond} that is to be
signaled whenever @var{x} becomes greater than @var{y}.

@smallexample
int x,y;
pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
pthread_cond_t cond = PTHREAD_COND_INITIALIZER;
@end smallexample

Waiting until @var{x} is greater than @var{y} is performed as follows:

@smallexample
pthread_mutex_lock(&mut);
while (x <= y) @{
        pthread_cond_wait(&cond, &mut);
@}
/* operate on x and y */
pthread_mutex_unlock(&mut);
@end smallexample

Modifications on @var{x} and @var{y} that may cause @var{x} to become greater than
@var{y} should signal the condition if needed:

@smallexample
pthread_mutex_lock(&mut);
/* modify x and y */
if (x > y) pthread_mutex_broadcast(&cond);
pthread_mutex_unlock(&mut);
@end smallexample

If it can be proved that at most one waiting thread needs to be waken
up (for instance, if there are only two threads communicating through
@var{x} and @var{y}), @code{pthread_cond_signal} can be used as a slightly more
efficient alternative to @code{pthread_cond_broadcast}. In doubt, use
@code{pthread_cond_broadcast}.

To wait for @var{x} to becomes greater than @var{y} with a timeout of 5
seconds, do:

@smallexample
struct timeval now;
struct timespec timeout;
int retcode;

pthread_mutex_lock(&mut);
gettimeofday(&now);
timeout.tv_sec = now.tv_sec + 5;
timeout.tv_nsec = now.tv_usec * 1000;
retcode = 0;
while (x <= y && retcode != ETIMEDOUT) @{
        retcode = pthread_cond_timedwait(&cond, &mut, &timeout);
@}
if (retcode == ETIMEDOUT) @{
        /* timeout occurred */
@} else @{
        /* operate on x and y */
@}
pthread_mutex_unlock(&mut);
@end smallexample

Condition attributes can be specified at condition creation time, by
passing a condition attribute object as second argument to
@code{pthread_cond_init}.  Passing @code{NULL} is equivalent to passing
a condition attribute object with all attributes set to their default
values.

The LinuxThreads implementation supports no attributes for
conditions. The functions on condition attributes are included only for
compliance with the POSIX standard.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_condattr_init (pthread_condattr_t *@var{attr})
@deftypefunx int pthread_condattr_destroy (pthread_condattr_t *@var{attr})
@code{pthread_condattr_init} initializes the condition attribute object
@var{attr} and fills it with default values for the attributes.
@code{pthread_condattr_destroy} destroys the condition attribute object
@var{attr}.

Both functions do nothing in the LinuxThreads implementation.

@code{pthread_condattr_init} and @code{pthread_condattr_destroy} always
return 0.
@end deftypefun

@node POSIX Semaphores
@section POSIX Semaphores

@vindex SEM_VALUE_MAX
Semaphores are counters for resources shared between threads. The
basic operations on semaphores are: increment the counter atomically,
and wait until the counter is non-null and decrement it atomically.

Semaphores have a maximum value past which they cannot be incremented.
The macro @code{SEM_VALUE_MAX} is defined to be this maximum value.  In
the GNU C library, @code{SEM_VALUE_MAX} is equal to @code{INT_MAX}
(@pxref{Range of Type}), but it may be much smaller on other systems.

The pthreads library implements POSIX 1003.1b semaphores.  These should
not be confused with System V semaphores (@code{ipc}, @code{semctl} and
@code{semop}).
@c !!! SysV IPC is not doc'd at all in our manual

All the semaphore functions and macros are defined in @file{semaphore.h}.

@comment semaphore.h
@comment POSIX
@deftypefun int sem_init (sem_t *@var{sem}, int @var{pshared}, unsigned int @var{value})
@code{sem_init} initializes the semaphore object pointed to by
@var{sem}. The count associated with the semaphore is set initially to
@var{value}. The @var{pshared} argument indicates whether the semaphore
is local to the current process (@var{pshared} is zero) or is to be
shared between several processes (@var{pshared} is not zero).

