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
|
==================================
Block Implementation Specification
==================================
.. contents::
:local:
History
=======
* 2008/7/14 - created.
* 2008/8/21 - revised, C++.
* 2008/9/24 - add ``NULL`` ``isa`` field to ``__block`` storage.
* 2008/10/1 - revise block layout to use a ``static`` descriptor structure.
* 2008/10/6 - revise block layout to use an unsigned long int flags.
* 2008/10/28 - specify use of ``_Block_object_assign`` and
``_Block_object_dispose`` for all "Object" types in helper functions.
* 2008/10/30 - revise new layout to have invoke function in same place.
* 2008/10/30 - add ``__weak`` support.
* 2010/3/16 - rev for stret return, signature field.
* 2010/4/6 - improved wording.
* 2013/1/6 - improved wording and converted to rst.
This document describes the Apple ABI implementation specification of Blocks.
The first shipping version of this ABI is found in Mac OS X 10.6, and shall be
referred to as 10.6.ABI. As of 2010/3/16, the following describes the ABI
contract with the runtime and the compiler, and, as necessary, will be referred
to as ABI.2010.3.16.
Since the Apple ABI references symbols from other elements of the system, any
attempt to use this ABI on systems prior to SnowLeopard is undefined.
High Level
==========
The ABI of ``Blocks`` consist of their layout and the runtime functions required
by the compiler. A ``Block`` of type ``R (^)(P...)`` consists of a structure of
the following form:
.. code-block:: c
struct Block_literal_1 {
void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
int flags;
int reserved;
R (*invoke)(struct Block_literal_1 *, P...);
struct Block_descriptor_1 {
unsigned long int reserved; // NULL
unsigned long int size; // sizeof(struct Block_literal_1)
// optional helper functions
void (*copy_helper)(void *dst, void *src); // IFF (1<<25)
void (*dispose_helper)(void *src); // IFF (1<<25)
// required ABI.2010.3.16
const char *signature; // IFF (1<<30)
} *descriptor;
// imported variables
};
The following flags bits are in use thusly for a possible ABI.2010.3.16:
.. code-block:: c
enum {
// Set to true on blocks that have captures (and thus are not true
// global blocks) but are known not to escape for various other
// reasons. For backward compatibility with old runtimes, whenever
// BLOCK_IS_NOESCAPE is set, BLOCK_IS_GLOBAL is set too. Copying a
// non-escaping block returns the original block and releasing such a
// block is a no-op, which is exactly how global blocks are handled.
BLOCK_IS_NOESCAPE = (1 << 23),
BLOCK_HAS_COPY_DISPOSE = (1 << 25),
BLOCK_HAS_CTOR = (1 << 26), // helpers have C++ code
BLOCK_IS_GLOBAL = (1 << 28),
BLOCK_HAS_STRET = (1 << 29), // IFF BLOCK_HAS_SIGNATURE
BLOCK_HAS_SIGNATURE = (1 << 30),
};
In 10.6.ABI the (1<<29) was usually set and was always ignored by the runtime -
it had been a transitional marker that did not get deleted after the
transition. This bit is now paired with (1<<30), and represented as the pair
(3<<29), for the following combinations of valid bit settings, and their
meanings:
.. code-block:: c
switch (flags & (3<<29)) {
case (0<<29): 10.6.ABI, no signature field available
case (1<<29): 10.6.ABI, no signature field available
case (2<<29): ABI.2010.3.16, regular calling convention, presence of signature field
case (3<<29): ABI.2010.3.16, stret calling convention, presence of signature field,
}
The signature field is not always populated.
The following discussions are presented as 10.6.ABI otherwise.
``Block`` literals may occur within functions where the structure is created in
stack local memory. They may also appear as initialization expressions for
``Block`` variables of global or ``static`` local variables.
When a ``Block`` literal expression is evaluated the stack based structure is
initialized as follows:
1. A ``static`` descriptor structure is declared and initialized as follows:
a. The ``invoke`` function pointer is set to a function that takes the
``Block`` structure as its first argument and the rest of the arguments (if
any) to the ``Block`` and executes the ``Block`` compound statement.
b. The ``size`` field is set to the size of the following ``Block`` literal
structure.
c. The ``copy_helper`` and ``dispose_helper`` function pointers are set to
respective helper functions if they are required by the ``Block`` literal.
