In the C++ bag of tricks: scoped locks | Hacker News Reader

In the C++ bag of tricks: scoped locks

blog.skanev.org

43 pointss_kanev51 comments

The real advantage of a scoped lock is that it promises to release the mutex, no matter how the scope is exited. If this LOCKED macro is used and an exception is thrown from within the block, that mutex now stays locked.

> The only gotcha is, there is some overhead involved in this approach. The class instance takes up some space on the stack (several bytes) for every lock acquisition.

Not so fast. A decent compiler will eliminate the overhead and not actually allocate stack space for the lock. Here's an example using one of the lock guards in C++:

    struct foo {
      int var;
      std::mutex m;
    };
    
    void process1(foo& f) {
      std::lock_guard<std::mutex> lock(f.m);
      ++f.var;  
    }

    void process2(foo& f) {
      f.m.lock();
      ++f.var;  
      f.m.unlock();
    }

GCC 4.8.1 generates identical code for process1 and process2.

This. For at least three typical 'best practice' reasons. I know there are times best practices can be nicely ignored, but this is not one of those occasions. On the contrary. Let me lay them out again:

- these are the days of C++11 where scoped locks are readily available

- premature optimization

- macros are evil

One of the reasons C++ gets bad reputation, is those Cisms that people insist on writing, when the language offers much better alternatives.

Which is mostly due to lack of education. If you don't ever spent time learning, but just coding, you don't know better. And sometimes it's all too easy to get thrapped into it due to deadlines and whatnot. A former collegue of mine, unfortunately, was the best example: programmed for like 15 years years and still wrote like it was all 'C with classes' (not that there ever was such a thing). You know, writing declarations at the top of functions etc. Missing out on C++11 is just as bad.

Fully agree.

Personally I never liked straight C.

For me it was just a short transition between Turbo Pascal and C++, only to be used when required to do so. Around 1993 or so.

I devoured every book and publication about C++ I could put my hands on. Always trying to educate others how to write safe and portable C++.

Nowadays I am into JVM and .NET mostly, but when I reach for C++ it is with regard to the latest standards.

Every now and then, I still see C++ code that is basically the C subset without using any improvements of C++ over C.

C with classes is what the google c++ style guide mandates: http://google-styleguide.googlecode.com/svn/trunk/cppguide.x...

No exceptions, no boost, no lambda, no rtti, streams only for logging, avoid operator overloading, etc.

The Google C++ style guide has its quirks, but describing it as "C with classes" is flat out wrong.

> no boost

Not true, select parts of Boost are available.

> no lambda

The C++11 features are being rolled in gradually. Lambda expressions are now permitted.

> no rtti

The guide says "avoid RTTI" and urges the developer to come up with a different design. This is very much in line with C++ best practice overall. There's no ban on RTTI.

Version 3.274 says:

Do not use lambda expressions, or the related std::function or std::bind utilities.

It's unfortunately not synched with the internal version.

I don't agree with Google's style guide, besides they are not perfect.

There was a C with Classes.

http://www.stroustrup.com/bs_faq.html#invention

There is no reason to code in 2014 as if CFront was just released today.

Wait... if it's identical, how can it be safe against exceptions? `process2` isn't.

gcc can statically determine that incrementing f.var will not throw.

In addition to twoodfin's answer, I will point out that even if the code could throw, the code for the function would still be identical for when a throw did not occur (as in, we would see identical output with the addition of some code that would seemingly never be used or executed), on "good architectures", due to zero-cost exceptions (which only incur overhead during unwind; if no exception is thrown the code executed is exactly identical to if exceptions were not possible).

Yeah, they even admit that their macro compiles to identical code as the original RAII version. Where's the "overhead"?

As a general rule, if your C++ compiler can statically see the layout of your structures and the implementation of your functions, it will produce code as efficient as inlining everything yourself. Sufficiently complex functions make the benefits of inlining a judgment call on the compiler's part, but for something as simple as an RAII scoped lock, the abstraction is free.

Indeed. Sorry OP, your solution:

(1) avoids best practice (RAII) using...

(2) a flawed rationale (ignoring the existence of compiler optimizations)...

(3) without measurement to demonstrate the supposed flaw in the best practice (which would have surfaced the flawed rationale)...

(4) and therefore implements a micro-optimization with no actual win...

(5) and introduces a bug in the process (it's not exception safe).

OP here. Totally agree. TIL about RAII. That particular codebase doesn't use c++ exceptions, but still not a reason for a roll-your-own solution if something as efficient is in the standard library.

Does it use the STL? The STL will throw. Does it use the new operator? new will throw unless you specifically tell it not to.

Ok, should've been more specific -- it doesn't catch any exceptions and lets them bring down the program and have the project maintainer check what they did wrong.

I fail to see the point of using a macro (ew) to accomplish something that's included, with several different implementations, in the c++11 standard library

Don't do this. The whole thing gave off a bad smell on first reading so I would never have done this.

Thank you vinkelhake for nailing the the real problem (exceptions) and bothering to check whether the stack overhead really exists.

Just wanted to chime in with a comment because HN doesn't show vote counts, so future readers will "see" my upvote on vinkelhake's comment.