On success @code{sem_init} returns 0.  On failure it returns -1 and sets
@var{errno} to one of the following values:

@table @code
@item EINVAL
@var{value} exceeds the maximal counter value @code{SEM_VALUE_MAX}

@item ENOSYS
@var{pshared} is not zero.  LinuxThreads currently does not support
process-shared semaphores.  (This will eventually change.)
@end table
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_destroy (sem_t * @var{sem})
@code{sem_destroy} destroys a semaphore object, freeing the resources it
might hold.  If any threads are waiting on the semaphore when
@code{sem_destroy} is called, it fails and sets @var{errno} to
@code{EBUSY}.

In the LinuxThreads implementation, no resources are associated with
semaphore objects, thus @code{sem_destroy} actually does nothing except
checking that no thread is waiting on the semaphore.  This will change
when process-shared semaphores are implemented.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_wait (sem_t * @var{sem})
@code{sem_wait} suspends the calling thread until the semaphore pointed
to by @var{sem} has non-zero count. It then atomically decreases the
semaphore count.

@code{sem_wait} is a cancellation point.  It always returns 0.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_trywait (sem_t * @var{sem})
@code{sem_trywait} is a non-blocking variant of @code{sem_wait}. If the
semaphore pointed to by @var{sem} has non-zero count, the count is
atomically decreased and @code{sem_trywait} immediately returns 0.  If
the semaphore count is zero, @code{sem_trywait} immediately returns -1
and sets errno to @code{EAGAIN}.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_post (sem_t * @var{sem})
@code{sem_post} atomically increases the count of the semaphore pointed to
by @var{sem}. This function never blocks.

@c !!! This para appears not to agree with the code.
On processors supporting atomic compare-and-swap (Intel 486, Pentium and
later, Alpha, PowerPC, MIPS II, Motorola 68k, Ultrasparc), the
@code{sem_post} function is can safely be called from signal handlers.
This is the only thread synchronization function provided by POSIX
threads that is async-signal safe.  On the Intel 386 and earlier Sparc
chips, the current LinuxThreads implementation of @code{sem_post} is not
async-signal safe, because the hardware does not support the required
atomic operations.

@code{sem_post} always succeeds and returns 0, unless the semaphore
count would exceed @code{SEM_VALUE_MAX} after being incremented.  In
that case @code{sem_post} returns -1 and sets @var{errno} to
@code{EINVAL}.  The semaphore count is left unchanged.
@end deftypefun

@comment semaphore.h
@comment POSIX
@deftypefun int sem_getvalue (sem_t * @var{sem}, int * @var{sval})
@code{sem_getvalue} stores in the location pointed to by @var{sval} the
current count of the semaphore @var{sem}.  It always returns 0.
@end deftypefun

@node Thread-Specific Data
@section Thread-Specific Data

Programs often need global or static variables that have different
values in different threads. Since threads share one memory space, this
cannot be achieved with regular variables. Thread-specific data is the
POSIX threads answer to this need.

Each thread possesses a private memory block, the thread-specific data
area, or TSD area for short. This area is indexed by TSD keys. The TSD
area associates values of type @code{void *} to TSD keys. TSD keys are
common to all threads, but the value associated with a given TSD key can
be different in each thread.

For concreteness, the TSD areas can be viewed as arrays of @code{void *}
pointers, TSD keys as integer indices into these arrays, and the value
of a TSD key as the value of the corresponding array element in the
calling thread.

When a thread is created, its TSD area initially associates @code{NULL}
with all keys.

@comment pthread.h
@comment POSIX
@deftypefun int pthread_key_create (pthread_key_t *@var{key}, void (*destr_function) (void *))
@code{pthread_key_create} allocates a new TSD key. The key is stored in
the location pointed to by @var{key}. There is a limit of
@code{PTHREAD_KEYS_MAX} on the number of keys allocated at a given
time. The value initially associated with the returned key is
@code{NULL} in all currently executing threads.

The @var{destr_function} argument, if not @code{NULL}, specifies a
destructor function associated with the key. When a thread terminates
via @code{pthread_exit} or by cancellation, @var{destr_function} is
called on the value associated with the key in that thread. The
@var{destr_function} is not called if a key is deleted with
@code{pthread_key_delete} or a value is changed with
@code{pthread_setspecific}.  The order in which destructor functions are
called at thread termination time is unspecified.

