Empty loop not responding to exit condition (also: how to multi-thread on Windows)

I had the following code

HANDLE * threadHandles
bool exitThreads

void threadEntryPoint(void * data)
{
    while (exitThreads == false){}
    _endthreadex(1);
}

void launchThreads(int nThreads)
{
    exitThreads = false;
    threadHandles = new HANDLE[nThreads];
    for (uint32_t i = 0; i < nThreads; ++i) {
        threadHandles[i] =
            (HANDLE)_beginthreadex(0,
            0,
            (uint32_t(__stdcall *)(void*))threadEntryPoint,
            NULL,
            0,
            0);
    }
}

void killThreads(int nThreads)
{
    exitThreads = true;
    for (uint32_t i = 0; i < nThreads; ++i) {
        WaitForSingleObject(threadHandles[i], INFINITE);
        CloseHandle(threadHandles[i]);
    }
    delete[] threadHandles;
}

This code will launch nThreads threads which will loop pointlessly until you call killThreads(). Ignore why I chose beginthreadex() over beginthread(). This is a good way to max out the cores on your CPU.

Conceptually this should work. And yet it didn't. The program was hanging it seemed as there was none of the expected output. Thankfully I had only launched it with 3 threads on a quadcore machine - I suspect launching it with 4 would've caused the machine to hang. The first thing I tried was debug mode. Multi-threading can get tricky, so might as well start by debugging it. Oddly enough it worked as expected in debug mode. That was weird, maybe the threads aren't printing properly? I then polluted my code with a bunch of cout statements with flushes in release mode, but I found that it would get to WaitForSingleObject() and then stop printing. So for whatever reason the threads were not exiting.

A quick google search turned up this beauty. It seems that the compiler tried to optimize the release version by not loading the value of exitThreads every loop iteration and instead assume it's always false. By adding the the keyword volatile before the bool exitThreads declaration, the compiler is forced to have the loop load the value every iteration - because the value is possibly volatile. I was aware of this concept before, but it's not something I deal with often so it's easy to slip the mind.

Project Euler #10: Summation of primes

This post is copied from another blog I run. I'll be shutting that blog down shortly and all future posts of that kind will go on this blog. These are solutions to various programming brain teasers. Optimizing your solution is the real challenge.

The problem reads as follows:

The sum of the primes below 10 is 2 + 3 + 5 + 7 = 17.
Find the sum of all the primes below two million.

Right from the start I can think of two methods:

1. Using a similar solution as for PE-7 but adding the primes instead of storing them
2. Sieve of Eratosthenes

I opted for the first method since I'm lazy. With minimal modifications, I get 173518 microseconds as my runtime. I was curious about the second method, so I copied and modified a solution found in the problem discussion thread. You can find that one under sieve.cpp. It has a runtime of 221631 microseconds. That's still pretty good.

The third way I can think of is to combine the first two methods. As with PE-7, automatically exclude all multiples of 2 and 3 for a 2/3 reduction in numbers considered. Then find all the primes below the square root of 2 million (so anything below and including 1415) and sieve on the remaining 1/3 with those prime numbers. Could result in a nice speedup over the first method. As usual, another thing you could do is to remove the use of the modulus operand - it's expensive.

The code can be found here 

Installing libclc on Ubuntu 14.04 LTS

libclc is needed for using Clang with OpenCL. See here for more details. Unfortunately it requires LLVM to be 3.7 or higher, while Ubuntu 14.04 only has up to 3.6.

Make sure you don't have LLVM installed before proceeding.To get around this you first have to get LLVM 3.7 by adding this line:

deb http://llvm.org/apt/precise/ llvm-toolchain-precise-3.7 main

 to this file:

/etc/apt/sources.list.d/llvm.list

I used nano. The original instructions using output redirection didn't work for me. You can find the address for other versions of Ubuntu or LLVM here. Now install llvm-3.7 and clang-3.7

sudo apt-get install llvm-3.7 clang-3.7

It will complain that it can't verify the source, but we added the source so go ahead and ignore it. Next get libclc if you don't already have it:

git clone http://llvm.org/git/libclc.git

Go through the readme and use llvm-config-3.7 instead of llvm-config. You may get an error like so:

/usr/bin/ld: cannot find -ledit

In this case just install libedit:

 sudo apt-get install libedit-dev

 However I still got many warnings of the following type:

WARNING: Linking two modules of different data layouts: 'amdgcn--/lib/image/write_image_impl.ll.tahiti.bc' is '' whereas 'llvm-link' is 'e-p:32:32-p1:64:64-p2:64:64-p3:32:32-p4:64:64-p5:32:32-p24:64:64-i64:64-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024-v2048:2048-n32:64'

A quick google search returned nothing. Because the data layout of the first module is empty I'm going to proceed with the assumption that I can ignore this.

Mixing C and C++

First, use g++ as it can compile C++ AND C - gcc cannot do that. If for whatever reason you need to use gcc, then just make object files to link with using g++ and the cpp files. 

If you're using malloc and get an error like this,

Process.cpp:45:53: error: invalid conversion from void* to char* [-fpermissive]
     char *linkCopy = malloc(strlen(&line[count]) + 1);

 it's because you need to cast the return value in C++ but not C. See this SO Q&A

Linker complains that a function doesn't exist when it does. First verify with nm that the function does indeed exist. You can use it on .o object and .a archive (static library) files. If it does, the your problem is you're trying to include a header for a C file in a C++ file. There's some problems with not enough information being generated from the C file compared to what the C++ file needs. See this SO Q&A for a solution

Issues when installing GPU Ocelot

Although I encountered all of these problems, most of them are not specific to GPU Ocelot

1. 'PTXLexer' is not a member of 'parser'
As per here, just edit ocelot/.release_build/ptxgrammar.hpp (make sure to use sudo) and comment out line 352.

 2. typedef 'GlobalMap' locally defined but not used
Turn off Werror by using "--no_werr" when running ./build.py

3. /usr/local/lib/libocelot.so: undefined reference to `tigetnum'
As per here, edit  SConscript (after running sudo ./build.py --install) on line 138 to add "-ltinfo" to ocelot_libs before it's used in OcelotConfig

4. /usr/bin/ld: cannot find -lboost_system-mt
According to this, -mt has been removed from Boost. As suggested I made the necessary symbolic links like so:

sudo ln -s /usr/lib/x86_64-linux-gnu/libboost_system.so /usr/lib/x86_64-linux-gnu
/libboost_system-mt.so

Skype calls or Steam are/is silencing my games/music/whatever

This isn't actually Skype's fault - it's Window's. The default setting in Windows is that a call will lower the volume of other applications by 80%! Thankfully, this is a quick fix. To solve this do the following

1. Right click on the volume icon (the little speaker in your notification tray)
2. Select "Sounds"
3. Choose the "Communications" tab
4. Pick whatever option you'd like.

For more details see here.

Excel hangs when selecting the file menu

Also known as "<insert MS Office product here> hangs when selecting the file menu"

What happened in my case was the printer was offline which causes a problem when you select the file menu. Why you ask? Because Excel tries to communicate with your printer to figure out the appropriate page margins or something of the sort. Of course since your printer is offline you can't do that, so the program hangs. Forever.

You can do one of two things to fix this:

1. Turn on your printer. You'll have to do this each time you encounter this problem.
2. Set the Microsoft XPS Document Writer as the default printer.

That's what fixed it for me.