1 Block Implementation Specification
3 Copyright 2008-2010 Apple, Inc.
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25 2008/8/21 - revised, C++
26 2008/9/24 - add NULL isa field to __block storage
27 2008/10/1 - revise block layout to use a static descriptor structure
28 2008/10/6 - revise block layout to use an unsigned long int flags
29 2008/10/28 - specify use of _Block_object_assign/dispose for all "Object" types in helper functions
30 2008/10/30 - revise new layout to have invoke function in same place
31 2008/10/30 - add __weak support
33 2010/3/16 - rev for stret return, signature field
34 2010/4/6 - improved wording
36 This document describes the Apple ABI implementation specification of Blocks.
38 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.
40 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.
44 The ABI of blocks consist of their layout and the runtime functions required by the compiler.
45 A Block consists of a structure of the following form:
47 struct Block_literal_1 {
48 void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
51 void (*invoke)(void *, ...);
52 struct Block_descriptor_1 {
53 unsigned long int reserved; // NULL
54 unsigned long int size; // sizeof(struct Block_literal_1)
55 // optional helper functions
56 void (*copy_helper)(void *dst, void *src); // IFF (1<<25)
57 void (*dispose_helper)(void *src); // IFF (1<<25)
58 // required ABI.2010.3.16
59 const char *signature; // IFF (1<<30)
64 The following flags bits are in use thusly for a possible ABI.2010.3.16:
67 BLOCK_HAS_COPY_DISPOSE = (1 << 25),
68 BLOCK_HAS_CTOR = (1 << 26), // helpers have C++ code
69 BLOCK_IS_GLOBAL = (1 << 28),
70 BLOCK_HAS_STRET = (1 << 29), // IFF BLOCK_HAS_SIGNATURE
71 BLOCK_HAS_SIGNATURE = (1 << 30),
74 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<<30), for the following combinations of valid bit settings, and their meanings.
76 switch (flags & (3<<29)) {
77 case (0<<29): 10.6.ABI, no signature field available
78 case (1<<29): 10.6.ABI, no signature field available
79 case (2<<29): ABI.2010.3.16, regular calling convention, presence of signature field
80 case (3<<29): ABI.2010.3.16, stret calling convention, presence of signature field,
83 The signature field is not always populated.
85 The following discussions are presented as 10.6.ABI otherwise.
87 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.
89 When a Block literal expression is evaluated the stack based structure is initialized as follows:
91 1) static descriptor structure is declared and initialized as follows:
92 1a) 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.
93 1b) the size field is set to the size of the following Block literal structure.
94 1c) the copy_helper and dispose_helper function pointers are set to respective helper functions if they are required by the Block literal
95 2) a stack (or global) Block literal data structure is created and initialized as follows:
96 2a) 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.
97 2) 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.
100 As an example, the Block literal expression
101 ^ { printf("hello world\n"); }
102 would cause to be created on a 32-bit system:
104 struct __block_literal_1 {
108 void (*invoke)(struct __block_literal_1 *);
109 struct __block_descriptor_1 *descriptor;
112 void __block_invoke_1(struct __block_literal_1 *_block) {
113 printf("hello world\n");
116 static struct __block_descriptor_1 {
117 unsigned long int reserved;
118 unsigned long int Block_size;
119 } __block_descriptor_1 = { 0, sizeof(struct __block_literal_1), __block_invoke_1 };
121 and where the block literal appeared
123 struct __block_literal_1 _block_literal = {
124 &_NSConcreteStackBlock,
125 (1<<29), <uninitialized>,
127 &__block_descriptor_1
130 Blocks import other Block references, const copies of other variables, and variables marked __block. In Objective-C variables may additionally be objects.
132 When a Block literal expression used as the initial value of a global or static local variable it is initialized as follows:
133 struct __block_literal_1 __block_literal_1 = {
134 &_NSConcreteGlobalBlock,
135 (1<<28)|(1<<29), <uninitialized>,
137 &__block_descriptor_1
139 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.
142 2. Imported Variables
144 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.
