Question

struct thread_info
{
    int       id;
    pthread_t thread_id;
    int       thread_num; 
    int       gpu;
};

struct thread_info *tinfo;

static void *GPUMon(void *userdata)
{
    struct thread_info *mythr = userdata;
    const int thread = mythr->id;

    while(1)
    {
        printf("Thread: %d\n", thread);
        Sleep(4000);
    }

    return NULL;
}

int main(void)
{
        struct thread_info *thr;

        tinfo = calloc(2, sizeof(*thr));
        for(int ka = 0; ka < 2; ka++)
        {           

            thr = &tinfo[ka];
            thr->id = ka;

            if (pthread_create(thr, NULL,GPUMon, thr))
            {
                return 0;
            }
        }   

                // some more code


    return 0;
}

Basically, the GPUMon thread should print the ID of my thread which is thr->id = ka; however what I get are enormously huge numbers such as

Thread: 7793336
Thread: 7792616

I do not understand what I am doing wrong.

Answer 1

pthread_create() takes a reference to pthread_id as its first argument.

So (as a quick&dirty fix) I’d propose you change struct thread_info to be:

struct thread_info
{
  pthread_t thread_id;
  int       id;
  int       thread_num; 
  int       gpu;
};

Or (much nicer) do call pthread_create() like this:

if (pthread_create(&thr->thread_id, NULL, GPUMon, thr))
{
  return 0;
}

The results your solution prints out are the values for the pthread_id assigned by pthread_create() (or at least parts of it depending on your platform) which have accidently been written into the member id of struct thread_info as you passed a pointer to the wrong structure.

And to say it again: At least when developing do turn all compiler warnings on as (also in this case they would have) they could point you to your mistake(s) (gcc option -Wall)