adding Multithreading pthread.h

no-reply@allegro.cc (CIRCLE) — Wed, 02 May 2007 22:15:35 +0000

I have been messing around for some time with a small game and now trying to add multithreading to the equation. Sure it is not needed but I just wanted to test it out this is what I got. Problem lies with in where at game start if just freezes my computer and I have to hit the reset button. I am unsure to what I am missing or making my mistake on, but right at program start it tries to do the multithreading area of the program which it shouldn't. Even removing the calls doesn't stop it from running.

skipping most of the non needed stuff Full program link can be added if needed

#SelectExpand
  1#include 
  2pthread_mutex_t threadsafe = PTHREAD_MUTEX_INITIALIZER;
  3for (a = 1; a < maxa; a++){
   xi[a] = rand() % 5 + 1;
   yi[a] = rand() % 5 + 1; 
   x[a] = rand() % 500 + 20;
   y[a] = rand() %400 + 20;
   c[a] = makecol(rand() % 253 + 1, rand() % 253 + 1, rand() % 253 + 1);
   }
 10void* thread0(void* data)
 11{
    int my_thread_id = *((int*)data);
   //For Pixelate Demo Addition
   while(lives > 0)
   {
      if (pthread_mutex_lock(&threadsafe))
         textprintf_ex(screen,font,SCREEN_W/2 - (4*19),SCREEN_H/2,makecol(255,255,255),-1,"Error: Thread mutex was locked");
      for (a = 1; a < 100; a++){
         if (x[a] > SCREEN_W - 4 - xi[a] || x[a] < 3 - xi[a])
            xi[a] = -xi[a];
         if (y[a] > Floors.y - 4 - yi[a] || y[a] < 13 - yi[a])
            yi[a] = -yi[a];
         x[a] = x[a] + xi[a];
         y[a] = y[a] + yi[a];
         //putpixel(active_page,x[a],y[a],c[a]);
         circlefill(active_page,x[a],y[a],BallSize / 5,c[a]);
         //putpixel(active_page,x[a] - xi[a],y[a] - yi[a],makecol(0,0,0));
         }
      if (pthread_mutex_unlock(&threadsafe))
         textprintf_ex(screen,font,SCREEN_W/2 - (4*19),SCREEN_H/2,makecol(255,255,255),-1,"Error: Thread mutex was unlocked");
      //For Pixelate demo addition
   }
   pthread_exit(NULL);
   return NULL;
 35}
 36int id;
  pthread_t pthread0;
  int threadid0 = 0;
  id = pthread_create(&pthread0, NULL, thread0, (void*)&threadid0);
 40while(!key[KEY_ESC] && lives > 0)
  {
      pthread_mutex_lock(&threadsafe);
      multicheck();
      singlecheck();
      movemouse();
      mousehold();
      drawback();
      checkdead();
      
      
      show_video_bitmap(active_page);
      if (active_page == buffer)
       active_page = buffer2;
      else
         active_page = buffer;           
      clear_to_color(active_page, makecol(0, 0, 0));
      pthread_mutex_unlock(&threadsafe);
      rest(1);
      
  }
 61pthread_mutex_destroy(&threadsafe);
  //destroy_bitmap(active_page);

  
  allegro_exit();
  return;
   
 68}
 69END_OF_MAIN()

Most of the multithreading was taken directly from a book I was reading a while back. Game Programming All In One pg 405-413 for those who have the book.

