question about interleaving of threads #6405

#include <pthread.h>
// #include <stdbool.h>
// #include <math.h>

int data;

void* a_task( void* arg )
{
  int dummy;
  //--------------------------------------------------
  data = 10;
  dummy = 3;
  ++dummy;
  ++dummy;
  ++dummy;
  __ESBMC_assert( data == 10, "a: 10" );
  data = 20;
  ++dummy;
  ++dummy;
  ++dummy;
  ++dummy;
  __ESBMC_assert( data == 20, "a: 20" );
  data = 30;
  ++dummy;
  ++dummy;
  ++dummy;
  ++dummy;
  __ESBMC_assert( data == 30, "a: 30" );
  data = 40;
  ++dummy;
  ++dummy;
  ++dummy;
  ++dummy;
  __ESBMC_assert( data == 40, "a: 40" );
  //--------------------------------------------------
  return NULL;    
}

void* b_task( void* arg )
{
  //--------------------------------------------------
  data = 1;
  __ESBMC_assert( data == 1, "b: 1" );
  data = 2;
  __ESBMC_assert( data == 2, "b: 2" );
  data = 3;
  __ESBMC_assert( data == 3, "b: 3" );
  data = 4;
  __ESBMC_assert( data == 4, "b: 4" );
  //--------------------------------------------------
  return NULL;    
}

int main()
{
  pthread_t a_th, b_th;
  pthread_create( &a_th, NULL, a_task, NULL );
  pthread_create( &b_th, NULL, b_task, NULL );
  pthread_join( a_th, NULL );
  pthread_join( b_th, NULL );
}