146 2.1 Imported const copy variables
148 Automatic storage variables not marked with __block are imported as const copies.
150 The simplest example is that of importing a variable of type int.
153 void (^vv)(void) = ^{ printf("x is %d\n", x); }
159 struct __block_literal_2 {
163 void (*invoke)(struct __block_literal_2 *);
164 struct __block_descriptor_2 *descriptor;
168 void __block_invoke_2(struct __block_literal_2 *_block) {
169 printf("x is %d\n", _block->x);
172 static struct __block_descriptor_2 {
173 unsigned long int reserved;
174 unsigned long int Block_size;
175 } __block_descriptor_2 = { 0, sizeof(struct __block_literal_2) };
179 struct __block_literal_2 __block_literal_2 = {
180 &_NSConcreteStackBlock,
181 (1<<29), <uninitialized>,
183 &__block_descriptor_2,
187 In summary, scalars, structures, unions, and function pointers are generally imported as const copies with no need for helper functions.
189 2.2 Imported const copy of Block reference
191 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 Section 5.0 Runtime Helper Functions.
195 void (^existingBlock)(void) = ...;
196 void (^vv)(void) = ^{ existingBlock(); }
199 struct __block_literal_3 {
200 ...; // existing block
203 struct __block_literal_4 {
207 void (*invoke)(struct __block_literal_4 *);
208 struct __block_literal_3 *const existingBlock;
211 void __block_invoke_4(struct __block_literal_2 *_block) {
212 __block->existingBlock->invoke(__block->existingBlock);
215 void __block_copy_4(struct __block_literal_4 *dst, struct __block_literal_4 *src) {
216 //_Block_copy_assign(&dst->existingBlock, src->existingBlock, 0);
217 _Block_object_assign(&dst->existingBlock, src->existingBlock, BLOCK_FIELD_IS_BLOCK);
220 void __block_dispose_4(struct __block_literal_4 *src) {
221 // was _Block_destroy
222 _Block_object_dispose(src->existingBlock, BLOCK_FIELD_IS_BLOCK);
225 static struct __block_descriptor_4 {
226 unsigned long int reserved;
227 unsigned long int Block_size;
228 void (*copy_helper)(struct __block_literal_4 *dst, struct __block_literal_4 *src);
229 void (*dispose_helper)(struct __block_literal_4 *);
230 } __block_descriptor_4 = {
232 sizeof(struct __block_literal_4),
239 struct __block_literal_4 _block_literal = {
240 &_NSConcreteStackBlock,
241 (1<<25)|(1<<29), <uninitialized>
243 & __block_descriptor_4
247 2.2.1 Importing __attribute__((NSObject)) variables.
249 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.
251 For example, block xyzzy in the following
253 struct Opaque *__attribute__((NSObject)) objectPointer = ...;
255 void (^xyzzy)(void) = ^{ CFPrint(objectPointer); };
257 would have helper functions
259 void __block_copy_xyzzy(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
260 _Block_object_assign(&dst->objectPointer, src-> objectPointer, BLOCK_FIELD_IS_OBJECT);
263 void __block_dispose_xyzzy(struct __block_literal_5 *src) {
264 _Block_object_dispose(src->objectPointer, BLOCK_FIELD_IS_OBJECT);
270 2.3 Imported __block marked variables.
272 2.3.1 Layout of __block marked variables
274 The compiler must embed variables that are marked __block in a specialized structure of the form:
276 struct _block_byref_xxxx {
278 struct Block_byref *forwarding;
279 int flags; //refcount;
281 typeof(marked_variable) marked_variable;
284 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:
286 struct _block_byref_xxxx {
288 struct _block_byref_xxxx *forwarding;
289 int flags; //refcount;
291 // helper functions called via Block_copy() and Block_release()
292 void (*byref_keep)(void *dst, void *src);
293 void (*byref_dispose)(void *);
294 typeof(marked_variable) marked_variable;
297 The structure is initialized such that
298 a) the forwarding pointer is set to the beginning of its enclosing structure,
299 b) the size field is initialized to the total size of the enclosing structure,
300 c) the flags field is set to either 0 if no helper functions are needed or (1<<25) if they are,
301 d) the helper functions are initialized (if present)
302 e) the variable itself is set to its initial value.