2. A stack (or global) ``Block`` literal data structure is created and
initialized as follows:
a. The ``isa`` field is set to the address of the external
``_NSConcreteStackBlock``, which is a block of uninitialized memory supplied
in ``libSystem``, or ``_NSConcreteGlobalBlock`` if this is a static or file
level ``Block`` literal.
b. The ``flags`` field is set to zero unless there are variables imported
into the ``Block`` that need helper functions for program level
``Block_copy()`` and ``Block_release()`` operations, in which case the
(1<<25) flags bit is set.
As an example, the ``Block`` literal expression:
.. code-block:: c
^ { printf("hello world\n"); }
would cause the following to be created on a 32-bit system:
.. code-block:: c
struct __block_literal_1 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_1 *);
struct __block_descriptor_1 *descriptor;
};
void __block_invoke_1(struct __block_literal_1 *_block) {
printf("hello world\n");
}
static struct __block_descriptor_1 {
unsigned long int reserved;
unsigned long int Block_size;
} __block_descriptor_1 = { 0, sizeof(struct __block_literal_1) };
and where the ``Block`` literal itself appears:
.. code-block:: c
struct __block_literal_1 _block_literal = {
&_NSConcreteStackBlock,
(1<<29), <uninitialized>,
__block_invoke_1,
&__block_descriptor_1
};
A ``Block`` imports other ``Block`` references, ``const`` copies of other
variables, and variables marked ``__block``. In Objective-C, variables may
additionally be objects.
When a ``Block`` literal expression is used as the initial value of a global
or ``static`` local variable, it is initialized as follows:
.. code-block:: c
struct __block_literal_1 __block_literal_1 = {
&_NSConcreteGlobalBlock,
(1<<28)|(1<<29), <uninitialized>,
__block_invoke_1,
&__block_descriptor_1
};
that is, a different address is provided as the first value and a particular
(1<<28) bit is set in the ``flags`` field, and otherwise it is the same as for
stack based ``Block`` literals. This is an optimization that can be used for
any ``Block`` literal that imports no ``const`` or ``__block`` storage
variables.
Imported Variables
==================
Variables of ``auto`` storage class are imported as ``const`` copies. Variables
of ``__block`` storage class are imported as a pointer to an enclosing data
structure. Global variables are simply referenced and not considered as
imported.
Imported ``const`` copy variables
---------------------------------
Automatic storage variables not marked with ``__block`` are imported as
``const`` copies.
The simplest example is that of importing a variable of type ``int``:
.. code-block:: c
int x = 10;
void (^vv)(void) = ^{ printf("x is %d\n", x); }
x = 11;
vv();
which would be compiled to:
.. code-block:: c
struct __block_literal_2 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_2 *);
struct __block_descriptor_2 *descriptor;
const int x;
};
void __block_invoke_2(struct __block_literal_2 *_block) {
printf("x is %d\n", _block->x);
}
static struct __block_descriptor_2 {
unsigned long int reserved;
unsigned long int Block_size;
} __block_descriptor_2 = { 0, sizeof(struct __block_literal_2) };
and:
.. code-block:: c
struct __block_literal_2 __block_literal_2 = {
&_NSConcreteStackBlock,
(1<<29), <uninitialized>,
__block_invoke_2,
&__block_descriptor_2,
x
};
In summary, scalars, structures, unions, and function pointers are generally
imported as ``const`` copies with no need for helper functions.
Imported ``const`` copy of ``Block`` reference
----------------------------------------------
The first case where copy and dispose helper functions are required is for the
case of when a ``Block`` itself is imported. In this case both a
``copy_helper`` function and a ``dispose_helper`` function are needed. The
``copy_helper`` function is passed both the existing stack based pointer and the
pointer to the new heap version and should call back into the runtime to
actually do the copy operation on the imported fields within the ``Block``. The
runtime functions are all described in :ref:`RuntimeHelperFunctions`.
