/* 
   - Creation of the threads with pthread_create()
   - Exchange with the threads of a structure with different values for each thread
   - Exchange with the main of the returned value of a thread
   - Wait a thread with pthread_join()
   - Global variables and threads
*/

#include <stdio.h>
#include <unistd.h>
#include <ctype.h>
#include <pthread.h>

#define N_THREAD 3


/* Global variable, all the threads can see this variable, if a thread modify the value of the variable, all the other threads see the modified value. Important: variables cannot be modified by more than one thread at a time, otherwise a race condition is generated. */
int global_variable = 10;


typedef struct {
  int thread_num;
  char letter;
} my_t;


static void *process (void *arg)
{
  int i;
  my_t *param = (my_t *) arg;	// cast the void argument to the real type

  if (param->thread_num == 0) global_variable = 20;
  fprintf (stdout, "Starting thread %d, letter=%c, global_variable=%d\n", param->thread_num, param->letter, global_variable);
  for (i = 0; i < 10; i++) 
    write (STDOUT_FILENO, &(param->letter), 1); /* By default, when a process is executed the file descriptors number 0, 1 and 2 are automatically opened, and assigned to stdin, stdout and stderr, respectivelly */
  printf("\n");
  param->letter = toupper(param->letter);
  pthread_exit(&(param->letter));
}


int main (void)
{
  int retcode;
  pthread_t th[N_THREAD];
  char *retval;
  my_t value[N_THREAD];
  int i = 0;
  
  for(i=0; i<N_THREAD; i++){
    value[i].letter = 'a'+i;
  }

  for(i=0; i<N_THREAD; i++){
    value[i].thread_num = i;
    retcode = pthread_create (&th[i], NULL, process, (void *) &(value[i]));
    if (retcode != 0)
      fprintf (stderr, "pthread_create failed %d\n", retcode);
  }


  for(i=0; i<N_THREAD; i++){

    retcode = pthread_join (th[i], (void *)&retval);
    if (retcode != 0)
      fprintf (stderr, "pthread_join failed %d\n", retcode);

    fprintf(stdout, "retval : %c\n", *(retval) );
 }

  return 0;
}