303 f) the isa field is set to NULL
305 2.3.2 Access to __block variables from within its lexical scope.
307 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:
312 would be rewritten to be:
314 struct _block_byref_i {
316 struct _block_byref_i *forwarding;
317 int flags; //refcount;
320 } i = { NULL, &i, 0, sizeof(struct _block_byref_i), 11 };
322 i.forwarding->captured_i = 11;
324 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:
326 __block void (voidBlock)(void) = blockA;
331 struct _block_byref_voidBlock {
333 struct _block_byref_voidBlock *forwarding;
334 int flags; //refcount;
336 void (*byref_keep)(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src);
337 void (*byref_dispose)(struct _block_byref_voidBlock *);
338 void (^captured_voidBlock)(void);
341 void _block_byref_keep_helper(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
342 //_Block_copy_assign(&dst->captured_voidBlock, src->captured_voidBlock, 0);
343 _Block_object_assign(&dst->captured_voidBlock, src->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
346 void _block_byref_dispose_helper(struct _block_byref_voidBlock *param) {
347 //_Block_destroy(param->captured_voidBlock, 0);
348 _Block_object_dispose(param->captured_voidBlock, BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER)}
351 struct _block_byref_voidBlock voidBlock = {( .forwarding=&voidBlock, .flags=(1<<25), .size=sizeof(struct _block_byref_voidBlock *),
352 .byref_keep=_block_byref_keep_helper, .byref_dispose=_block_byref_dispose_helper,
353 .captured_voidBlock=blockA )};
355 voidBlock.forwarding->captured_voidBlock = blockB;
358 2.3.3 Importing __block variables into Blocks
360 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.
365 functioncall(^{ i = 10; });
369 struct _block_byref_i {
370 void *isa; // set to NULL
371 struct _block_byref_voidBlock *forwarding;
372 int flags; //refcount;
374 void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
375 void (*byref_dispose)(struct _block_byref_i *);
380 struct __block_literal_5 {
384 void (*invoke)(struct __block_literal_5 *);
385 struct __block_descriptor_5 *descriptor;
386 struct _block_byref_i *i_holder;
389 void __block_invoke_5(struct __block_literal_5 *_block) {
390 _block->forwarding->captured_i = 10;
393 void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
394 //_Block_byref_assign_copy(&dst->captured_i, src->captured_i);
395 _Block_object_assign(&dst->captured_i, src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
398 void __block_dispose_5(struct __block_literal_5 *src) {
399 //_Block_byref_release(src->captured_i);
400 _Block_object_dispose(src->captured_i, BLOCK_FIELD_IS_BYREF | BLOCK_BYREF_CALLER);
403 static struct __block_descriptor_5 {
404 unsigned long int reserved;
405 unsigned long int Block_size;
406 void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
407 void (*dispose_helper)(struct __block_literal_5 *);
408 } __block_descriptor_5 = { 0, sizeof(struct __block_literal_5) __block_copy_5, __block_dispose_5 };
412 struct _block_byref_i i = {( .forwarding=&i, .flags=0, .size=sizeof(struct _block_byref_i) )};
413 struct __block_literal_5 _block_literal = {
414 &_NSConcreteStackBlock,
415 (1<<25)|(1<<29), <uninitialized>,
417 &__block_descriptor_5,
421 2.3.4 Importing __attribute__((NSObject)) __block variables
423 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.
425 2.3.5 __block escapes
427 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:
429 _Block_object_dispose(&_block_byref_xxx, BLOCK_FIELD_IS_BYREF);
434 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.
436 3. Objective C Extensions to Blocks
438 3.1 Importing Objects
440 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.
443 3.2 Blocks as Objects
445 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.
