multithreading - How to declare a thread id in c? -

i trying write 2 codes. 1 mutex , without mutex. in program, have 2 threads. while declaring thread id have declared pthread_t tid[2]; not getting why value 2 here? have got sample code. if make value 1 still works , gives me correct value. if make pthread_t tid[0], not work. if put value 10000, not getting right answer program. so, question value inside brace after tid means? have provided code below:

     #include<stdio.h> #include<string.h> #include<pthread.h> #include<stdlib.h> #include<unistd.h>  pthread_t tid[10000]; int counter;   void* dosomething(void *arg) {       unsigned = 0;     counter += 1;     printf("\njob %d started\n", counter);      for(i=0; i<(0xffffffff);i++); //     printf(" \njob %d finished\n", counter);      return null; }  int main() {     int = 0;     int err;     while(i < 2)     {         err = pthread_create(&(tid[i]), null, &dosomething, null);         if (err != 0)             printf("\ncan't create thread :[%s]", strerror(err));         i++;     }     //printf("value of pthread %d\n",err);      pthread_join(tid[0], null);     pthread_join(tid[1], null);      return 0; }