Can a C program modify its executable file?

Question

I had a little too much time on my hands and started wondering if I could write a self-modifying program. To that end, I wrote a \"Hello World\" in C, then used a hex editor

Answer 1

On newer versions of Windows CE (atleast 5.x an newer) where apps run in user space, (compared to earlier versions where all apps ran in supervisor mode), apps cannot even read it's own executable file.

Answer 2

There are non-portable ways to do this on many platforms. In Windows you can do this with WriteProcessMemory(), for example. However, in 2010 it's usually a very bad idea to do this. This isn't the days of DOS where you code in assembly and do this to save space. It's very hard to get right, and you're basically asking for stability and security problems. Unless you are doing something very low-level like a debugger I would say don't bother with this, the problems you will introduce are not worth whatever gain you might have.

Answer 3

If you operating on Windows, I believe it locks the file to prevent it from being modified while its being run. Thats why you often needs to exit a program in order to install an update. The same is not true on a linux system.

Answer 4

It is very operating system dependent. Some operating systems lock the file, so you could try to cheat by making a new copy of it somewhere, but the you're just running another compy of the program.

Other operating systems do security checks on the file, e.g. iPhone, so writing it will be a lot of work, plus it resides as a readonly file.

With other systems you might not even know where the file is.

Answer 5

All present answers more or less revolve around the fact that today you cannot easily do self-modifying machine code anymore. I agree that that is basically true for today's PCs.

However, if you really want to see own self-modifying code in action, you have some possibilities available:

• Try out microcontrollers, the simpler ones do not have advanced pipelining. The cheapest and quickest choice I found is an MSP430 USB-Stick

• If an emulation is ok for you, you can run an emulator for an older non-pipelined platform.

• If you wanted self-modifying code just for the fun of it, you can have even more fun with self-destroying code (more exactly enemy-destroying) at Corewars.

• If you are willing to move from C to say a Lisp dialect, code that writes code is very natural there. I would suggest Scheme which is intentionally kept small.

Answer 6

On Windows, when a program is run the entire *.exe file is mapped into memory using the memory-mapped-file functions in Windows. This means that the file isn't necessarily all loaded at once, but instead the pages of the file are loaded on-demand as they are accessed.

When the file is mapped in this way, another application (including itself) can't write to the same file to change it while it's running. (Also, on Windows the running executable can't be renamed either, but it can on Linux and other Unix systems with inode-based filesystems).

It is possible to change the bits mapped into memory, but if you do this the OS does it using "copy-on-write" semantics, which means that the underlying file isn't changed on disk, but a copy of the page(s) in memory is made with your modifications. Before being allowed to do this though, you usually have to fiddle with protection bits on the memory in question (e.g. VirtualProtect).

At one time, it used to be common for low-level assembly programs that were in very constrained memory environments to use self-modifying code. However, nobody does this anymore because we're not running in the same constrained environments, and modern processors have long pipelines that get very upset if you start changing code from underneath them.