1.截获自动变量

int value = 1;
    void (^blk)(void) = ^{
        printf("Block function :%d",value);
      };
    value = 2;
    blk();

将上面代码进行clang转换

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int value;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int _value, int flags=0) : value(_value) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int value = __cself->value; // bound by copy

        printf("Block function :%d",value);
      }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, char * argv[]) {

    int value = 1;
    void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, value));
    value = 2;
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

    return 0;

}

我们可以看到 value 被捕获到__main_block_impl_0结构体中了

2.__block 说明符

我们先看一下 静态全局变量 全局变量 和静态变量在clang中的表示

int globalValue = 0;
static int staticGlobalValue = 1;

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  int *staticValue;
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, int *_staticValue, int flags=0) : staticValue(_staticValue) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  int *staticValue = __cself->staticValue; // bound by copy

        globalValue += 1;
        staticGlobalValue += 1;
        (*staticValue) += 1;
        printf("Block function :%d",(*staticValue));
      }

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0)};
int main(int argc, char * argv[]) {

    static int staticValue = 2;
    void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, &staticValue));
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

    return 0;

}

我们看到只有静态变量进行了转换，全局和动态变量都和之前完全相同，我们再来看把静态变量转成__block

struct __main_block_impl_0 {
  struct __block_impl impl;
  struct __main_block_desc_0* Desc;
  __Block_byref_staticValue_0 *staticValue; // by ref
  __main_block_impl_0(void *fp, struct __main_block_desc_0 *desc, __Block_byref_staticValue_0 *_staticValue, int flags=0) : staticValue(_staticValue->__forwarding) {
    impl.isa = &_NSConcreteStackBlock;
    impl.Flags = flags;
    impl.FuncPtr = fp;
    Desc = desc;
  }
};
static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_staticValue_0 *staticValue = __cself->staticValue; // bound by ref

        globalValue += 1;
        staticGlobalValue += 1;
        (staticValue->__forwarding->staticValue) += 1;
        printf("Block function :%d",(staticValue->__forwarding->staticValue));
      }
static void __main_block_copy_0(struct __main_block_impl_0*dst, struct __main_block_impl_0*src) {_Block_object_assign((void*)&dst->staticValue, (void*)src->staticValue, 8/*BLOCK_FIELD_IS_BYREF*/);}

static void __main_block_dispose_0(struct __main_block_impl_0*src) {_Block_object_dispose((void*)src->staticValue, 8/*BLOCK_FIELD_IS_BYREF*/);}

static struct __main_block_desc_0 {
  size_t reserved;
  size_t Block_size;
  void (*copy)(struct __main_block_impl_0*, struct __main_block_impl_0*);
  void (*dispose)(struct __main_block_impl_0*);
} __main_block_desc_0_DATA = { 0, sizeof(struct __main_block_impl_0), __main_block_copy_0, __main_block_dispose_0};
int main(int argc, char * argv[]) {

   __attribute__((__blocks__(byref))) __Block_byref_staticValue_0 staticValue = {(void*)0,(__Block_byref_staticValue_0 *)&staticValue, 0, sizeof(__Block_byref_staticValue_0), 2};
    void (*blk)(void) = ((void (*)())&__main_block_impl_0((void *)__main_block_func_0, &__main_block_desc_0_DATA, (__Block_byref_staticValue_0 *)&staticValue, 570425344));
    ((void (*)(__block_impl *))((__block_impl *)blk)->FuncPtr)((__block_impl *)blk);

    return 0;

}

此时__block 竟然变成__Block_byref_staticValue_0结构体，给staticValue静态变量赋值的时候，使用了指向该静态变量的指针，而向__block变量赋值的时候__Block_byref_staticValue_0 结构体的成员变量__forwarding 持有该实例自身的指针

static void __main_block_func_0(struct __main_block_impl_0 *__cself) {
  __Block_byref_staticValue_0 *staticValue = __cself->staticValue; // bound by ref

        globalValue += 1;
        staticGlobalValue += 1;
        (staticValue->__forwarding->staticValue) += 1;
        printf("Block function :%d",(staticValue->__forwarding->staticValue));
      }