Does attaching to a process make it behave differently?

Question

While I am aware of the differences between debug and release builds, I am curious if attaching the debugger to a process (built release or debug) changes that processes behaviour?

For reference, I'm developing on HP 11.31 Itanium but still am curious for the general case.

Answer 1

http://en.wikipedia.org/wiki/Heisenbug#Heisenbug

Of course, attaching a debugger will change the timing (which can change eg thread race conditions), and also some system calls can detect if a debugger is attached.

Answer 2

It certainly can, depending on the platform and the method of debugging. For example, when debugging on Windows , there is actually the IsDebuggerPresent function. As noted that function can be circumvented, but then there are other means. So basically, it's complicated.

Answer 3

Yep, lots of things inside the Windows data structures change when a debugger is attached. It changes how memory is allocated/freed, it adds additional housekeeping code and "markers" on the stack (Ever noticed the F00D values in newly allocated memory) in fact many of the changes are used by anti-debuggers to detect if an application is being debugged.

In interpreted languages (Java, .NET) the runtime will often generate different machine instructions when running under a debugger to help it trap and display exceptions, show the original code, etc. It will usually generate unoptimized code as well when a debugger is attached.

Some of these changes affect the way the software behaves and can result complicate transient bugs that are caused by optimizations or extremely fine timinig dependencies.

Answer 4

Sure, in multithreaded apps, attaching a debugger can yield different result. However, how about the codes which are not related to threads?

I have seen a release build, which has a debugger attached, doesn't have problems. But, when a debugger is not attached, it has problems.
If it is launched first and a debugger is attached to it, it also shows the same problems.

Answer 5

Yes, I've often found that attaching a debugger to a process instantly makes bugs disappear, only to have them reappear when I compile my app in release mode. Unfortunately I usually can't really ask all my users to open a debugger just to run my app, so it can be quite frustrating.

Answer 6

Another thing to keep in mind is that for multithreaded apps attaching the debugger definitely can yield very different results. These are the kind of things referred to as "Heisenbugs."