看了“我对C++中THUNK一种实现技术的分析”一文，有点不同的想法。
原代码如下：
#pragma pack(push,1)
// structure to store the machine code
struct Thunk
{
char m_jmp; // op code of jmp instruction
unsigned long m_relproc; // relative jmp
};
#pragma pack(pop)
//This type of structure can contain then thunk code, which can be executed on the fly. Let's take a look at the simple case in which we are going to execute our required function by thunk.
//Program 77
#include <iostream>
#include <windows.h>
using namespace std;
class C;
C* g_pC = NULL;
typedef void(*pFUN)();
class C
{
public:
int a;
int b;
float bbj;
Thunk m_thunk;
//virtual void g(){};
void Init(pFUN pFun, void* pThis)
{
// op code of jump instruction
m_thunk.m_jmp = 0xe9;
// address of the appripriate function
/*@@@@*/ m_thunk.m_relproc = (int)pFun- ((int)&m_thunk+sizeof(Thunk));
FlushInstructionCache(GetCurrentProcess(),
&m_thunk, sizeof(m_thunk));
}
// this is cour call back function
static void CallBackFun()
{
C* pC = g_pC;
// initilize the thunk
pC->Init(StaticFun, pC);
// get the address of thunk code
pFUN pFun = (pFUN)&(pC->m_thunk);
// start executing thunk code which will call StaticFun
pFun();
cout << "C::CallBackFun" << endl;
}
static void StaticFun()
{
cout << "C::StaticFun" << endl;
}
};
int main()
{
C objC;
g_pC = &objC;
C::CallBackFun();
return 0;
}
首先我们知道段内直接跳转的JMP指令长5个字节，所以sizeof（Thunk）必须是五字节，这也是#pragma pack(push,1)保持struct 1字节对齐的原因。
再来看static member function StaticFun的地址pFun，在VC编译器下，实际上该地址指向一条段内JMP指令，该JMP指令跳转到StaticFun的执行代码，比如pFun＝0040106E，即指向E9 ED 04 00 00，所以当前EIP=0040106E+5=401073（即下一条指令的地址）,执行JMP,EIP=EIP+04ED＝401560,401560即是StaticFun执行代码的起始地址。
接着看：
// get the address of thunk code
pFUN pFun = (pFUN)&(pC->m_thunk);
// start executing thunk code which will call StaticFun
pFun();
此处的段内直接函数call，将直接跳转到&m_thunk所指的代码执行，也即
JMP (int)pFun - ((int)this+sizeof(Thunk)),
而此时EIP=&m_thunk＋5＝this＋5，所以这个JMP将跳转到
EIP=EIP+(int)pFun - ((int)this+sizeof(Thunk))=pFun，
这样就最终跳到StaticFun执行代码的起始地址。
所以如果m_thunk前还有其他变量，比如int a；则必须使
m_thunk.m_relproc = (int)pFun - ((int)this+sizeof(Thunk)+sizeof(int));
当pFun();时，EIP=&m_thunk＋5＝this＋sizeof(int)+5,
EIP=EIP+(int)pFun - ((int)this+sizeof(Thunk)+sizeof(int))=pFun
这样才能最终跳到StaticFun执行代码的起始地址。
所以，最好写成：
m_thunk.m_relproc = (int)pFun - ((int)&m_thunk+sizeof(Thunk));
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