如何在Win32 C ++ Dll中连接int和char *并返回char *？

Question

I have written a win32 DLL by using visual studio 2012.

My function return type is char*:

extern "C" TESTDLL_API char* SelectColor()
{

}

I want to return string in my main application (C#):

[return: MarshalAs(UnmanagedType.LPStr)]
[DllImport("TESTDll.dll", CharSet = CharSet.Auto, SetLastError = true, CallingConvention = CallingConvention.Cdecl)]
public static extern string SelectColor();

in body of the SelectColor method I must to concat some char* and int. I tested this codes until now:

char* cid0 = GetCpuID(0);
char* cid1 = GetCpuID(1);
char* result = new char[1000];
sprintf(result,"%s%s", result ,cid0);
sprintf(result,"%s%s", result ,cid1);
return result;

This returns wrong string;

char* cid0 = GetCpuID(0);
char* cid1 = GetCpuID(1);
char * result = (char*)calloc(strlen(cid0) + strlen(cid1) + 1, sizeof(char));
strcat(result, cid0);
strcat(result, cid1);
return result;

This will fail;

char* cid0 = GetCpuID(0);
char* cid1 = GetCpuID(1);
stringstream ss;
ss << cid0 << cid1 << number;
return (char*)ss.str().c_str();

This returns empty string;

What I must do? Sorry for bad english. Thanks.

Answer 1

i guess by :

char* cid0 = GetCpuID(0);
char* cid1 = GetCpuID(1);
char* result = new char[1000];
sprintf(result,"%s%s", result ,cid0);
sprintf(result,"%s%s", result ,cid1);
return result;

you want to do this;

char* cid0 = GetCpuID(0);
char* cid1 = GetCpuID(1);
char* result = new char[1000];
sprintf(result,"%s%s", cid0,cid1);
return result;

Answer 2

If you absolutely cannot change the signature of the function, then you will have to return a pointer to a static memory buffer to ensure the pointer is not leaked (in the case of your new[] and calloc() examples) or freed prematurely (in the case of your stringstream example), eg:

char result[1000];

extern "C" TESTDLL_API char* SelectColor()
{
    char* cid0 = GetCpuID(0);
    char* cid1 = GetCpuID(1);
    StringCchPrintfA(result, 1000, "%s%s", cid0, cid1);
    return result;
}

Or:

std::string result;

extern "C" TESTDLL_API char* SelectColor()
{
    char* cid0 = GetCpuID(0);
    char* cid1 = GetCpuID(1);
    std::ostringstream oss;
    oss << cid0 << cid1;
    result = oss.str();
    return result.c_str();
}

Note that this is not thread-safe. If multiple threads need to call the function at the same time, use a memory buffer in thread-local storage, eg:

__declspec(thread) char result[1000];

extern "C" TESTDLL_API char* SelectColor()
{
    char* cid0 = GetCpuID(0);
    char* cid1 = GetCpuID(1);
    StringCchPrintfA(result, 1000, "%s%s", cid0, cid1);
    return result;
}

Update : If the content of the buffer never changes, you can initialize the buffer once and just keep returning it as-is:

struct sResult
{
    char buffer[1000];

    sResult()
    {
        char* cid0 = GetCpuID(0);
        char* cid1 = GetCpuID(1);
        StringCchPrintfA(buffer, 1000, "%s%s", cid0, cid1);
    }
};

extern "C" TESTDLL_API char* SelectColor()
{
    static sResult result;
    return result.buffer;
}

Or:

static char result[1000];

BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpReserved)
{
    if (fdwReason == DLL_PROCESS_ATTACH)
    {
        char* cid0 = GetCpuID(0);
        char* cid1 = GetCpuID(1);
        StringCchPrintfA(buffer, 1000, "%s%s", cid0, cid1);
    }
    return TRUE;
};

extern "C" TESTDLL_API char* SelectColor()
{
    return result;
}