Atomic Operations

Last Updated: May 28th, 2024

Semaphores operate at an operating system level. Atomic operations are a way to lock data at an efficient CPU level. Here we'll be locking a critical section using CPU spinlocks.

//Data access spin lock
SDL_SpinLock gDataLock = 0;

//The "data buffer"
int gData = -1;

Instead of a semaphore we'll be using a spinlock to protect our data buffer.

bool loadMedia()
{
    //Loading success flag
    bool success = true;
    
    //Load splash texture
    if( !gSplashTexture.loadFromFile( "48_atomic_operations/splash.png" ) )
    {
        printf( "Failed to load splash texture!\n" );
        success = false;
    }

    return success;
}

void close()
{
    //Free loaded images
    gSplashTexture.free();

    //Destroy window    
    SDL_DestroyRenderer( gRenderer );
    SDL_DestroyWindow( gWindow );
    gWindow = NULL;
    gRenderer = NULL;

    //Quit SDL subsystems
    IMG_Quit();
    SDL_Quit();
}

Unlike semaphores, spin locks do not need to be allocated and deallocated.

int worker( void* data )
{
    printf( "%s starting...\n", data );

    //Pre thread random seeding
    srand( SDL_GetTicks() );
    
    //Work 5 times
    for( int i = 0; i < 5; ++i )
    {
        //Wait randomly
        SDL_Delay( 16 + rand() % 32 );
        
        //Lock
        SDL_AtomicLock( &gDataLock );

        //Print pre work data
        printf( "%s gets %d\n", data, gData );

        //"Work"
        gData = rand() % 256;

        //Print post work data
        printf( "%s sets %d\n\n", data, gData );
        
        //Unlock
        SDL_AtomicUnlock( &gDataLock );

        //Wait randomly
        SDL_Delay( 16 + rand() % 640 );
    }

    printf( "%s finished!\n\n", data );

    return 0;
}

Here our critical section is protected by SDL_AtomicLock and SDL_AtomicUnlock.

In this case it may seem like semaphores and atomic locks are the same, but remember that semaphores can allow access beyond a single thread. Atomic operations are for when you want a strict locked/unlocked state.

Download the media and source code for this demo here.

Download SDL3 port of this demo (graciously provided by Sam Lantinga, creator of SDL) here.

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