C++: Safe to use longjmp and setjmp?

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+4 Q:

C++: Safe to use longjmp and setjmp?

Hi,

Is it safe to use longjmp and setjmp in C++ on linux/gcc with regards to the following?

Exception handling (I'm not implementing exception handling using longjmp/setjmp. I want to know what side effects longjmp/setjmp will have on standard exception handling)
*this pointer
Signals
Smart pointers (boost's shared and intrusive pointers)
Anything else you can think of.

Thanks, James

+22 A:

setjmp()/longjmp() completely subvert stack unwinding and therefore exception handling as well as RAII (destructors in general).

From 18.7/4 "Other runtime support" in the standard:

If any automatic objects would be destroyed by a thrown exception transferring control to another (destination) point in the program, then a call to longjmp(jbuf, val) at the throw point that transfers control to the same (destination) point has undefined behavior.

So the bottom line is that setjmp()/longjmp() do not play well in C++.

Michael Burr 2009-09-03 21:36:55

Can you explain how longjmp messes with explicit memory deletion and destructors?

jameszhao00 2009-09-03 22:08:38

Generally, whenever there's some way to exit a scope in C++ (return, throw, or whatever), the compiler will place instructions to call the dtors for any automatic variables that need to be destroyed as a result of leaving that block. `longjmp()` just jumps to a new location in the code, so it will not provide any chance for the dtors to be called. The standard is actually less specific than that - the standard doesn't say that dtors won't be called - it says that all bets are off. You can't depend on any particular behavior in this case.

Michael Burr 2009-09-03 22:37:58

I'm not too familiar with smart pointers. If I have a smart pointer on the stack, when I do a longjmp does that mean the smart pointer won't remove a refcount? Also, am I safe if I disable exceptions and write code that doesn't go crazy with longjmp?

jameszhao00 2009-09-03 22:52:25

Since smart pointers depend on being destroyed, you will get undefined behavior. It's likely that that undefined behavior would include a refcount not getting decremented. You're 'safe' using `longjmp()` as long as you don't longjmp out of code that should cause dtors to be invoked. However, as David Thornley noted in a comment, setjmp()/longjmp() can be tricky to use right even in straight C - in C++ they're downright dangerous. Avoid them if at all possible.

Michael Burr 2009-09-03 23:08:48

@jameszhao00: If you don't know smart pointers well enough, (get familiar with them ASAP an for now) think other things: `std::vector` and `std::string` not freeing its memory, `std::fstream` not closing its file and things like this.

sbi 2009-09-04 08:14:48

Can you take a quick look at http://stackoverflow.com/questions/1381937/setjmp-longjmp-why-is-this-throwing-a-segfault ?

jameszhao00 2009-09-04 23:25:48

I have only learned about those commands, and never seen them in action in real applications.

IMHO, it's safe to say that it's not safe to use them: developers will not understand what those "unpopular" APIs do.

elcuco 2009-09-03 21:52:23

This is for a coroutines implementation (which require randomly jumping around)

jameszhao00 2009-09-03 21:55:33

Experienced developers will absolutely know what these "unpopular" APIs do. They have been a part of the standard C library for decades.

Novelocrat 2009-09-03 21:57:27

Experienced developers will absolutely know what these functions do. They will also know that they are tricky things to use right in C (although sometimes necessary), and extremely dangerous in C++.

David Thornley 2009-09-03 22:00:17

It's not specific to Linux or gcc; setjmp / longjmp and C++ don't work too well together if you're using longjmp to leave a context where there are automatic variables with destructors.

The destructors won't run which may result in a memory leak or other bad behaviour.

MarkR 2009-09-04 21:51:42

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C++: Safe to use longjmp and setjmp?

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