A quick example:
.. code-block:: c
void (^existingBlock)(void) = ...;
void (^vv)(void) = ^{ existingBlock(); }
vv();
struct __block_literal_3 {
...; // existing block
};
struct __block_literal_4 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_4 *);
struct __block_literal_3 *const existingBlock;
};
void __block_invoke_4(struct __block_literal_2 *_block) {
__block->existingBlock->invoke(__block->existingBlock);
}
void __block_copy_4(struct __block_literal_4 *dst, struct __block_literal_4 *src) {
//_Block_copy_assign(&dst->existingBlock, src->existingBlock, 0);
_Block_object_assign(&dst->existingBlock, src->existingBlock, BLOCK_FIELD_IS_BLOCK);
}
void __block_dispose_4(struct __block_literal_4 *src) {
// was _Block_destroy
_Block_object_dispose(src->existingBlock, BLOCK_FIELD_IS_BLOCK);
}
static struct __block_descriptor_4 {
unsigned long int reserved;
unsigned long int Block_size;
void (*copy_helper)(struct __block_literal_4 *dst, struct __block_literal_4 *src);
void (*dispose_helper)(struct __block_literal_4 *);
} __block_descriptor_4 = {
0,
sizeof(struct __block_literal_4),
__block_copy_4,
__block_dispose_4,
};
and where said ``Block`` is used:
.. code-block:: c
struct __block_literal_4 _block_literal = {
&_NSConcreteStackBlock,
(1<<25)|(1<<29), <uninitialized>
__block_invoke_4,
& __block_descriptor_4
existingBlock,
};
Importing ``__attribute__((NSObject))`` variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
GCC introduces ``__attribute__((NSObject))`` on structure pointers to mean "this
is an object". This is useful because many low level data structures are
declared as opaque structure pointers, e.g. ``CFStringRef``, ``CFArrayRef``,
etc. When used from C, however, these are still really objects and are the
second case where that requires copy and dispose helper functions to be
generated. The copy helper functions generated by the compiler should use the
``_Block_object_assign`` runtime helper function and in the dispose helper the
``_Block_object_dispose`` runtime helper function should be called.
For example, ``Block`` foo in the following:
.. code-block:: c
struct Opaque *__attribute__((NSObject)) objectPointer = ...;
...
void (^foo)(void) = ^{ CFPrint(objectPointer); };
would have the following helper functions generated:
.. code-block:: c
void __block_copy_foo(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
_Block_object_assign(&dst->objectPointer, src-> objectPointer, BLOCK_FIELD_IS_OBJECT);
}
void __block_dispose_foo(struct __block_literal_5 *src) {
_Block_object_dispose(src->objectPointer, BLOCK_FIELD_IS_OBJECT);
}
Imported ``__block`` marked variables
-------------------------------------
Layout of ``__block`` marked variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The compiler must embed variables that are marked ``__block`` in a specialized
structure of the form:
.. code-block:: c
struct _block_byref_foo {
void *isa;
struct Block_byref *forwarding;
int flags; //refcount;
int size;
typeof(marked_variable) marked_variable;
};
Variables of certain types require helper functions for when ``Block_copy()``
and ``Block_release()`` are performed upon a referencing ``Block``. At the "C"
level only variables that are of type ``Block`` or ones that have
``__attribute__((NSObject))`` marked require helper functions. In Objective-C
objects require helper functions and in C++ stack based objects require helper
functions. Variables that require helper functions use the form:
.. code-block:: c
struct _block_byref_foo {
void *isa;
struct _block_byref_foo *forwarding;
int flags; //refcount;
int size;
// helper functions called via Block_copy() and Block_release()
void (*byref_keep)(void *dst, void *src);
void (*byref_dispose)(void *);
typeof(marked_variable) marked_variable;
};
The structure is initialized such that:
a. The ``forwarding`` pointer is set to the beginning of its enclosing
structure.
b. The ``size`` field is initialized to the total size of the enclosing
structure.
c. The ``flags`` field is set to either 0 if no helper functions are needed
or (1<<25) if they are.
d. The helper functions are initialized (if present).
e. The variable itself is set to its initial value.
f. The ``isa`` field is set to ``NULL``.