#SelectExpand
   1
   2#include 
   3#include "allegro.h"
   4
   5#define MODE GFX_AUTODETECT_FULLSCREEN
   6#define WIDTH 640
   7#define HEIGHT 480
   8#define BLACK makecol(0,0,0)
   9#define WHITE makecol(255,255,255)
  10
  11//define the sprite structure
  12typedef struct SPRITE
  13{
  14    int dir, alive;
  15    int x,y;
  16    int width,height;
  17    int xspeed,yspeed;
  18    int xdelay,ydelay;
  19    int xcount,ycount;
  20    int curframe,maxframe,animdir;
  21    int framecount,framedelay;
  22}SPRITE;
  23
  24//variables
  25BITMAP *buffer;
  26BITMAP *ballimg[32];
  27SPRITE theballs[2];
  28SPRITE *balls[2];
  29int done;
  30int n;
  31
  32//create a new thread mutex to protect variables
  33pthread_mutex_t threadsafe = PTHREAD_MUTEX_INITIALIZER;
  34
  35
  36void erasesprite(BITMAP *dest, SPRITE *spr)
  37{
  38    //erase the sprite
  39    rectfill(dest, spr->x, spr->y, spr->x + spr->width, 
  40        spr->y + spr->height, BLACK);
  41}
  42
  43void updatesprite(SPRITE *spr)
  44{
  45    //update x position
  46    if (++spr->xcount > spr->xdelay)
  47    {
  48        spr->xcount = 0;
  49        spr->x += spr->xspeed;
  50    }
  51
  52    //update y position
  53    if (++spr->ycount > spr->ydelay)
  54    {
  55        spr->ycount = 0;
  56        spr->y += spr->yspeed;
  57    }
  58
  59    //update frame based on animdir
  60    if (++spr->framecount > spr->framedelay)
  61    {
  62        spr->framecount = 0;
  63        if (spr->animdir == -1)
  64        {
  65            if (--spr->curframe < 0)
  66                spr->curframe = spr->maxframe;
  67        }
  68        else if (spr->animdir == 1)
  69        {
  70            if (++spr->curframe > spr->maxframe)
  71                spr->curframe = 0;
  72        }
  73    }
  74}
  75
  76//this version doesn't change speed, just direction
  77void bouncesprite(SPRITE *spr)
  78{
  79    //simple screen bouncing behavior
  80    if (spr->x < 0)
  81    {
  82        spr->x = 0;
  83        spr->xspeed = -spr->xspeed;
  84        spr->animdir *= -1;
  85    }
  86
  87    else if (spr->x > SCREEN_W - spr->width)
  88    {
  89        spr->x = SCREEN_W - spr->width;
  90        spr->xspeed = -spr->xspeed;
  91        spr->animdir *= -1;
  92    }
  93
  94    if (spr->y < 0)
  95    {
  96        spr->y = 0;
  97        spr->yspeed = -spr->yspeed;
  98        spr->animdir *= -1;
  99    }
 100
 101    else if (spr->y > SCREEN_H - spr->height)
 102    {
 103        spr->y = SCREEN_H - spr->height;
 104        spr->yspeed = -spr->yspeed;
 105        spr->animdir *= -1;
 106    }
 107}
 108
 109BITMAP *grabframe(BITMAP *source, 
 110                  int width, int height, 
 111                  int startx, int starty, 
 112                  int columns, int frame)
 113{
 114    BITMAP *temp = create_bitmap(width,height);
 115    int x = startx + (frame % columns) * width;
 116    int y = starty + (frame / columns) * height;
 117    blit(source,temp,x,y,0,0,width,height);
 118    return temp;
 119}
 120
 121void loadsprites()
 122{
 123    BITMAP *temp;
 124
 125    //load sprite images
 126    temp = load_bitmap("sphere.bmp", NULL);
 127    for (n=0; n<32; n++)
 128        ballimg[n] = grabframe(temp,64,64,0,0,8,n);
 129    destroy_bitmap(temp);
 130
 131    //initialize the sprite
 132    for (n=0; n<2; n++)
 133    {
 134        balls[n] = &theballs[n];
 135        balls[n]->x = rand() % (SCREEN_W - ballimg[0]->w);
 136        balls[n]->y = rand() % (SCREEN_H - ballimg[0]->h);
 137        balls[n]->width = ballimg[0]->w;
 138        balls[n]->height = ballimg[0]->h;
 139        balls[n]->xdelay = 0;
 140        balls[n]->ydelay = 0;
 141        balls[n]->xcount = 0;
 142        balls[n]->ycount = 0;
 143        balls[n]->xspeed = 5;
 144        balls[n]->yspeed = 5;
 145        balls[n]->curframe = rand() % 32;
 146        balls[n]->maxframe = 31;
 147        balls[n]->framecount = 0;
 148        balls[n]->framedelay = 0;
 149        balls[n]->animdir = 1;
 150    }
 151}
 152
 153//this thread updates sprite 0
 154void* thread0(void* data)
 155{
 156    //get this thread id
 157    int my_thread_id = *((int*)data);
 158
 159    //thread's main loop
 160    while(!done)
 161    {
 162        //lock the mutex to protect variables
