Comment by haberman | Hacker News Reader

Comment by haberman | Hacker News Reader

I concur with the author, it's an absolute mess. And I would love to see a single unified API for this. It's been a wish of mine for many years.

Here are even more interfaces than the three the author listed (libunwind, __register_frame, GDB):

- oprofile has a JIT interface: http://oprofile.sourceforge.net/doc/devel/jit-interface.html

- perf has a JIT interface: https://github.com/torvalds/linux/blob/master/tools/perf/Doc...

I wrote a blog article about the craziness around unwinding a few years back: http://blog.reverberate.org/2013/05/deep-wizardry-stack-unwi...

One of the questions I answer in my article which isn't addressed as much here is: why do we care about generating backtraces in the first place? It might seem obvious, but I identified four separate reasons which are all important: to support runtime exceptions (a la C++), to offer backtraces in debuggers, to sample the stack for profilers, and to print a backtrace from the program itself. So if you have a JIT and you don't offer proper backtraces, your users are going to be annoyed when they use any of these tools.

You probably knew this, but just to add in case others weren't aware: Those reasons are the reasons you want unwinding on Unix, but on Windows it's even more important. The Windows equivalent of Unix signals (very roughly speaking) is Structured Exception Handling (SEH), in which the kernel actually arranges for your stack to be unwound to deliver you signals such as segfaults [1]. So unless you maintain proper unwinding info, your app isn't in a position to receive any signals sent by the kernel (at least, the way you're intended to).

[1]: http://www.nynaeve.net/?p=201

That blog looks very interesting, thanks.

I found out the hard way that Microsoft has two sets of ARM compilers for WinCE7, and only one of them generates code that's compatible with the SEH unwinder.

I don't think SEH actually calls destructors when unwinding C++, unlike C++ exceptions. This makes it leaky to use for anything other than fatal exceptions.

> I don't think SEH actually calls destructors when unwinding C++, unlike C++ exceptions. This makes it leaky to use for anything other than fatal exceptions.

How do you come to the conclusion that SEH is a leaky abstraction instead of the behavior of C++ destructors?

He means it causes resource leaks in any classes that implement RAII.

SEH was there before C++ was fully standardized (first standardized version was C++98). In this sense one can argue that the definition of RAII is leaky since it "forgot" that SEH was already there and not the other way round. A proper solution would have been that functions where RAII is used must not call functions that can throw an SEH exception.

All kinds of functions can throw SEH exceptions, because they're machine check exceptions (divide by zero, reference invalid page/null pointer).

I went and looked this up to refresh my memory: https://msdn.microsoft.com/en-us/library/swezty51.aspx

Microsoft recommend you don't mix the use of SEH exceptions with C++, or that you use the flag which turns SEH into C++ exceptions (and therefore calls destructors). In our case we didn't want to do that, because we're doing our own crash reporting and all we want is to log a backtrace while exiting the program, and do not want to call destructors which may themselves crash because the program is in an invalid state.

To me this all looks like stuff that is undefined behavior in C++, so a C++ program for which such a problem occurs is by definition invalid. In other words: You have a large problem in your code.

Like I said: it's crash logging code, to be used for detecting problems in the code. Undefined behaviour is not invalid: if C++ compilers rejected all UB it would be considerably easier to ship (harder to write!) reliable programs.

No: The C++ standard requires from programmer that it will not trigger UB. This is a kind of contract between the programmer and the compiler.

The compiler vendor is free to extend the standard and define UB.

C++ exception handling is implemented using SEH (so is CLR's IIRC, and Chakra's). SEH is implemented by the OS and is available to non-C++ code too (including C).

We mainly care about those reasons because many popular languages have exceptions and meaningful backtraces. If you remove exceptions, or add asynchronous/lazy/continuationful code, then the game becomes very different.

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