Access to ``__block`` variables from within its lexical scope
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
In order to "move" the variable to the heap upon a ``copy_helper`` operation the
compiler must rewrite access to such a variable to be indirect through the
structures ``forwarding`` pointer. For example:
.. code-block:: c
int __block i = 10;
i = 11;
would be rewritten to be:
.. code-block:: c
struct _block_byref_i {
void *isa;
struct _block_byref_i *forwarding;
int flags; //refcount;
int size;
int captured_i;
} i = { NULL, &i, 0, sizeof(struct _block_byref_i), 10 };
i.forwarding->captured_i = 11;
In the case of a ``Block`` reference variable being marked ``__block`` the
helper code generated must use the ``_Block_object_assign`` and
``_Block_object_dispose`` routines supplied by the runtime to make the
copies. For example:
.. code-block:: c
__block void (voidBlock)(void) = blockA;
voidBlock = blockB;
would translate into:
.. code-block:: c
struct _block_byref_voidBlock {
void *isa;
struct _block_byref_voidBlock *forwarding;
int flags; //refcount;
int size;
void (*byref_keep)(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src);
void (*byref_dispose)(struct _block_byref_voidBlock *);
void (^captured_voidBlock)(void);
};
void _block_byref_keep_helper(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
//_Block_copy_assign(&dst->captured_voidBlock, src->captured_voidBlock, 0);
_Block_object_assign(&dst->captured_voidBlock, src->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
}
void _block_byref_dispose_helper(struct _block_byref_voidBlock *param) {
//_Block_destroy(param->captured_voidBlock, 0);
_Block_object_dispose(param->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER)}
and:
.. code-block:: c
struct _block_byref_voidBlock voidBlock = {( .forwarding=&voidBlock, .flags=(1<<25), .size=sizeof(struct _block_byref_voidBlock *),
.byref_keep=_block_byref_keep_helper, .byref_dispose=_block_byref_dispose_helper,
.captured_voidBlock=blockA )};
voidBlock.forwarding->captured_voidBlock = blockB;
Importing ``__block`` variables into ``Blocks``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A ``Block`` that uses a ``__block`` variable in its compound statement body must
import the variable and emit ``copy_helper`` and ``dispose_helper`` helper
functions that, in turn, call back into the runtime to actually copy or release
the ``byref`` data block using the functions ``_Block_object_assign`` and
``_Block_object_dispose``.
For example:
.. code-block:: c
int __block i = 2;
functioncall(^{ i = 10; });
would translate to:
.. code-block:: c
struct _block_byref_i {
void *isa; // set to NULL
struct _block_byref_voidBlock *forwarding;
int flags; //refcount;
int size;
void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
void (*byref_dispose)(struct _block_byref_i *);
int captured_i;
};
struct __block_literal_5 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_5 *);
struct __block_descriptor_5 *descriptor;
struct _block_byref_i *i_holder;
};
void __block_invoke_5(struct __block_literal_5 *_block) {
_block->forwarding->captured_i = 10;
}
void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
//_Block_byref_assign_copy(&dst->captured_i, src->captured_i);
_Block_object_assign(&dst->captured_i, src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
}
void __block_dispose_5(struct __block_literal_5 *src) {
//_Block_byref_release(src->captured_i);
_Block_object_dispose(src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
}
static struct __block_descriptor_5 {
unsigned long int reserved;
unsigned long int Block_size;
void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
void (*dispose_helper)(struct __block_literal_5 *);
} __block_descriptor_5 = { 0, sizeof(struct __block_literal_5) __block_copy_5, __block_dispose_5 };
and:
.. code-block:: c
struct _block_byref_i i = {( .isa=NULL, .forwarding=&i, .flags=0, .size=sizeof(struct _block_byref_i), .captured_i=2 )};
struct __block_literal_5 _block_literal = {
&_NSConcreteStackBlock,
(1<<25)|(1<<29), <uninitialized>,
__block_invoke_5,
&__block_descriptor_5,
&i,
};
Importing ``__attribute__((NSObject))`` ``__block`` variables
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
A ``__block`` variable that is also marked ``__attribute__((NSObject))`` should
have ``byref_keep`` and ``byref_dispose`` helper functions that use
``_Block_object_assign`` and ``_Block_object_dispose``.
