Tag Archives: boost

Coroutines in C++/Boost

Starting with 1.56, boost/asio provides asio::spawn() to work with coroutines. Just paste the sample code here, with minor modifications:

The Python in my previous article can be used to work with the code above. I also tried to write a TCP server with only boost::coroutines classes. select() is used, since I want the code to be platform independent. NOTE: with coroutines, we have only _one_ thread.

Basic Usage of Boost MultiIndex Containers

Just take a simple note here.
The Boost Multi-index Containers Library provides a class template named multi_index_container which enables the construction of containers maintaining one or more indices with different sorting and access semantics.

Output:

To use with pointer values, only limited change needed as highlighted:

Spurious Wakeups

http://vladimir_prus.blogspot.com/2005/07/spurious-wakeups.html

One of the two basic synchronisation primitives in multithreaded programming is called “condition variables”. Here’s a small example:

Here, the call to “c.wait()” unlocks the mutex (allowing the other thread to eventually lock it), and suspends the calling thread. When another thread calls ‘notify’, the first thread wakes up, locks the mutex again (implicitly, inside ‘wait’), sees that variable is set to ‘true’ and goes on.

But why do we need the while loop, can’t we write:

We can’t. And the killer reason is that ‘wait’ can return without any ‘notify’ call. That’s called spurious wakeup and is explicitly allowed by POSIX. Essentially, return from ‘wait’ only indicates that the shared data might have changed, so that data must be evaluated again.

Okay, so why this is not fixed yet? The first reason is that nobody wants to fix it. Wrapping call to ‘wait’ in a loop is very desired for several other reasons. But those reasons require explanation, while spurious wakeup is a hammer that can be applied to any first year student without fail.

The second reason is that fixing this is supposed to be hard. Most sources I’ve seen say that fixing that would require very large overhead on certain architectures. Strangely, no details were ever given, which made me wonder if avoiding spurious wakeups is simple, but all the threading experts secretly decided to tell everybody it’s hard.

After asking on comp.programming.thread, I at least know the reason for Linux (thanks to Ben Hutchings). Internally, wait is implemented as a call to the ‘futex’ system call. Each blocking system call on Linux returns abruptly when the process receives a signal — because calling signal handler from kernel call is tricky. What if the signal handler calls some other system function? And a new signal arrives? It’s easy to run out of kernel stack for a process. Exactly because each system call can be interrupted, when glibc calls any blocking function, like ‘read’, it does it in a loop, and if ‘read’ returns EINTR, calls ‘read’ again.

Can the same trick be used to conditions? No, because the moment we return from ‘futex’ call, another thread can send us notification. And since we’re not waiting inside ‘futex’, we’ll miss the notification(A third thread can get it, and change the value of predicate. — gonwan). So, we need to return to the caller, and have it reevaluate the predicate. If another thread indeed set it to true, we’ll break out of the loop.

So much for spurious wakeups on Linux. But I’m still very interested to know what the original reasons were.

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Also see the explanation for spurious wakeups on the linux man page: pthread_cond_signal.
Last note: PulseEvent() in windows(manual-reset) = pthread_cond_signal() in linux, while SetEvent() in windows(auto-reset) = pthread_cond_broadcast() in linux, see here and here. And spurious wakeups are also possible on windows when using condition variables.

Exception Safety with shared_ptr

Code snippet:

Output:

Exception safety is ensured, when using shared_ptr. Memory allocated by m_a is freed even when an exception is thrown. The trick is: the destructor of class shared_ptr is invoked after the destructor of class C.

Writing UTF-8 String Using ofstream in C++

I’ve googled a lot to find the answer. But none really solve the problem simply and gracefully, even on stackoverflow. So we’ll do ourselves here 🙂

Actually, std::string supports operation using multibytes characters. This is the base of our solution:

g_cs is a Chinese word(“你好” which means hello) encoded in UTF-8. The code works under both Windows(WinXP+VS2005) and Linux(Ubuntu12.04+gcc4.6). You may wanna open a.txt to check whether the string is correctly written.

NOTE: Under Linux, we print the string directly since the default console encoding is UTF-8, and we can view the string. While under Window, the console DOES NOT support UTF-8(codepage 65001) encoding. Printing to it simply causes typo. We just convert it to a std::wstring and use MessageBox() API to check the result. I will cover the encoding issue in windows console in my next post, maybe.

I began to investigate the problem, since I cannot find a solution to read/write a UTF-8 string to XML file using boost::property_tree. Actually, it’s a bug and is already fixed in boost 1.47 and later versions. Unfortunately, Ubuntu 12.04 came with boost 1.46.1. When reading non-ASCII characters, some bytes are incorrectly skipped. The failure function is boost::property_tree::detail::rapidxml::internal::get_index(). My test code looks like:

Almost the same structure with the previous function. And finally the utf8_to_ucs2() function:

Please add header files yourselves to make it compile 🙂