Thread and conditional variable example

SDL Development
1

I think I’ve managed to hammer out how conditional variables are supposed to
be used in SDL… it seems to work. does anyone see anything fundamentally
wrong with it?

/**

  • Demonstration of mutexes, semaphores, and conditional variables
  • in SDL, by Tyler Montbriand, 2003.*
  • This code is freeware. Do whatever you want with it.
    */
    #include <SDL/SDL.h>
    #include <SDL/SDL_thread.h>
    #include <stdlib.h>

SDL_sem *sem=NULL;
SDL_mutex *mutex=NULL;
SDL_cond *launch=NULL;

/* Function that will be called by threads */
int zig(void *t);

int main(int argc, char *argv[])
{
int n;
const int zigs=4;
SDL_Thread *t[zigs];

if(SDL_Init(SDL_INIT_EVENTTHREAD)<0)
{
fprintf(stderr,“Couldn’t init SDL\n”);
return(1);
}

mutex=SDL_CreateMutex();
sem=SDL_CreateSemaphore(0);
launch=SDL_CreateCond();

if(mutex && launch && sem)
{
fprintf(stderr,“main] Take off every zig!\n”);

for(n=0; n<zigs; n++)
{
  t[n]=SDL_CreateThread(zig,(void *)n);

  if(t[n]!=NULL) SDL_SemWait(sem); /* wait until zig has locked mutex */
}

fprintf(stderr,"main] MOVE ZIG!\n");

SDL_mutexP(mutex);
  SDL_CondBroadcast(launch); /* move zig! */
SDL_mutexV(mutex);


for(n=0; n<zigs; n++)
  if(t[n]!=NULL) SDL_WaitThread(t[n],NULL);

fprintf(stderr,"main] You know what you doing\n");

}
else
fprintf(stderr,“Couldn’t create signalling devices\n”);

if(sem!=NULL) SDL_DestroySemaphore(sem);
if(mutex!=NULL) SDL_DestroyMutex(mutex);
if(launch!=NULL) SDL_DestroyCond(launch);
return(0);
}

int zig(void *t)
{
if((mutex==NULL)||(launch==NULL)||(sem==NULL)) return(-1);

SDL_mutexP(mutex); /* Need to lock mutex before calling SDL_CondWait */
{
fprintf(stderr,"%02d] Zig ready for launch.\n",t);

SDL_SemPost(sem); /* Notify main that we have locked mutex */

SDL_CondWait(launch,mutex); /* Wait until main triggers all threads */

/* Critical section - no other threads are running in this part */
{
  fprintf(stderr,"%02d] Zig launched!\n",t);
  SDL_Delay(100);
}

}
SDL_mutexV(mutex); /* exit critical section - let next zig launch */

SDL_Delay(1000); /* Can happen the same time as other threads */

fprintf(stderr,"%02d] Zig out of fuel\n",t);

return(0);
}