Before the destructor function is called, the @code{NULL} value is
associated with the key in the current thread.  A destructor function
might, however, re-associate non-@code{NULL} values to that key or some
other key.  To deal with this, if after all the destructors have been
called for all non-@code{NULL} values, there are still some
non-@code{NULL} values with associated destructors, then the process is
repeated.  The LinuxThreads implementation stops the process after
@code{PTHREAD_DESTRUCTOR_ITERATIONS} iterations, even if some
non-@code{NULL} values with associated descriptors remain.  Other
implementations may loop indefinitely.

@code{pthread_key_create} returns 0 unless @code{PTHREAD_KEYS_MAX} keys
have already been allocated, in which case it fails and returns
@code{EAGAIN}.
@end deftypefun


@comment pthread.h
@comment POSIX
@deftypefun int pthread_key_delete (pthread_key_t @var{key})
@code{pthread_key_delete} deallocates a TSD key. It does not check
whether non-@code{NULL} values are associated with that key in the
currently executing threads, nor call the destructor function associated
with the key.

If there is no such key @var{key}, it returns @code{EINVAL}.  Otherwise
it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setspecific (pthread_key_t @var{key}, const void *@var{pointer})
@code{pthread_setspecific} changes the value associated with @var{key}
in the calling thread, storing the given @var{pointer} instead.

If there is no such key @var{key}, it returns @code{EINVAL}.  Otherwise
it returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun {void *} pthread_getspecific (pthread_key_t @var{key})
@code{pthread_getspecific} returns the value currently associated with
@var{key} in the calling thread.

If there is no such key @var{key}, it returns @code{NULL}.
@end deftypefun

The following code fragment allocates a thread-specific array of 100
characters, with automatic reclaimation at thread exit:

@smallexample
/* Key for the thread-specific buffer */
static pthread_key_t buffer_key;

/* Once-only initialisation of the key */
static pthread_once_t buffer_key_once = PTHREAD_ONCE_INIT;

/* Allocate the thread-specific buffer */
void buffer_alloc(void)
@{
  pthread_once(&buffer_key_once, buffer_key_alloc);
  pthread_setspecific(buffer_key, malloc(100));
@}

/* Return the thread-specific buffer */
char * get_buffer(void)
@{
  return (char *) pthread_getspecific(buffer_key);
@}

/* Allocate the key */
static void buffer_key_alloc()
@{
  pthread_key_create(&buffer_key, buffer_destroy);
@}

/* Free the thread-specific buffer */
static void buffer_destroy(void * buf)
@{
  free(buf);
@}
@end smallexample

@node Threads and Signal Handling
@section Threads and Signal Handling

@comment pthread.h
@comment POSIX
@deftypefun int pthread_sigmask (int @var{how}, const sigset_t *@var{newmask}, sigset_t *@var{oldmask})
@code{pthread_sigmask} changes the signal mask for the calling thread as
described by the @var{how} and @var{newmask} arguments. If @var{oldmask}
is not @code{NULL}, the previous signal mask is stored in the location
pointed to by @var{oldmask}.

The meaning of the @var{how} and @var{newmask} arguments is the same as
for @code{sigprocmask}. If @var{how} is @code{SIG_SETMASK}, the signal
mask is set to @var{newmask}. If @var{how} is @code{SIG_BLOCK}, the
signals specified to @var{newmask} are added to the current signal mask.
If @var{how} is @code{SIG_UNBLOCK}, the signals specified to
@var{newmask} are removed from the current signal mask.

Recall that signal masks are set on a per-thread basis, but signal
actions and signal handlers, as set with @code{sigaction}, are shared
between all threads.

The @code{pthread_sigmask} function returns 0 on success, and one of the
following error codes on error:
@table @code
@item EINVAL
@var{how} is not one of @code{SIG_SETMASK}, @code{SIG_BLOCK}, or @code{SIG_UNBLOCK}

@item EFAULT
@var{newmask} or @var{oldmask} point to invalid addresses
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_kill (pthread_t @var{thread}, int @var{signo})
@code{pthread_kill} sends signal number @var{signo} to the thread
@var{thread}.  The signal is delivered and handled as described in
@ref{Signal Handling}.