 163        if (pthread_mutex_lock(&threadsafe))
 164            textout(buffer,font,"ERROR: thread mutex was locked",0,0,WHITE);
 165        
 166        //erase sprite 0
 167        erasesprite(buffer, balls[0]);
 168
 169        //update sprite 0
 170        updatesprite(balls[0]);
 171
 172        //bounce sprite 0
 173        bouncesprite(balls[0]);
 174
 175        //draw sprite 0
 176        draw_sprite(buffer, ballimg[balls[0]->curframe], 
 177            balls[0]->x, balls[0]->y);
 178
 179        //print sprite number
 180        textout(buffer, font, "0", balls[0]->x, balls[0]->y,WHITE);
 181
 182        //display sprite position
 183        textprintf(buffer,font,0,10,WHITE,"THREAD ID %d, SPRITE (%3d,%3d)",
 184            my_thread_id, balls[0]->x, balls[0]->y);
 185
 186        //unlock the mutex
 187        if (pthread_mutex_unlock(&threadsafe))
 188            textout(buffer,font,"ERROR: thread mutex unlock error",0,0,WHITE);
 189
 190        //slow down (this thread only!)
 191      rest(10);
 192    }
 193
 194    // terminate the thread
 195    pthread_exit(NULL);
 196
 197    return NULL;
 198}
 199
 200//this thread updates sprite 1
 201void* thread1(void* data)
 202{
 203    //get this thread id
 204    int my_thread_id = *((int*)data);
 205
 206    //thread's main loop
 207    while(!done)
 208    {
 209        //lock the mutex to protect variables
 210        if (pthread_mutex_lock(&threadsafe))
 211            textout(buffer,font,"ERROR: thread mutex was locked",0,0,WHITE);
 212
 213        //erase sprite 1
 214        erasesprite(buffer, balls[1]);
 215
 216        //update sprite 1
 217        updatesprite(balls[1]);
 218
 219        //bounce sprite 1
 220        bouncesprite(balls[1]);
 221
 222        //draw sprite 1
 223        draw_sprite(buffer, ballimg[balls[1]->curframe], 
 224            balls[1]->x, balls[1]->y);
 225
 226        //print sprite number
 227        textout(buffer, font, "1", balls[1]->x, balls[1]->y,WHITE);
 228
 229        //display sprite position
 230        textprintf(buffer,font,0,20,WHITE,"THREAD ID %d, SPRITE (%3d,%3d)",
 231            my_thread_id, balls[1]->x, balls[1]->y);
 232
 233        //unlock the mutex
 234        if (pthread_mutex_unlock(&threadsafe))
 235            textout(buffer,font,"ERROR: thread mutex unlock error",0,0,WHITE);
 236
 237        //slow down (this thread only!)
 238      rest(20);
 239    }
 240
 241    // terminate the thread
 242    pthread_exit(NULL);
 243
 244    return NULL;
 245}
 246
 247//program's primary thread
 248void main(void)
 249{
 250    int id;
 251    pthread_t pthread0;
 252    pthread_t pthread1;
 253    int threadid0 = 0;
 254    int threadid1 = 1;
 255
 256    //initialize
 257    allegro_init();
 258    set_color_depth(16);
 259    set_gfx_mode(MODE, WIDTH, HEIGHT, 0, 0);
 260    srand(time(NULL));
 261    install_keyboard();
 262    install_timer();
 263
 264    //create double buffer
 265    buffer = create_bitmap(SCREEN_W,SCREEN_H);
 266
 267    //load ball sprite 
 268    loadsprites();
 269
 270    //create the thread for sprite 0
 271    id = pthread_create(&pthread0, NULL, thread0, (void*)&threadid0);
 272
 273    //create the thread for sprite 1
 274    id = pthread_create(&pthread1, NULL, thread1, (void*)&threadid1);
 275
 276    //main loop
 277    while (!key[KEY_ESC])
 278    {
 279        //lock the mutex to protect double buffer
 280        pthread_mutex_lock(&threadsafe);
 281
 282        //display title
 283        textout(buffer, font, "MultiThread Program (ESC to quit)", 0, 0, WHITE);
 284
 285        //update the screen
 286        acquire_screen();
 287        blit(buffer,screen,0,0,0,0,SCREEN_W-1,SCREEN_H-1);
 288        release_screen();
 289
 290        //unlock the mutex
 291        pthread_mutex_unlock(&threadsafe);
 292
 293        //note there is no delay
 294    }
 295
 296    //tell threads it's time to quit
 297    done++;
 298    rest(100);
 299
 300    //kill the mutex (thread protection)
 301    pthread_mutex_destroy(&threadsafe);
 302    
 303    //remove objects from memory
 304    destroy_bitmap(buffer);
 305
 306    //delete sprites
 307    for (n=0; n<32; n++)
 308        destroy_bitmap(ballimg[n]);
 309
 310    return;
 311}
 312
 313END_OF_MAIN()

