- 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.
The ABI of Blocks
consist of their layout and the runtime functions required
by the compiler. A Block
consists of a structure of the following form:
struct Block_literal_1 {
void *isa; // initialized to &_NSConcreteStackBlock or &_NSConcreteGlobalBlock
int flags;
int reserved;
void (*invoke)(void *, ...);
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:
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 compatiblity 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<<30), for the following combinations of valid bit settings, and their meanings:
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:
- A
static
descriptor structure is declared and initialized as follows:
a. The
invoke
function pointer is set to a function that takes theBlock
structure as its first argument and the rest of the arguments (if any) to theBlock
and executes theBlock
compound statement.b. The
size
field is set to the size of the followingBlock
literal structure.c. The
copy_helper
anddispose_helper
function pointers are set to respective helper functions if they are required by theBlock
literal.
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 inlibSystem
, or_NSConcreteGlobalBlock
if this is a static or file levelBlock
literal.b. The
flags
field is set to zero unless there are variables imported into theBlock
that need helper functions for program levelBlock_copy()
andBlock_release()
operations, in which case the (1<<25) flags bit is set.
As an example, the Block
literal expression:
^ { printf("hello world\n"); }
would cause the following to be created on a 32-bit system:
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), __block_invoke_1 };
and where the Block
literal itself appears:
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:
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.
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.
Automatic storage variables not marked with __block
are imported as
const
copies.
The simplest example is that of importing a variable of type int
:
int x = 10;
void (^vv)(void) = ^{ printf("x is %d\n", x); }
x = 11;
vv();
which would be compiled to:
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:
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.
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:
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:
struct __block_literal_4 _block_literal = {
&_NSConcreteStackBlock,
(1<<25)|(1<<29), <uninitialized>
__block_invoke_4,
& __block_descriptor_4
existingBlock,
};
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:
struct Opaque *__attribute__((NSObject)) objectPointer = ...;
...
void (^foo)(void) = ^{ CFPrint(objectPointer); };
would have the following helper functions generated:
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);
}
The compiler must embed variables that are marked __block
in a specialized
structure of the form:
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:
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.
- The helper functions are initialized (if present).
- The variable itself is set to its initial value.
- The
isa
field is set toNULL
.
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:
int __block i = 10;
i = 11;
would be rewritten to be:
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:
__block void (voidBlock)(void) = blockA;
voidBlock = blockB;
would translate into:
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:
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;
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:
int __block i = 2;
functioncall(^{ i = 10; });
would translate to:
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:
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,
};
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
.
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:
_Block_object_dispose(&_block_byref_foo, BLOCK_FIELD_IS_BYREF);
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.
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.
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.
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:
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:
_Block_object_assign(&dest->_block_byref_i, src-> _block_byref_i, BLOCK_FIELD_IS_WEAK | BLOCK_FIELD_IS_BYREF);
and:
_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:
_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:
_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:
__block __weak id obj = <initialization expression>;
functioncall(^{ [obj somemessage]; });
would translate to:
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:
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:
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));
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:
{
FOO foo;
void (^block)(void) = ^{ printf("%d\n", foo.value()); };
}
The compiler would synthesize:
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_literal_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:
{
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,
__block FOO blockStorageFoo;
requires the normal constructor for the embedded blockStorageFoo
object:
FOO_ctor(& _block_byref_blockStorageFoo->blockStorageFoo);
and at scope termination the destructor:
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:
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.
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:
_Block_object_assign(&dst->target, src->target, BLOCK_FIELD_<apropos>);
in the copy helper and:
_Block_object_dispose(->target, BLOCK_FIELD_<apropos>);
in the dispose helper where <apropos>
is:
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:
/* 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);
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