``__block`` escapes
^^^^^^^^^^^^^^^^^^^
Because ``Blocks`` referencing ``__block`` variables may have ``Block_copy()``
performed upon them the underlying storage for the variables may move to the
heap. In Objective-C Garbage Collection Only compilation environments the heap
used is the garbage collected one and no further action is required. Otherwise
the compiler must issue a call to potentially release any heap storage for
``__block`` variables at all escapes or terminations of their scope. The call
should be:
.. code-block:: c
_Block_object_dispose(&_block_byref_foo, BLOCK_FIELD_IS_BYREF);
Nesting
^^^^^^^
``Blocks`` may contain ``Block`` literal expressions. Any variables used within
inner blocks are imported into all enclosing ``Block`` scopes even if the
variables are not used. This includes ``const`` imports as well as ``__block``
variables.
Objective C Extensions to ``Blocks``
====================================
Importing Objects
-----------------
Objects should be treated as ``__attribute__((NSObject))`` variables; all
``copy_helper``, ``dispose_helper``, ``byref_keep``, and ``byref_dispose``
helper functions should use ``_Block_object_assign`` and
``_Block_object_dispose``. There should be no code generated that uses
``*-retain`` or ``*-release`` methods.
``Blocks`` as Objects
---------------------
The compiler will treat ``Blocks`` as objects when synthesizing property setters
and getters, will characterize them as objects when generating garbage
collection strong and weak layout information in the same manner as objects, and
will issue strong and weak write-barrier assignments in the same manner as
objects.
``__weak __block`` Support
--------------------------
Objective-C (and Objective-C++) support the ``__weak`` attribute on ``__block``
variables. Under normal circumstances the compiler uses the Objective-C runtime
helper support functions ``objc_assign_weak`` and ``objc_read_weak``. Both
should continue to be used for all reads and writes of ``__weak __block``
variables:
.. code-block:: c
objc_read_weak(&block->byref_i->forwarding->i)
The ``__weak`` variable is stored in a ``_block_byref_foo`` structure and the
``Block`` has copy and dispose helpers for this structure that call:
.. code-block:: c
_Block_object_assign(&dest->_block_byref_i, src-> _block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
and:
.. code-block:: c
_Block_object_dispose(src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
In turn, the ``block_byref`` copy support helpers distinguish between whether
the ``__block`` variable is a ``Block`` or not and should either call:
.. code-block:: c
_Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_OBJECT | BLOCK_BYREF_CALLER);
for something declared as an object or:
.. code-block:: c
_Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
for something declared as a ``Block``.