447 3.3 __weak __block Support
449 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:
450 objc_read_weak(&block->byref_i->forwarding->i)
452 The __weak variable is stored in a _block_byref_xxxx structure and the Block has copy and dispose helpers for this structure that call:
453 _Block_object_assign(&dest->_block_byref_i, src-> _block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
455 _Block_object_dispose(src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
458 In turn, the block_byref copy support helpers distinguish between whether the __block variable is a Block or not and should either call:
459 _Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_OBJECT | BLOCK_BYREF_CALLER);
460 for something declared as an object or
461 _Block_object_assign(&dest->_block_byref_i, src->_block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BLOCK | BLOCK_BYREF_CALLER);
462 for something declared as a Block.
464 A full example follows:
467 __block __weak id obj = <initialization expression>;
468 functioncall(^{ [obj somemessage]; });
472 struct _block_byref_obj {
473 void *isa; // uninitialized
474 struct _block_byref_obj *forwarding;
475 int flags; //refcount;
477 void (*byref_keep)(struct _block_byref_i *dst, struct _block_byref_i *src);
478 void (*byref_dispose)(struct _block_byref_i *);
482 void _block_byref_obj_keep(struct _block_byref_voidBlock *dst, struct _block_byref_voidBlock *src) {
483 //_Block_copy_assign(&dst->captured_obj, src->captured_obj, 0);
484 _Block_object_assign(&dst->captured_obj, src->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
487 void _block_byref_obj_dispose(struct _block_byref_voidBlock *param) {
488 //_Block_destroy(param->captured_obj, 0);
489 _Block_object_dispose(param->captured_obj, BLOCK_FIELD_IS_OBJECT | BLOCK_FIELD_IS_WEAK | BLOCK_BYREF_CALLER);
492 for the block byref part and
494 struct __block_literal_5 {
498 void (*invoke)(struct __block_literal_5 *);
499 struct __block_descriptor_5 *descriptor;
500 struct _block_byref_obj *byref_obj;
503 void __block_invoke_5(struct __block_literal_5 *_block) {
504 [objc_read_weak(&_block->byref_obj->forwarding->captured_obj) somemessage];
507 void __block_copy_5(struct __block_literal_5 *dst, struct __block_literal_5 *src) {
508 //_Block_byref_assign_copy(&dst->byref_obj, src->byref_obj);
509 _Block_object_assign(&dst->byref_obj, src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
512 void __block_dispose_5(struct __block_literal_5 *src) {
513 //_Block_byref_release(src->byref_obj);
514 _Block_object_dispose(src->byref_obj, BLOCK_FIELD_IS_BYREF | BLOCK_FIELD_IS_WEAK);
517 static struct __block_descriptor_5 {
518 unsigned long int reserved;
519 unsigned long int Block_size;
520 void (*copy_helper)(struct __block_literal_5 *dst, struct __block_literal_5 *src);
521 void (*dispose_helper)(struct __block_literal_5 *);
522 } __block_descriptor_5 = { 0, sizeof(struct __block_literal_5), __block_copy_5, __block_dispose_5 };
524 and within the compound statement:
526 struct _block_byref_obj obj = {( .forwarding=&obj, .flags=(1<<25), .size=sizeof(struct _block_byref_obj),
527 .byref_keep=_block_byref_obj_keep, .byref_dispose=_block_byref_obj_dispose,
528 .captured_obj = <initialization expression> )};
530 struct __block_literal_5 _block_literal = {
531 &_NSConcreteStackBlock,
532 (1<<25)|(1<<29), <uninitialized>,
534 &__block_descriptor_5,
535 &obj, // a reference to the on-stack structure containing "captured_obj"
539 functioncall(_block_literal->invoke(&_block_literal));
544 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.
