Program only crashes as release build -- how to debug?

No answer so far has tried to give a serious overview about the available techniques for debugging release applications:

1. Release and Debug builds behave differently for many reasons. Here is an excellent overview. Each of these differences might cause a bug in the Release build that doesn't exist in the Debug build.

2. The presence of a debugger may change the behavior of a program too, both for release and debug builds. See this answer. In short, at least the Visual Studio Debugger uses the Debug Heap automatically when attached to a program. You can turn the debug heap off by using environment variable _NO_DEBUG_HEAP . You can specify this either in your computer properties, or in the Project Settings in Visual Studio. That might make the crash reproducible with the debugger attached.

   More on debugging heap corruption here.

3. If the previous solution doesn't work, you need to catch the unhandled exception and attach a post-mortem debugger the instance the crash occurs. You can use e.g. WinDbg for this, details about the avaiable post-mortem debuggers and their installation at MSDN

4. You can improve your exception handling code and if this is a production application, you should:

   a. Install a custom termination handler using std::set_terminate

   If you want to debug this problem locally, you could run an endless loop inside the termination handler and output some text to the console to notify you that std::terminate has been called. Then attach the debugger and check the call stack. Or you print the stack trace as described in this answer.

   In a production application you might want to send an error report back home, ideally together with a small memory dump that allows you to analyze the problem as described here.

   b. Use Microsoft's structured exception handling mechanism that allows you to catch both hardware and software exceptions. See MSDN. You could guard parts of your code using SEH and use the same approach as in a) to debug the problem. SEH gives more information about the exception that occurred that you could use when sending an error report from a production app.

When I have encountered problems like this before it has generally been due to variable initialization. In debug mode, variables and pointers get initialized to zero automatically but in release mode they do not. Therefore, if you have code like this

int* p;
....
if (p == 0) { // do stuff }

In debug mode the code in the if is not executed but in release mode p contains an undefined value, which is unlikely to be 0, so the code is executed often causing a crash.

I would check your code for uninitialized variables. This can also apply to the contents of arrays.

In 100% of the cases I've seen or heard of, where a C or C++ program runs fine in the debugger but fails when run outside, the cause has been writing past the end of a function local array. (The debugger puts more on the stack, so you're less likely to overwrite something important.)