有错误127的代码如下:

#include<windows.h>
#include<stdio.h>

//#pragma comment(linker,"/export:add=dllA_old.add")

HMODULE hModule=NULL; //模块句柄
FARPROC proc; //函数入口指针
typedef int (__cdecl *Add)(int a,int b);
Add padd;
BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved
                     )
{
    if(NULL==hModule)
    {
        hModule=LoadLibrary("dllA_old.dll");
    }
    if(NULL==hModule)
    {
        MessageBoxA(NULL,"获取dllA_old.dll失败","caption",0);
        return 0;
    }
    return TRUE;
}

int add(int a,int b)
{
    padd=(Add)GetProcAddress(hModule,"add");  //这里获取函数地址失败 127
    if(NULL==padd)
    {
        char msg[256];
        sprintf(msg,"获取add()失败:%d",GetLastError());
        MessageBoxA(NULL,msg,"caption",0);
        return 0;
    }
    return padd(a,b);
}

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修改方法，就是在add函数中，进行加装dll操作，而不是在dll的入口处加载dll文件,代码如下:

#include<windows.h>
#include<stdio.h>

//#pragma comment(linker,"/export:add=dllA_old.add")

HMODULE hModule=NULL; //模块句柄
FARPROC proc; //函数入口指针
typedef int (__cdecl *Add)(int a,int b);
Add padd;
BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved
                     )
{
    if(NULL==hModule)
    {
        hModule=LoadLibrary("dllA_old.dll");
    }
    if(NULL==hModule)
    {
        MessageBoxA(NULL,"获取dllA_old.dll失败","caption",0);
        return 0;
    }
    return TRUE;
}

int add(int a,int b)
{
    hModule=LoadLibrary("dllA_old.dll");    //在这里加载哈
    padd=(Add)GetProcAddress(hModule,"add");
    if(NULL==padd)
    {
        char msg[256];
        sprintf(msg,"获取add()失败:%d",GetLastError());
        MessageBoxA(NULL,msg,"caption",0);
        return 0;
    }
    return padd(a,b);
}

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您的十分满意是我追求的宗旨。

您的一点建议是我后续的动力。