C / C++ / MFC

George_George wrote:

When we use OLESTR to allocate a variable from string literal, like OLESTR("Hello World"), the memory space for the string literal is on stack, right?

George_George wrote:

I am more interested in which option in MSVC could be used to control such optimization?

CPallini wrote:

I think it is always safe (i.e. it maybe unsafe only for not-standard compilers).

George_George wrote:

Do you have any documents support this claim? Even if we turn off string pooling?

George_George wrote:

I read your referred article, I think it has nothing to do with OLESTR, but dealing with write to const data? Right?

The C++ standard (section lex.string) states:

    1 A  string  literal  is  a  sequence  of  characters  (as  defined   in
      _lex.ccon_) surrounded by double quotes, optionally beginning with the
      letter L, as in "..." or L"...".  A string literal that does not begin
      with  L  is  an  ordinary string literal, also referred to as a narrow
      string literal.  An ordinary string literal has type "array of n const
      char"  and  static storage duration [...] 

// IsOnStack.cpp : Defines the entry point for the console application.
//
 
#include "stdafx.h"
#include <iostream>
#include <windows.h>
using namespace std;
 
bool IsOnStack(const void *pData)
{
	DWORD StackBase;
	DWORD StackLimit;
	__asm 
	{
		MOV EAX, DWORD PTR FS:[0x04]
		MOV StackBase, EAX
		MOV EAX, DWORD PTR FS:[0x08]
		MOV StackLimit, EAX
	}
 
	DWORD Address = reinterpret_cast<DWORD>(pData);
 
	return (Address >= StackLimit) && (Address < StackBase);
}
 
const char* PrintStack(const void *pData)
{
	return IsOnStack(pData) ? ": Stack" : ": Not on Stack";
}
 
int main(int argc, char* argv[])
{
	const char *pString1 = "String1";
	cout << pString1 << PrintStack(pString1) << endl;
 
	char String2[] = "String2";
	cout << String2 << PrintStack(String2) << endl;
 
	return 0;
}

String1: Not on Stack
String2: Stack

MOV EAX, DWORD PTR FS:[0x04]
MOV StackBase, EAX
MOV EAX, DWORD PTR FS:[0x08]
MOV StackLimit, EAX