http://www.geocities.com/jensen_305/Fallingballerr.zip

no-reply@allegro.cc (CGamesPlay) — Wed, 02 May 2007 23:37:39 +0000

~~pthread_mutex_lock and pthread_mutex_unlock return 0 if they were successful but you treat that as an error condition.~~ This is wrong, you don't treat it as an error condition.
Not to mention that your textprintf'ing to the screen when the mutex fails to lock defeats the purpose of using the mutex (you are using a mutex to say that it is okay to use Allegro's resources, so if the mutex locking fails, you can't use Allegro's resources).
Also, note that pthread_mutex_lock will not fail because the mutex was already locked. It will only fail if the mutex was destroyed. pthread_mutex_unlock will also only ever fail if the mutex was destroyed.
Your call to pthread_exit isn't needed, also.

Quote:

Game Programming All In One pg 405-413 for those who have the book.

We hear a ton of horrible programming practices gotten from this book.

[edit]
Edited for correctness.

no-reply@allegro.cc (GullRaDriel) — Wed, 02 May 2007 23:53:27 +0000

Nothing else to say than what CGamesPlay said. Too slow :p

no-reply@allegro.cc (CIRCLE) — Thu, 03 May 2007 08:42:18 +0000

Fixed a few things. But still remains that right at start the multithreaded part of the program is trying to start and just freezes. I used a graphical feature to show what was being multithreaded. At the start of the game is a main menu the "bouncing balls" should be applied when the game is started but instead they are starting at the main screen it gets (guessing) 3-5 loops and locks up. What I do not understand is what am I doing that starts the calls for the multithreaded parts of my program.

Compiles no errors just locks up on run.

no-reply@allegro.cc (GullRaDriel) — Thu, 03 May 2007 12:41:54 +0000

Yeah, you must have a dead lock somewhere in your code. That is typically why threaded program froze.

no-reply@allegro.cc (CIRCLE) — Thu, 03 May 2007 12:58:18 +0000

What should I be looking for in my program. What kind of things might cause this. It really doesn't effect the game play to have it out now, but perhaps later when there is more going on I may want to have it implemented just in case.

I code in a simple form "Newbish" so to speak.

1if (Balls[n].y > Floors.y - BallSize)
  {
      spls = 0;
      splashed[n].x = Balls[n].x;
      splashed[n].y = Balls[n].y;
      s = n; //Mark location of Splash
      play_sample((SAMPLE *)data[EXPLODE2_WAV].dat, volume, pan, pitch, 0);
      splashes();
      dead[n] = 1; //mark ball as dead
      Balls[n].y = 0 + BallSize;
      dropball();
      lives--;
      FrC = FrC - 25; //Changes the floor color on dropped ball
      FbC = FbC + 25;
      FgC = FgC;
  }

Showing how simplistic I keep things.

Maybe I should add everything that adds multithreading until I get the error. Let me see what causes it exactly.

no-reply@allegro.cc (GullRaDriel) — Thu, 03 May 2007 12:59:47 +0000

You should add some TRACE(...) in your code to see where you stop.

