C# 不安全代码固定指针作为参数传递

Question

我在 msdn 上看到了以下代码:

        unsafe static void SquarePtrParam (int* p) 
        {
            *p *= *p;
        }

        unsafe static void Main() 
        {
            Point pt = new Point();
            pt.x = 5;
            pt.y = 6;
            // Pin pt in place:
            fixed (int* p = &pt.x) 
            {
                SquarePtrParam (p);
            }
            // pt now unpinned.
            Console.WriteLine ("{0} {1}", pt.x, pt.y);
        }

我只是想知道，我们在SquarePtrParam函数中直接访问指针，它是否从调用方法继承了数组固定的信息？

为什么我们不需要在 SquarePtrParam 中显式地在本地将其设置为 fixed。

我想我可以对这个 fixed 语句进行一些详细说明。

Answer 1

固定语句以与“using”语句在using(FileStream stream = new FileStream(..))构造中关闭打开的文件相同的方式实现unpin内存区域。内存将被固定，直到您离开固定代码块。

在 IL 代码中，它将创建虚拟 PINNED 局部变量并将指针存储到其中。这将不允许 GC 移动包含此指针的内存区域。在您离开固定 block 后，它会将零存储到此 PINNED 变量中。就像这样:

public static unsafe void TestInternal(byte* pointer)
{
    Console.WriteLine((IntPtr)pointer);
}

public static void FixedDemo()
{
    Byte[] newArray = new Byte[1024];

    unsafe
    {
        fixed (Byte* pointer = &newArray[0])
        {
            TestInternal(pointer);
        }
    }

    Console.WriteLine("Test Complete");
}

IL 代码中的 FixedDemo:

.method public hidebysig static void  FixedDemo() cil managed
{
  // Code size       47 (0x2f)
  .maxstack  2
  .locals init ([0] uint8[] newArray,
           [1] uint8& pinned pointer)
  IL_0000:  nop
  IL_0001:  ldc.i4     0x400 // Put 1024 on the stack
  IL_0006:  newarr     [mscorlib]System.Byte // allocate new array of 1024 length
  IL_000b:  stloc.0    // Store it in local variable 0
  IL_000c:  nop 
  IL_000d:  ldloc.0    // Put local variable 0 on the stack
  IL_000e:  ldc.i4.0   // Put zero on the stack
  IL_000f:  ldelema    [mscorlib]System.Byte // Load address of zero index from array
  IL_0014:  stloc.1    // !!! Here we pin memory by storing it in pinned variable
  IL_0015:  nop
  IL_0016:  ldloc.1    // Load function argument
  IL_0017:  conv.i     // Perform conversion 
  IL_0018:  call       void FinMath.Tests.Program::TestInternal(uint8*)
  IL_001d:  nop
  IL_001e:  nop
  IL_001f:  ldc.i4.0   // Load zero on the stack
  IL_0020:  conv.u     // Perform conversion 
  IL_0021:  stloc.1    // !!!! Here we unpin memory
  IL_0022:  nop
  IL_0023:  ldstr      "Test Complete" // Load string
  IL_0028:  call       void [mscorlib]System.Console::WriteLine(string) // Out message
  IL_002d:  nop
  IL_002e:  ret
} // end of method Program::FixedDemo

更多信息请访问:

1. MSDN

2. MSDN Magazine: Garbage Collection: Automatic Memory Management in the Microsoft .NET Framework

3. Common language Runtime standard ECMA 335第三部分，1.1.4.2托管指针(type &)