Problem on dword ptr[...] with visual studio 2010

3
votes

I'm trying to compile a program which worked with the compiler of Visual C++ 6.0. Now, I'm compiling it using the compiler of Visual Studio 2010. It compiles but my program crashes on assignment of a variable:

sapHigh = ctx->saphigh;

In assembler code (using Visual Studio Debugger), this instruction is decomposed by:

00410DF3  mov         eax,dword ptr [ctx]  
00410DF6  mov         ecx,dword ptr [eax+20h]  
00410DF9  mov         dword ptr [sapHigh],ecx

The instruction "mov eax, dword ptr[ctx]" return 0x00000000 in eax and the program crashes. But the debugger can see the real value of ctx pointer which is "0x0172287a". If I change the value of the eax register with its real value, "0x0172287a", the program still working fine until the next assignment.

Does anybody know why this instruction doesn't work ? Is this a problem with compilation options ?

Thank you for your help.

3
c++visual-studiovisual-studio-2010assembly
Does your program use any external libraries or dlls which were compiled to work with the old application? We had some problems like this when using stl strings between dlls compiled with VC6 and visual studio 2003 I remember. – NotJarvis
Have you tried to make a breakpoint on ctx location? – eugene_che
Can you post the source of the program? It's kind of hard to guess from these snippets... – RedX
@NotJarvis Normally, all external libraries and dlls are compiled with the new compiler. – Cedekasme
@RedX Unfortunately, I can't post the source of the program, sorry... – Cedekasme

3 Answers

2
votes

That's a long gap of 10-12 years. I recommend you compile the program in VC10 with ALL compiler warnings enabled, and attempt to remove all of them. It includes all warnings related to deprecated stuff, 64-bit warnings, performance warnings, and C++ compliant warnings.

0
votes

Sounds like the memory for the variable ctx is not longer valid. If ctx is for example part of some dynamically allocated struct that got freed earlier, or if it's a reference or pointer to an out-of-scope stack variable, all kinds of undefined behaviors can happen.

Using invalid memory might not crash if (by chance) no other functions overwrote that memory, but using a different compiler and a different runtime library can easily lead to different results.

0
votes

What if you try lea eax, dword ptr[ctx] ?

Also I've seen problems like that happen if the pointer to the array is passed to a function. When the array is defined within a function everything's fine.