Step by step debugging could help too.

no-reply@allegro.cc (CIRCLE) — Thu, 03 May 2007 13:06:08 +0000

Trace is new to me should try it out. I am fairly new to the programming thing and havent done any debugging stuff with my compiler yet will have to try that aswell.

hitting the post button to test

ok with just this line commented out the program works.

circlefill(active_page,x[a],y[a],BallSize / 5,c[a]);

if I add
textprintf_ex(screen,font,0,10,makecol(255,255,255),-1,"%i",x[1]);

instead of the circle movements it shows that it works but again starts right off the bat before it should. sorry I edit a lot.

no-reply@allegro.cc (Marco Radaelli) — Thu, 03 May 2007 20:53:50 +0000

Quote:

What kind of things might cause this.

A deadlock happens when two subjects cross they need for objects. Example, you have P1, P2 which are processes and R1, R2 which are resources.

P1 owns R1 and needs R2, P2 owns R2 and needs R1. You have a deadlock because none of the processes can go on until the other releases the resource it's owning.

no-reply@allegro.cc (ImLeftFooted) — Thu, 03 May 2007 23:49:49 +0000

I would suggest you skip threading entirely and then come back to it later.

If you can't do that, then I would start off with some dead simple thread programs and work your way up. This app is a tad complicated.

no-reply@allegro.cc (CIRCLE) — Fri, 04 May 2007 08:08:12 +0000

Taking the easy way is never my thing. I got it working and have isolated the problem. Now working around the problem is something else.

If I use...

circlefill(screen,xx[z],yy[z],BallSize / 5,c[z]);

instead of...

circlefill(active_page,xx[z],yy[z],BallSize / 5,c[z]);

So my next question is how do I tell the multithread to hold off and wait till the main program is done. I assumed that would be done with pthread_mutex_lock and pthread_mutex_unlock but I guess I am wrong.

Where would be a good place to learn about pthread?

no-reply@allegro.cc (ImLeftFooted) — Fri, 04 May 2007 08:14:28 +0000

It makes no sense whatsoever to use threads to handle paging.

no-reply@allegro.cc (CIRCLE) — Fri, 04 May 2007 08:25:47 +0000

I am using the thread to handle background graphics. Having some fun. The paging is done out side the thread.

        show_video_bitmap(active_page);
        if (active_page == buffer)
         active_page = buffer2;
        else
           active_page = buffer;           
        clear_to_color(active_page, makecol(0, 0, 0));

question was on how to tell the threaded area of my program to hold on while the main program uses a resource.

if I use

for (z = 1; z < 1000; z++){
        circlefill(active_page,xx[z],yy[z],BallSize / 5,c[z]);
        }

outside the threaded program it works fine. But the point is to get it to work inside the threaded area.

no-reply@allegro.cc (nonnus29) — Fri, 04 May 2007 09:03:57 +0000

Quote:

Taking the easy way is never my thing. I got it working and have isolated the problem. Now working around the problem is something else.

If you're not into 'easy things' then why are you asking these guys to write your code for you instead of you studying the relevant subject ie threads and communicating process with semaphores and mutexes?

no-reply@allegro.cc (CIRCLE) — Fri, 04 May 2007 09:25:45 +0000

Wow tons of help. I understand as to how they work not to why it is not working.

another example which SHOULD lock variables till it is done but doesn't

1void* thread0(void* data)
2{
    int my_thread_id = *((int*)data);
   //For Pixelate Demo Addition
   while (lives > 0)
   {
      pthread_mutex_lock(&threadsafe); 
      for (z = 1; z < 20; z++){
         if (xx[z] > SCREEN_W - 4 - xi[z] || xx[z] < 3 - xi[z])
            xi[z] = -xi[z];
         if (yy[z] > Floors.y - 4 - yi[z] || yy[z] < 13 - yi[z])
            yi[z] = -yi[z];
         xx[z] = xx[z] + xi[z];
         yy[z] = yy[z] + yi[z];
         //trying to get the active_page to work in here
         circlefill(active_page,xx[z],yy[z],BallSize / 5,c[z]);
         }
         pthread_mutex_unlock(&threadsafe);
         rest(1);
      //For Pixelate demo addition
   }
   //pthread_exit(NULL);
   return NULL;

but instead of helping find what seems to everyone else like a small mistake I get the run around.

no-reply@allegro.cc (nonnus29) — Fri, 04 May 2007 09:32:33 +0000

The point is it's not small, multi threading is really hard. Can anyone gurantee that allegro, or direct x or anything else isn't running a thread in the background that's causing deadlocks with your thread? I can't.

no-reply@allegro.cc (GullRaDriel) — Fri, 04 May 2007 11:02:55 +0000

I can. I use threads a lot with allegro, and all the deadlock I had were caused by me.

no-reply@allegro.cc (CIRCLE) — Fri, 04 May 2007 11:17:18 +0000

This also a deadlock caused by me. Simple comment out showed me what was doing it aswell.

