text: #ifndef TIMER_H
  #define TIMER_H
  
  #include <sys/time.h>
  #include "decls.h"
  
  #define USEC_PER_SEC    1000000L
  
  struct xtimer {
      struct timeval elapsed_time;
      struct timeval timestamp;
  };
  
  typedef struct xtimer   xtimer_t;
  
  __BEGIN_C_DECLS
  
  void timer_clear(xtimer_t *timer);
  void timer_start(xtimer_t *timer);
  void timer_stop(xtimer_t *timer);
  double timer_elapsed_time(xtimer_t *timer);
  
  __END_C_DECLS
  
  #endif  /* TIMER_H */
  