C++ chars stored in the LPCSTR .. broken? - c++

Question

LPCSTR dllPath = ExePath().append("\\").append(DEF_INJECT_DLL).c_str();
DWORD dwBufSize = (DWORD)(strlen(dllPath) + 1) * sizeof(LPCSTR);
/* test */
char tbuf[1024]= {0,};
sprintf_s(tbuf, "dllPath : %s\r\ndwBufSize : %d", dllPath, dwBufSize);
MessageBoxA(NULL, tbuf, "TEST", MB_OK);
part of the code to inject my dll.
ExePath() is a function to get AbsolutePath of std::string data type using GetModuleFileNameA API and so on.
DEF_INJECT_DLL is defined by #define "MyDll.dll"
But when I run this code, it shows me broken strings....
And, when I change the MessageBoxA to this:
MessageBoxA(NULL,
ExePath().append("\\").append(DEF_INJECT_DLL).c_str(),
"TEST",
MB_OK);
it shows properly?
Also, I tried in this way:
MessageBoxA(NULL,dllPath, "TEST", MB_OK);
but it shows to me like first screenshot.
What is the problem?

Answer 1

The problem is in this line:
LPCSTR dllPath = ExePath().append("\\").append(DEF_INJECT_DLL).c_str();
here you call ExePath(), which returns a std::string instance, modify it, and finally call c_str() to get the raw data buffer.
However, the return value is a temporary object. After that line, the returned std::string is deleted, and will clean its memory. Therefore, the address where dllPath points to is no longer valid!
You could store the return value in a local instance, e.g.
std::string str = ExePath().append("\\").append(DEF_INJECT_DLL);
LPCSTR dllPath = str.c_str();