@code{pthread_kill} returns 0 on success, one of the following error codes
on error:
@table @code
@item EINVAL
@var{signo} is not a valid signal number

@item ESRCH
The thread @var{thread} does not exist (e.g. it has already terminated)
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int sigwait (const sigset_t *@var{set}, int *@var{sig})
@code{sigwait} suspends the calling thread until one of the signals in
@var{set} is delivered to the calling thread. It then stores the number
of the signal received in the location pointed to by @var{sig} and
returns. The signals in @var{set} must be blocked and not ignored on
entrance to @code{sigwait}. If the delivered signal has a signal handler
function attached, that function is @emph{not} called.

@code{sigwait} is a cancellation point.  It always returns 0.
@end deftypefun

For @code{sigwait} to work reliably, the signals being waited for must be
blocked in all threads, not only in the calling thread, since
otherwise the POSIX semantics for signal delivery do not guarantee
that it's the thread doing the @code{sigwait} that will receive the signal.
The best way to achieve this is block those signals before any threads
are created, and never unblock them in the program other than by
calling @code{sigwait}.

Signal handling in LinuxThreads departs significantly from the POSIX
standard. According to the standard, ``asynchronous'' (external) signals
are addressed to the whole process (the collection of all threads),
which then delivers them to one particular thread. The thread that
actually receives the signal is any thread that does not currently block
the signal.

In LinuxThreads, each thread is actually a kernel process with its own
PID, so external signals are always directed to one particular thread.
If, for instance, another thread is blocked in @code{sigwait} on that
signal, it will not be restarted.

The LinuxThreads implementation of @code{sigwait} installs dummy signal
handlers for the signals in @var{set} for the duration of the
wait. Since signal handlers are shared between all threads, other
threads must not attach their own signal handlers to these signals, or
alternatively they should all block these signals (which is recommended
anyway).

@node Miscellaneous Thread Functions
@section Miscellaneous Thread Functions

@comment pthread.h
@comment POSIX
@deftypefun {pthread_t} pthread_self (@var{void})
@code{pthread_self} returns the thread identifier for the calling thread.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_equal (pthread_t thread1, pthread_t thread2)
@code{pthread_equal} determines if two thread identifiers refer to the same
thread.

A non-zero value is returned if @var{thread1} and @var{thread2} refer to
the same thread. Otherwise, 0 is returned.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_detach (pthread_t @var{th})
@code{pthread_detach} puts the thread @var{th} in the detached
state. This guarantees that the memory resources consumed by @var{th}
will be freed immediately when @var{th} terminates. However, this
prevents other threads from synchronizing on the termination of @var{th}
using @code{pthread_join}.

A thread can be created initially in the detached state, using the
@code{detachstate} attribute to @code{pthread_create}. In contrast,
@code{pthread_detach} applies to threads created in the joinable state,
and which need to be put in the detached state later.

After @code{pthread_detach} completes, subsequent attempts to perform
@code{pthread_join} on @var{th} will fail. If another thread is already
joining the thread @var{th} at the time @code{pthread_detach} is called,
@code{pthread_detach} does nothing and leaves @var{th} in the joinable
state.

On success, 0 is returned. On error, one of the following codes is
returned:
@table @code
@item ESRCH
No thread could be found corresponding to that specified by @var{th}
@item EINVAL
The thread @var{th} is already in the detached state
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_atfork (void (*@var{prepare})(void), void (*@var{parent})(void), void (*@var{child})(void))

@code{pthread_atfork} registers handler functions to be called just
before and just after a new process is created with @code{fork}. The
@var{prepare} handler will be called from the parent process, just
before the new process is created. The @var{parent} handler will be
called from the parent process, just before @code{fork} returns. The
@var{child} handler will be called from the child process, just before
@code{fork} returns.

@code{pthread_atfork} returns 0 on success and a non-zero error code on
error.

One or more of the three handlers @var{prepare}, @var{parent} and
@var{child} can be given as @code{NULL}, meaning that no handler needs
to be called at the corresponding point.

@code{pthread_atfork} can be called several times to install several
sets of handlers. At @code{fork} time, the @var{prepare} handlers are
called in LIFO order (last added with @code{pthread_atfork}, first
called before @code{fork}), while the @var{parent} and @var{child}
handlers are called in FIFO order (first added, first called).

