#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

int sum; //global variable used by both threads
void *runner(void *param); // the thread function

int main(int argc, char * argv[])
{
  // main is the initial thread of execution
  pthread_t tid; //the id of a created thread later
  pthread_attr_t attr; //attributes to create a thread

  if(argc != 2) { //do we have the right number of arguments?
    fprintf(stderr,"Syntax:./thread value\n");
    return -1;
  }
  if(atoi(argv[1]) < 0) { // we're doing a sum from 0 to "n"
    fprintf(stderr,"Argument %d must be >= 0\n",atoi(argv[1]));
    return -1;
  }
  
  pthread_attr_init(&attr); //set up the initial attributes

  //pthread_create creates a new thread
  //the id of the new thread is "tid"
  //the new thread will run the function "runner"
  //with an an argument "argv[1]"
  //the argument is passed as a void * and thus must be handled.
  // ALL arguments to a thread function must be an address
  pthread_create(&tid,&attr,runner,argv[1]); //create the new thread

  //pthread_join is like a "wait" This call says:
  //wait until the thread with id "tid" is finished before continuing
  //this thread of execution
  pthread_join(tid,NULL);
  printf("sum = %d\n",sum);
}


void *runner(void *param)
{
  int i,upper;
  char *p = ((char *)param); //transform the input param
  upper = atoi(p); //get the integer assoc with this string
  sum = 0;
  for(i = 1; i <= upper; i++) //calc sum from i = 1 to upper
    sum+=i;
  
  pthread_exit(0); // exit this thread
}