546 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:
550 void (^block)(void) = ^{ printf("%d\n", foo.value()); };
553 The compiler would synthesize
555 struct __block_literal_10 {
559 void (*invoke)(struct __block_literal_10 *);
560 struct __block_descriptor_10 *descriptor;
564 void __block_invoke_10(struct __block_literal_10 *_block) {
565 printf("%d\n", _block->foo.value());
568 void __block_literal_10(struct __block_literal_10 *dst, struct __block_literal_10 *src) {
569 FOO_ctor(&dst->foo, &src->foo);
572 void __block_dispose_10(struct __block_literal_10 *src) {
576 static struct __block_descriptor_10 {
577 unsigned long int reserved;
578 unsigned long int Block_size;
579 void (*copy_helper)(struct __block_literal_10 *dst, struct __block_literal_10 *src);
580 void (*dispose_helper)(struct __block_literal_10 *);
581 } __block_descriptor_10 = { 0, sizeof(struct __block_literal_10), __block_copy_10, __block_dispose_10 };
583 and the code would be:
586 comp_ctor(&foo); // default constructor
587 struct __block_literal_10 _block_literal = {
588 &_NSConcreteStackBlock,
589 (1<<25)|(1<<26)|(1<<29), <uninitialized>,
591 &__block_descriptor_10,
593 comp_ctor(&_block_literal->foo, &foo); // const copy into stack version
594 struct __block_literal_10 &block = &_block_literal; // assign literal to block variable
595 block->invoke(block); // invoke block
596 comp_dtor(&_block_literal->foo); // destroy stack version of const block copy
597 comp_dtor(&foo); // destroy original version
601 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,
603 __block FOO blockStorageFoo;
605 requires the normal constructor for the embedded blockStorageFoo object
607 FOO_ctor(& _block_byref_blockStorageFoo->blockStorageFoo);
609 and at scope termination the destructor:
611 FOO_dtor(& _block_byref_blockStorageFoo->blockStorageFoo);
613 Note that the forwarding indirection is NOT used.
615 The compiler would need to generate (if used from a block literal) the following copy/dispose helpers:
617 void _block_byref_obj_keep(struct _block_byref_blockStorageFoo *dst, struct _block_byref_blockStorageFoo *src) {
618 FOO_ctor(&dst->blockStorageFoo, &src->blockStorageFoo);
621 void _block_byref_obj_dispose(struct _block_byref_blockStorageFoo *src) {
622 FOO_dtor(&src->blockStorageFoo);
625 for the appropriately named constructor and destructor for the class/struct FOO.
627 To support member variable and function access the compiler will synthesize a const pointer to a block version of the "this" pointer.
629 5.0 Runtime Helper Functions
631 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.
633 The block copy helper function should, for each of the variables of the type mentioned above, call
634 _Block_object_assign(&dst->target, src->target, BLOCK_FIELD_<appropo>);
635 in the copy helper and
636 _Block_object_dispose(->target, BLOCK_FIELD_<appropo>);
637 in the dispose helper where
641 BLOCK_FIELD_IS_OBJECT = 3, // id, NSObject, __attribute__((NSObject)), block, ...
642 BLOCK_FIELD_IS_BLOCK = 7, // a block variable
643 BLOCK_FIELD_IS_BYREF = 8, // the on stack structure holding the __block variable
645 BLOCK_FIELD_IS_WEAK = 16, // declared __weak
647 BLOCK_BYREF_CALLER = 128, // called from byref copy/dispose helpers
650 and of course the CTORs/DTORs for const copied C++ objects.
652 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.
654 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_<appropo> field within the block_byref copy/dispose helper calls.
656 The prototypes, and summary, of the helper functions are
658 /* Certain field types require runtime assistance when being copied to the heap. The following function is used
659 to copy fields of types: blocks, pointers to byref structures, and objects (including __attribute__((NSObject)) pointers.
660 BLOCK_FIELD_IS_WEAK is orthogonal to the other choices which are mutually exclusive.
661 Only in a Block copy helper will one see BLOCK_FIELD_IS_BYREF.
663 void _Block_object_assign(void *destAddr, const void *object, const int flags);
665 /* Similarly a compiler generated dispose helper needs to call back for each field of the byref data structure.
666 (Currently the implementation only packs one field into the byref structure but in principle there could be more).
667 The same flags used in the copy helper should be used for each call generated to this function:
669 void _Block_object_dispose(const void *object, const int flags);