If I just use a BITMAP that my main program does not have access to like "screen" then everything works fine. Only problem is it still slows the main program down where to my understanding each thread should have its own speed.

no-reply@allegro.cc (GullRaDriel) — Fri, 04 May 2007 11:26:53 +0000

Yeah, but if they both access the same critical section, they will wait each other to free the locked ressource before doing some stuff.

no-reply@allegro.cc (CIRCLE) — Fri, 04 May 2007 11:35:31 +0000

I was also under the impression when I pthread_mutex_lock it would lock the resources it was using till pthread_mutex_unlock was passed both my main loop and my thread use this. While screen works both can not use active_page even when locking for some reason.

If I could lock out the main loop from using the BITMAP active_page when the thread was using it things would be easier and vice versa.

no-reply@allegro.cc (Kitty Cat) — Sat, 05 May 2007 02:24:07 +0000

Locking a mutex prevents other threads from locking the same mutex. If a thread locks a mutex, and another tries to lock the same mutex, the second will block (wait) until the first thread unlocks the mutex. THis makes it so two threads can't touch the same data at the same time.

Quote:

my understanding each thread should have its own speed.

When two threads handle the same data, they have to do syncronization (usually through mutexes) so they don't step on each others toes. This quickly leads to performance degradation on both parts. Using threads is not a recipe for speed. It's a tool to help make more efficient use of resources when properly handled.

no-reply@allegro.cc (CIRCLE) — Sat, 05 May 2007 20:35:09 +0000

Thanks to those that helped and gave advice. I was able to get my program to work. Although not entirely the way I wanted but work none the less.

To GullRaDriel for pointing out that I may have a deadlock. Once figured out where was fairly easy to fix. Thanks.

To Marco Radaelli for the explination of exactly what happens when a deadlock occurs. Thanks

To Dustin Dettmer helping to get me to realize I am over my head

To nonnus29 with that awsome link with all sorts of information about threads and much more. Thanks.

Thanks for all the help.

-Consider this thread closed.

1	if (Balls[n].y > Floors.y - BallSize)
2	{
3	spls = 0;
4	splashed[n].x = Balls[n].x;
5	splashed[n].y = Balls[n].y;
6	s = n; //Mark location of Splash
7	play_sample((SAMPLE *)data[EXPLODE2_WAV].dat, volume, pan, pitch, 0);
8	splashes();
9	dead[n] = 1; //mark ball as dead
10	Balls[n].y = 0 + BallSize;
11	dropball();
12	lives--;
13	FrC = FrC - 25; //Changes the floor color on dropped ball
14	FbC = FbC + 25;
15	FgC = FgC;
16	}

1	void* thread0(void* data)
2	{
3	int my_thread_id = ((int)data);
4	//For Pixelate Demo Addition
5	while (lives > 0)
6	{
7	pthread_mutex_lock(&threadsafe);
8	for (z = 1; z < 20; z++){
9	if (xx[z] > SCREEN_W - 4 - xi[z] \|\| xx[z] < 3 - xi[z])
10	xi[z] = -xi[z];
11	if (yy[z] > Floors.y - 4 - yi[z] \|\| yy[z] < 13 - yi[z])
12	yi[z] = -yi[z];
13	xx[z] = xx[z] + xi[z];
14	yy[z] = yy[z] + yi[z];
15	//trying to get the active_page to work in here
16	circlefill(active_page,xx[z],yy[z],BallSize / 5,c[z]);
17	}
18	pthread_mutex_unlock(&threadsafe);
19	rest(1);
20	//For Pixelate demo addition
21	}
22	//pthread_exit(NULL);
23	return NULL;