Exception issues aside, an odd definition of "scoped". A very non-C++ solution. Use RAII and lock_guard (if possible) as vinkelhake demonstrates. Consider,

    LOCKED(lk_var) {
        return 42; // whoops
    }

You're also consuming the variable name i, which everyone else is probably using.

Speaking of which, this means you can't have one of your locks scoped inside another one.

SUre you could. The{} make it work.

EDIT: I sit corrected. My post is wrong.

No, you can't, and no they don't.

    lock(lk_var) {
        lock(lk_var2) {
            var = var2;
        }
    }

That expands to:

    for(int i=0; (i < 1) && !lk_lock(lk_var); lk_unlock(lk_var), i++)
    {
        for(int i=0; (i < 1) && !lk_lock(lk_var2); lk_unlock(lk_var2), i++)
        {
            var = var2;
        }
    }

You end up with an i scoped inside another i.

C++ has variable shadowing, so that works correctly.

    for (int i = 0; i < 5; ++i) {
        for (int i = 0; i < 5; ++i) {
            printf("hello\n");
        }
    }

prints hello 25 times.

Oh right. My brain was stuck in another language. Wow, I forgot how much I'd forgotten about C++.

Actually that's just C (name nesting)

Solvable by creating a Preprocessor macro which creates a unique and complex-named identifier.

  #define LOCKED(lk) for(int UNIQUE_ID=0; UNIQUE_ID<1 && !lk_lock(lk); lk_unlock(lk), UNIQUE_ID++)

Use __LINE__ to help make the id unique.

Then you still can't forget and do this:

    lock(lk_var) lock(lk_var2)
    {
        lk_var = lk_var2;
    }

Or this:

    lock(lk_var) { lock(lk_var2 { lk_var = lk_var2; } }

Using the pre-processor to solve a regular, every day, hum-drum, solved-a-billion-times RAII problem is bad news.

This kind of thing is one area I'm excited to see work in Rust: you can use Rust's safety guarantees to ensure that all access goes through the lock.

Works, but requires {} for scope. The local-object version can be scoped however you like. It optimizes away just fine, similar to the for loop.

C++11 has scoped locks built-in, doesn't it?

It offers a variety of them, simplest of which is lock_guard:

http://en.cppreference.com/w/cpp/thread/lock_guard

What about:

     #define LOCKED(lk) for(int i=lk_lock(lk); i == 0; lk_unlock(lk), i++)

Probably better to test for 0 exactly, in case it fails. You don't want it to unlock twice just because it returns -1.

This is so wrong on so many levels.

It's not any worse than what was suggested in the linked article...

A very important gotcha with this approach is that any variable named "i" will be reset to 0 inside of the lock, and modifying "i" inside of the lock can cause an infinite loop.

This can be mitigated by generating a unique variable name with the C preprocessor: http://stackoverflow.com/questions/1132751/how-can-i-generat...

That being said, it's much better to use a standard method of scoped locking instead of trying to reinvent the wheel.

I learned about the for loop hack from avr-libc[1][2]. It even does some preprocessor abuse to implement different kinds of atomic and non-atomic sections.

[1] http://www.nongnu.org/avr-libc/user-manual/group__util__atom...

[2] http://svn.savannah.nongnu.org/viewvc/trunk/avr-libc/include...

Wouldn't this be exception-safe, provided lock_iter defines a ctor, dtor, operator bool() and operator ++()?

    #define LOCKED(lk) for (lock_iter i = lk; i; i++)

(Edit: I thought lock_iter could be an empty class but no, it can not, so I edited my post. lock_iter will have an internal flag that should be optimized away following the same logic int i is optimized in the OP's version)

People have already pointed out the specific problems with this approach so I won't repeat them.

It's worth reiterating, though: If you are writing c++ these days and find yourself reaching for a preprocessor macro, I think that's a good sign you should think really carefully about what you are actually trying to do. Chances are really good there is a (much) better way.

You can do some of this with the cleanup attribute in GCC, although it doesn't trigger in the case of a longjmp.

if you can rely on gcc, cleanup attribute is probably the best you can do. The solution in original post doesn't cope with "break", "continue", "return" (and of course, "goto", but I don't remember if cleanup attribute handles that either).

Looks like their project already uses C++ though, so why don't they just use C++ for this is beyond me.

Agreed.

Actually, it looks like C++ destructors don't run on longjmp either, though that's less of an issue since C++ actually has real exceptions.

longjmp is what the library and standards writers refer to as a "non-local goto" so it's kind of to be expected that it would manage to bypass destructors (I'm not sure how you'd unwind the stack enough to ensure that you've destructed everything created since the corresponding setjmp).

It's not terribly surprising, but it is worth noting since one use of setjmp/longjmp is to implement exceptions, and one would expect (and desire) destructors to run when unwinding on exceptions.

Boost.Scopexit addresses this point pretty well I think: http://www.boost.org/doc/libs/1_55_0/libs/scope_exit/doc/htm...

folly::Synchronized implements this in a correct way (and also prevents access without acquiring the lock):

SYNCHRONIZED(data) { // modify data }

https://github.com/facebook/folly/blob/master/folly/Synchron...

This is not exception-safe like scoped locks are.

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