If there is insufficient memory available to register the handlers,
@code{pthread_atfork} fails and returns @code{ENOMEM}.  Otherwise it
returns 0.
@end deftypefun

To understand the purpose of @code{pthread_atfork}, recall that
@code{fork} duplicates the whole memory space, including mutexes in
their current locking state, but only the calling thread: other threads
are not running in the child process. Thus, if a mutex is locked by a
thread other than the thread calling @code{fork}, that mutex will remain
locked forever in the child process, possibly blocking the execution of
the child process. To avoid this, install handlers with
@code{pthread_atfork} as follows: the @var{prepare} handler locks the
global mutexes (in locking order), and the @var{parent} and @var{child}
handlers unlock them (in reverse order). Alternatively, @var{prepare}
and @var{parent} can be set to @code{NULL} and @var{child} to a function
that calls @code{pthread_mutex_init} on the global mutexes.

@comment pthread.h
@comment GNU
@deftypefun void pthread_kill_other_threads_np (@var{void})
@code{pthread_kill_other_threads_np} is a non-portable LinuxThreads extension.
It causes all threads in the program to terminate immediately, except
the calling thread which proceeds normally. It is intended to be
called just before a thread calls one of the @code{exec} functions,
e.g. @code{execve}.

Termination of the other threads is not performed through
@code{pthread_cancel} and completely bypasses the cancellation
mechanism. Hence, the current settings for cancellation state and
cancellation type are ignored, and the cleanup handlers are not
executed in the terminated threads.

According to POSIX 1003.1c, a successful @code{exec*} in one of the
threads should automatically terminate all other threads in the program.
This behavior is not yet implemented in LinuxThreads.  Calling
@code{pthread_kill_other_threads_np} before @code{exec*} achieves much
of the same behavior, except that if @code{exec*} ultimately fails, then
all other threads are already killed.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_once (pthread_once_t *once_@var{control}, void (*@var{init_routine}) (void))

The purpose of @code{pthread_once} is to ensure that a piece of
initialization code is executed at most once. The @var{once_control}
argument points to a static or extern variable statically initialized
to @code{PTHREAD_ONCE_INIT}.

The first time @code{pthread_once} is called with a given
@var{once_control} argument, it calls @var{init_routine} with no
argument and changes the value of the @var{once_control} variable to
record that initialization has been performed. Subsequent calls to
@code{pthread_once} with the same @code{once_control} argument do
nothing.

@code{pthread_once} always returns 0.
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_setschedparam (pthread_t target_@var{thread}, int @var{policy}, const struct sched_param *@var{param})

@code{pthread_setschedparam} sets the scheduling parameters for the
thread @var{target_thread} as indicated by @var{policy} and
@var{param}. @var{policy} can be either @code{SCHED_OTHER} (regular,
non-realtime scheduling), @code{SCHED_RR} (realtime, round-robin) or
@code{SCHED_FIFO} (realtime, first-in first-out). @var{param} specifies
the scheduling priority for the two realtime policies.  See
@code{sched_setpolicy} for more information on scheduling policies.

The realtime scheduling policies @code{SCHED_RR} and @code{SCHED_FIFO}
are available only to processes with superuser privileges.

On success, @code{pthread_setschedparam} returns 0.  On error it returns
one of the following codes:
@table @code
@item EINVAL
@var{policy} is not one of @code{SCHED_OTHER}, @code{SCHED_RR},
@code{SCHED_FIFO}, or the priority value specified by @var{param} is not
valid for the specified policy

@item EPERM
Realtime scheduling was requested but the calling process does not have
sufficient privileges.

@item ESRCH
The @var{target_thread} is invalid or has already terminated

@item EFAULT
@var{param} points outside the process memory space
@end table
@end deftypefun

@comment pthread.h
@comment POSIX
@deftypefun int pthread_getschedparam (pthread_t target_@var{thread}, int *@var{policy}, struct sched_param *@var{param})

@code{pthread_getschedparam} retrieves the scheduling policy and
scheduling parameters for the thread @var{target_thread} and stores them
in the locations pointed to by @var{policy} and @var{param},
respectively.

@code{pthread_getschedparam} returns 0 on success, or one of the
following error codes on failure:
@table @code
@item ESRCH
The @var{target_thread} is invalid or has already terminated.

@item EFAULT
@var{policy} or @var{param} point outside the process memory space.

@end table
@end deftypefun