A full example follows:
.. code-block:: c
__block __weak id obj = <initialization expression>;
functioncall(^{ [obj somemessage]; });
would translate to:
.. code-block:: c
struct _block_byref_obj {
void *isa; // uninitialized
struct _block_byref_obj *forwarding;
int flags; //refcount;
int size;
void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
void (*byref_dispose)(struct _block_byref_i *);
id captured_obj;
};
void _block_byref_obj_keep(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
//_Block_copy_assign(&dst->captured_obj, src->captured_obj, 0);
_Block_object_assign(&dst->captured_obj, src->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
}
void _block_byref_obj_dispose(struct _block_byref_voidBlock *param) {
//_Block_destroy(param->captured_obj, 0);
_Block_object_dispose(param->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
};
for the block ``byref`` part and:
.. code-block:: c
struct __block_literal_5 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_5 *);
struct __block_descriptor_5 *descriptor;
struct _block_byref_obj *byref_obj;
};
void __block_invoke_5(struct __block_literal_5 *_block) {
[objc_read_weak(&_block->byref_obj->forwarding->captured_obj) somemessage];
}
void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
//_Block_byref_assign_copy(&dst->byref_obj, src->byref_obj);
_Block_object_assign(&dst->byref_obj, src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
}
void __block_dispose_5(struct __block_literal_5 *src) {
//_Block_byref_release(src->byref_obj);
_Block_object_dispose(src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
}
static struct __block_descriptor_5 {
unsigned long int reserved;
unsigned long int Block_size;
void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
void (*dispose_helper)(struct __block_literal_5 *);
} __block_descriptor_5 = { 0, sizeof(struct __block_literal_5), __block_copy_5, __block_dispose_5 };
and within the compound statement:
.. code-block:: c
truct _block_byref_obj obj = {( .forwarding=&obj, .flags=(1<<25), .size=sizeof(struct _block_byref_obj),
.byref_keep=_block_byref_obj_keep, .byref_dispose=_block_byref_obj_dispose,
.captured_obj = <initialization expression> )};
truct __block_literal_5 _block_literal = {
&_NSConcreteStackBlock,
(1<<25)|(1<<29), <uninitialized>,
__block_invoke_5,
&__block_descriptor_5,
&obj, // a reference to the on-stack structure containing "captured_obj"
};
functioncall(_block_literal->invoke(&_block_literal));
C++ Support
===========
Within a block stack based C++ objects are copied into ``const`` copies using
the copy constructor. It is an error if a stack based C++ object is used within
a block if it does not have a copy constructor. In addition both copy and
destroy helper routines must be synthesized for the block to support the
``Block_copy()`` operation, and the flags work marked with the (1<<26) bit in
addition to the (1<<25) bit. The copy helper should call the constructor using
appropriate offsets of the variable within the supplied stack based block source
and heap based destination for all ``const`` constructed copies, and similarly
should call the destructor in the destroy routine.
As an example, suppose a C++ class ``FOO`` existed with a copy constructor.
Within a code block a stack version of a ``FOO`` object is declared and used
within a ``Block`` literal expression:
.. code-block:: c++
{
FOO foo;
void (^block)(void) = ^{ printf("%d\n", foo.value()); };
}
The compiler would synthesize:
.. code-block:: c++
struct __block_literal_10 {
void *isa;
int flags;
int reserved;
void (*invoke)(struct __block_literal_10 *);
struct __block_descriptor_10 *descriptor;
const FOO foo;
};
void __block_invoke_10(struct __block_literal_10 *_block) {
printf("%d\n", _block->foo.value());
}
void __block_copy_10(struct __block_literal_10 *dst, struct __block_literal_10 *src) {
FOO_ctor(&dst->foo, &src->foo);
}
void __block_dispose_10(struct __block_literal_10 *src) {
FOO_dtor(&src->foo);
}
static struct __block_descriptor_10 {
unsigned long int reserved;
unsigned long int Block_size;
void (*copy_helper)(struct __block_literal_10 *dst, struct __block_literal_10 *src);
void (*dispose_helper)(struct __block_literal_10 *);
} __block_descriptor_10 = { 0, sizeof(struct __block_literal_10), __block_copy_10, __block_dispose_10 };
and the code would be:
.. code-block:: c++
{
FOO foo;
comp_ctor(&foo); // default constructor
struct __block_literal_10 _block_literal = {
&_NSConcreteStackBlock,
(1<<25)|(1<<26)|(1<<29), <uninitialized>,
__block_invoke_10,
&__block_descriptor_10,
};
comp_ctor(&_block_literal->foo, &foo); // const copy into stack version
struct __block_literal_10 &block = &_block_literal; // assign literal to block variable
block->invoke(block); // invoke block
comp_dtor(&_block_literal->foo); // destroy stack version of const block copy
comp_dtor(&foo); // destroy original version
}
C++ objects stored in ``__block`` storage start out on the stack in a
``block_byref`` data structure as do other variables. Such objects (if not
``const`` objects) must support a regular copy constructor. The ``block_byref``
data structure will have copy and destroy helper routines synthesized by the
compiler. The copy helper will have code created to perform the copy
constructor based on the initial stack ``block_byref`` data structure, and will
also set the (1<<26) bit in addition to the (1<<25) bit. The destroy helper
will have code to do the destructor on the object stored within the supplied
``block_byref`` heap data structure. For example,
.. code-block:: c++
__block FOO blockStorageFoo;
requires the normal constructor for the embedded ``blockStorageFoo`` object:
.. code-block:: c++
FOO_ctor(& _block_byref_blockStorageFoo->blockStorageFoo);
and at scope termination the destructor:
.. code-block:: c++
FOO_dtor(& _block_byref_blockStorageFoo->blockStorageFoo);
Note that the forwarding indirection is *NOT* used.
