How pinning is presented in IL

Question

I was wondering how field pinning is expressed in .Net IL, so I looked at an example code:

struct S { public fixed int buf[8]; } S s = default(S); public void MyMethod() { fixed (int* ptr = s.buf){ *ptr = 2; } } 

This generates IL:

 .method private hidebysig instance void MyMethod () cil managed { // Method begins at RVA 0x2050 // Code size 25 (0x19) .maxstack 2 .locals init ( [0] int32& pinned ) IL_0000: ldarg.0 // Load argument 0 onto the stack IL_0001: ldflda valuetype C/SC::s // Push the address of field of object obj on the stack IL_0006: ldflda valuetype C/S/'<buf>e__FixedBuffer' C/S::buf // Push the address of field of object obj on the stack IL_000b: ldflda int32 C/S/'<buf>e__FixedBuffer'::FixedElementField // Push the address of field of object obj on the stack IL_0010: stloc.0 // Pop a value from stack into local variable 0 IL_0011: ldloc.0 // Load local variable 0 onto stack IL_0012: conv.i // Convert to native int, pushing native int on stack IL_0013: ldc.i4.2 // Push 2 onto the stack as int32 IL_0014: stind.i4 // Store value of type int32 into memory at address IL_0015: ldc.i4.0 // Push 0 onto the stack as int32 IL_0016: conv.u // Convert to native unsigned int, pushing native int on stack IL_0017: stloc.0 // Pop a value from stack into local variable 0 IL_0018: ret // Return from method, possibly with a value } // end of method C::MyMethod 

I don’t see anything here, I will explicitly tell GC to bind an array, which instruction is actually responsible for committing? In addition, are there other basic operations that are associated with fixing “under the hood”?

Answer 1

 .locals init ( [0] int32& pinned ) 

Using pinned is responsible for pinning. This article explains this as follows: How does the 'fixed' keyword work? This article provides the following excerpt from Standard ECMA-335 Common Language Infrastructure (CLI) :

II.7.1.2 fixed

The signature encoding for pinned should appear only in signatures describing local variables (§II.15.4.1.3). Although a method with a fixed local variable is executed, VES should not move the object to which the local belongs. That is, if the CLI implementation uses a garbage collector that moves objects, the collector should not move objects referenced by the active pinned local variable.

[Rationale: if unmanaged pointers are used to dereference managed objects, these objects must be pinned. This happens, for example, when a managed object is passed to a method designed to work with unmanaged data. final rationale]

VES = Virtual Execution System, CLI = Common Language Infrastructure