Variable location in memory moves in between runs of program

[mford78]

I'll start by admitting that this may simply be my lack of knowledge about how modern OS/Compilers work together, but my coworker and I have run into an oddity that we cannot explain. A large amount of our debugging is done via Data Breakpoints (i.e. see when such and such variable changes), and we are seeing different behavior on our respective computers. Both are running Visual Studio 2010, and the latest Intel Fortran Compiler (IVF 12.1 XE rel. 9). The difference is that he is running Windows 7 64-bit and I am running Windows XP 32-bit as our respective operating systems.

On my machine, if I set a Data Breakpoint during a run of our software, and then allow that program to run to completetion, then restart the program my Data Breakpoint is still valid. I.e. the memory location of the variable has not changed between subsequent runnings of the program. On his machine, however, the location of a variable seems to move between subsequent runnings so Data Breakpoints he sets in Run 1, need to be reset in Run 2 because the location of the variable has changed.

I have whipped up a simple test program to demonstrate this. Using the following:
[fortran] program MemoryLocTest implicit none ! Variables double precision, dimension (1000000) :: array integer :: i, j, k ! Body of MemoryLocTest i = 0 j = 0 k = 0 do i = 1, 1000000 array(i) = i enddo i = 1 j = 2*i do i = 1, 10 k = k + i enddo print*, "I, J, K = ", i, j, k end program MemoryLocTest[/fortran] I have captured the three images showing the locations of the variable i, and the 1,000,000th index of the array. The image in the file 32bitNoDifference.png is from my machine and as one can see in the "Immediate Window" the variables location does not change between completion of one run of the program and the next (the delineation is marked by the red line). However, on my coworker's machine we see that one or both of the variables' addresses can change when run as either a 32-bit program, or a 64-bit program (32bitDifference.png and 64bitDifference.png, respectively).

In between the subsequent runs, the program was NOT recompiled. Before each of these sequences, the project was made clean, then built from scratch. And it is the same solution file/settings between both machines. Is this correct behavior? Is there a way to turn this off on his machine for debugging purposes? Thanks.

- Matt

[Steven_L_Intel1]

Is this code in a DLL? Or does the actual program allocate the array dynamically? In either case, Windows 7 (and Vista) will load things at a different address each time the program is run - a feature called Address Space Layout Randomization. It is designed to thwart malware. It is possible to disable the DLL loading variation, but not the memory allocation variation. You're absolutely right that this creates headaches when debugging.

[mford78]

Steve, the code is not in a DLL so it seems we'll just have to live with it. Thanks for the information!

[Steven_L_Intel1]

But it does use dynamic allocation?

[mford78]

The main program I was inquiring about, no. Nothing is dynamically allocated... It's been around for a while, part of my job is to modernize it, at least a bit.

Also, after some digging, I found the following option in Visual Studio: Property > Linker > Advanced > Fixed Base Address, if I set this to "Image must be loaded at a fixed base address" then my test program (see above) will maintain variable addresses between consecutive runs. This option adds the /FIXED flag to the linker, and (on the surface) seems to help things out. According to some other websites I read, this won't apply to things created on the heap, but since 99% of the code I am currently working with is allocated (almost) entirely on the stack, it should help.