Question

我在开源EDKII代码库中看到下面的代码片段：

某些背景信息：

#define VOID      void
#define SPIN_LOCK_RELEASED          ((UINTN) 1)
#define SPIN_LOCK_ACQUIRED          ((UINTN) 2)
typedef volatile UINTN              SPIN_LOCK;
typedef unsigned __int64    UINTN; //depending on the build options
typedef unsigned __int32    UINTN; //depending on the build options

和

BOOLEAN
EFIAPI
AcquireSpinLockOrFail (
  IN OUT  SPIN_LOCK                 *SpinLock
  )
{
  SPIN_LOCK   LockValue;
  VOID        *Result;

  ASSERT (SpinLock != NULL);

  LockValue = *SpinLock;
  ASSERT (LockValue == SPIN_LOCK_ACQUIRED || LockValue == SPIN_LOCK_RELEASED);

  _ReadWriteBarrier ();
  Result = InterlockedCompareExchangePointer (
             (VOID**)SpinLock, // <============= PLACE A
             (VOID*)SPIN_LOCK_RELEASED,
             (VOID*)SPIN_LOCK_ACQUIRED
           );

  _ReadWriteBarrier ();
  return (BOOLEAN) (Result == (VOID*) SPIN_LOCK_RELEASED);
}

InterlockedCompareExchangePointer的实施方式如下：

VOID *
EFIAPI
InterlockedCompareExchangePointer (
  IN OUT  VOID                      **Value,
  IN      VOID                      *CompareValue,
  IN      VOID                      *ExchangeValue
  )
{
  UINT8  SizeOfValue;

  SizeOfValue = (UINT8) sizeof (*Value); // <=========== PLACE B

  switch (SizeOfValue) {
    case sizeof (UINT32):
      return (VOID*)(UINTN)InterlockedCompareExchange32 (
                             (UINT32*)Value,
                             (UINT32)(UINTN)CompareValue,
                             (UINT32)(UINTN)ExchangeValue
                             );
    case sizeof (UINT64):
      return (VOID*)(UINTN)InterlockedCompareExchange64 (
                             (UINT64*)Value,
                             (UINT64)(UINTN)CompareValue,
                             (UINT64)(UINTN)ExchangeValue
                             );
    default:
      ASSERT (FALSE);
      return NULL;
  }
}

AFAIK，处理不同的位宽是一些逻辑。

对于地点A，我不知道为什么我们可以将UINTN *转换为void **。为什么要明确地将指针转换为 指针指向 ？

对于地方B，它是如何运作的？

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受@ UnholySheep的评论启发。如果这一切都归结为sizeof(void*)，为什么不写SizeOfValue = (UINT8) sizeof (void*);而不是在参数Value上使用类型转换？

为什么int *转换为void **？

ADD 1

0 个答案: