llvm-project/clang/docs/Block-ABI-Apple.rst

==================================
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<<30), 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_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:

.. 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);

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=========

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