How can there be static addresses in C/C++ programs?

Question

I\'ve been looking a bit into Cheat Engine, which allows you to inspect and manipulate the memory of running processes on Windows: You scan for variables based on their valu

Answer 1

As far as I understand it, variables declared static have a permanent offset within the program data. This means that when the program is loaded into RAM, the offset of the variable will always be the same. Because the beginning address of the program is known globally, finding a static variable based on offset, as you mentioned, should be a trivial task. Therefore, while a pointer to a static variable might be random in the scheme of things, its offset to the beginning of program memory should remain the same no matter when the program starts. So Cheat Engine (though I don't know the software) most likely stores the offset of the static variable, and then when the software starts, applies this logic to find that variable.

As to how it can tell it's a static variable in the first place... well, this is partially a guess, but when you declare a variable static in C, I'm assuming the compiler/linker puts some kind of flag so the OS knows that it's a static variable. It could also be that all static variables are stored in a certain way, or at a certain address offset, for all programs compiled for a certain target system. Again, not too sure about that, but from what I understand about memory management, that seems to make the most sense. With these assumptions, it's quite possible for a program to contain solely static variables. The difference is that memory is assigned statically at program runtime, as a opposed to dynamically (as with a call to malloc() or similar). If the variables were stored dynamically, I'm sure there'd be a way to find them easily, so I don't think it matters to Cheat Engine whether or not a variable is static or not. However, as I'm assuming Cheat Engine wants to modify a game upon startup (just like the old GameSharks used to... ahh, miss those days) it's probably more reliable to modify variables that are static, instead of trying to locate pointers and disassemble the code, etc. etc.

If you're interested in learning more, I'd recommend checking out something like this tutorial over at OSDev!

Answer 2

I will answer the bonus questions first because they introduce some concepts you may need to know to understand the answer for the main question.

Answering the first bonus question is easy if you know how an executable file works: all the global/static variables are inside the .data section, in which the .exe stores the address offset for the section so Cheat Engine just checks if the variable is in this address range (from this section to the next one).

For the second question, it is possible to use only static addresses, but that is nearly impossible for a game. Even the older ones. What the tutorial creator was probably trying to say is that all variables that he wants, actually had a static pointer pointing to them. But solely by the fact that you create a local variable, or even pass an argument to a function, their values are being stored into the stack. That's why it is nearly impossible to have a "static-only" program. Even if you compile a program that actually doesn't do anything, it will probably have some stuff being stored in the stack.

For the whole question itself, not all dynamic address variables are pointed by a global variable. It depends totally on the programmer. I can create a local variable and never assign its address to a global/static pointer in a C program, for example. The only certain way to find that address in this case is to actually know the code when the variable was first assigned a value in the stack.

Some variables have a dynamic address because they are just local variables, which are stored in the stack the first time they have a value assigned to them.

Some other variables have a static address because they are declared either as a global or a static variable to the compiler. These variables have a fixed address offset that is part of the .data section in the executable file.

The executable file has a fixed offset address for each section inside it, and the .data section is no exception.

But it is worth to note that the offset inside the executable itself is fixed. In the operating system things might be different (all random addresses), but that is the job of an OS, abstracting this kind of stuff for you (creating the executable's virtual address space in this case). So it just looks like static variables are actually static, but only inside the executable's memory space. On the RAM things might be anywhere.

Finally, it is difficult to try to explain this to you because you'll have to understand how executable files work. A good start would be to search for some explanations regarding low-level programming, like stack frame, calling conventions, the Assembly language itself and how compilers use some well-known techniques to manage functions (scopes in general), global/static/local/constant variables, and the memory system (sections, the stack, etc.), and maybe some research into PE (and even ELF) files.