The compiler would need to generate (if used from a block literal) the following
copy/dispose helpers:
.. code-block:: c++
void _block_byref_obj_keep(struct _block_byref_blockStorageFoo *dst, struct _block_byref_blockStorageFoo *src) {
FOO_ctor(&dst->blockStorageFoo, &src->blockStorageFoo);
}
void _block_byref_obj_dispose(struct _block_byref_blockStorageFoo *src) {
FOO_dtor(&src->blockStorageFoo);
}
for the appropriately named constructor and destructor for the class/struct
``FOO``.
To support member variable and function access the compiler will synthesize a
``const`` pointer to a block version of the ``this`` pointer.
.. _RuntimeHelperFunctions:
Runtime Helper Functions
========================
The runtime helper functions are described in
``/usr/local/include/Block_private.h``. To summarize their use, a ``Block``
requires copy/dispose helpers if it imports any block variables, ``__block``
storage variables, ``__attribute__((NSObject))`` variables, or C++ ``const``
copied objects with constructor/destructors. The (1<<26) bit is set and
functions are generated.
The block copy helper function should, for each of the variables of the type
mentioned above, call:
.. code-block:: c
_Block_object_assign(&dst->target, src->target, BLOCK_FIELD_<apropos>);
in the copy helper and:
.. code-block:: c
_Block_object_dispose(->target, BLOCK_FIELD_<apropos>);
in the dispose helper where ``<apropos>`` is:
.. code-block:: c
enum {
BLOCK_FIELD_IS_OBJECT = 3, // id, NSObject, __attribute__((NSObject)), block, ...
BLOCK_FIELD_IS_BLOCK = 7, // a block variable
BLOCK_FIELD_IS_BYREF = 8, // the on stack structure holding the __block variable
BLOCK_FIELD_IS_WEAK = 16, // declared __weak
BLOCK_BYREF_CALLER = 128, // called from byref copy/dispose helpers
};
and of course the constructors/destructors for ``const`` copied C++ objects.
The ``block_byref`` data structure similarly requires copy/dispose helpers for
block variables, ``__attribute__((NSObject))`` variables, or C++ ``const``
copied objects with constructor/destructors, and again the (1<<26) bit is set
and functions are generated in the same manner.
Under ObjC we allow ``__weak`` as an attribute on ``__block`` variables, and
this causes the addition of ``BLOCK_FIELD_IS_WEAK`` orred onto the
``BLOCK_FIELD_IS_BYREF`` flag when copying the ``block_byref`` structure in the
``Block`` copy helper, and onto the ``BLOCK_FIELD_<apropos>`` field within the
``block_byref`` copy/dispose helper calls.
The prototypes, and summary, of the helper functions are:
.. code-block:: c
/* Certain field types require runtime assistance when being copied to the
heap. The following function is used to copy fields of types: blocks,
pointers to byref structures, and objects (including
__attribute__((NSObject)) pointers. BLOCK_FIELD_IS_WEAK is orthogonal to
the other choices which are mutually exclusive. Only in a Block copy
helper will one see BLOCK_FIELD_IS_BYREF.
*/
void _Block_object_assign(void *destAddr, const void *object, const int flags);
/* Similarly a compiler generated dispose helper needs to call back for each
field of the byref data structure. (Currently the implementation only
packs one field into the byref structure but in principle there could be
more). The same flags used in the copy helper should be used for each
call generated to this function:
*/
void _Block_object_dispose(const void *object, const int flags);
Copyright
=========
Copyright 2008-2010 Apple, Inc.
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in
all copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.
|