IsDebuggerPresent – When To Attach a Debugger

This API is really helpful sometimes. And no, I’m not talking about using it for anti-debugging, com’on.
Suppose you have a complicated application that you would like to debug on special occasions.
Two concerns arise:
1 – you don’t want to always DebugBreak() at a certain point, which will nuke the application every time the code is running at that point (because 99% of the times you don’t have a debugger attached, or it’s a release code, obviously).
2 – on the other hand, you don’t want to miss that point in execution, if you choose you want to debug it.

An example would be, to set a key in the registry that each time it will be checked and if it is set (no matter the value), the code will DebugBreak().
A similar one would be to set a timeout, that on points of your interest inside the code, it will be read and wait for that amount of time, thus giving you enough time for attaching a debugger to the process.
Or setting an environment variable to indicate the need for a DebugBreak, but that might be a pain as well, cause environment blocks are inherited from parent process, and if you set a system one, it doesn’t mean your process will be affected, etc.
Another idea I can think of is pretty obvious, to create a file in some directory, say, c:\debugme, that the application will check for existence, and if so it will wait for attaching a debugger.

What’s in common for all the approaches above? Eventually they will DebugBreak or get stuck waiting for you to do the work (attaching a debugger).

But here I’m suggesting a different flow, check that a debugger is currently present, using IsDebuggerPresent (or thousands of other tricks, why bother?) and only then fire the DebugBreak. This way you can extend it to wait in certain points for a debugger-attach.

The algorithm would be:

read timeout from registry (or check an existence of a file, or whatever you’re up to. Which is most convenient for you)
if exists, while (timeout not passed)
if IsDebuggerPresent DebugBreak()
sleep(100) – or just wait a bit not to hog CPU

That’s it, so the application would always run normally, unless there’s some value set to hint you would like to attach a debugger in certain points, and if you don’t want to, it will timeout and continue normally. Of course, it’s possible to add some log messages, you will know it’s time to attach a debugger, in case you haven’t attached it earlier…

It’s always funny to see people call MessageBox, and then they attach a debugger, they then want to set a breakpoint at some function or instruction or even straight away at the caller itself, but can’t find that place easily without symbols or expertise. Instead, put a breakpoint at the end of the MessageBox function and step out of it.

Thanks to Yuval Kokhavi for this great idea. If you have a better idea or implementation please share it with us 😉

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3 Responses to “IsDebuggerPresent – When To Attach a Debugger”

  1. sylvainulg says:

    I guess that means there’s nothing like gdbserver in your environment … sigh.

  2. praddo says:

    On the other hand, you can use two separate builds and conditional compilation. For example define DEBUG in your debug build and use
    #ifdef DEBUG
    bla bla bla

  3. waliedassar says:

    I am not sure whether it is the best practice, but i usually do this for win32 builds, an infinite loop that follows a message box.

    jmp self

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