Maybe const struct timespec *start, const struct timespec *stop?

Should it be "1000000000UL" instead of "1000000000"?

This code is correct only if stop > start. If start is 0 and stop is 1ns, then the code above will produce result->tv_sec = -1 and result->tv_nsec = 999999999. If denormalized results are alright, then if ((stop->tv_nsec - start->tv_nsec) < 0) is not necessary at all.

Should it be "1000000000UL" instead of "1000000000"?

It should be L, not UL, given that struct timespec::tv_nsec is of type long.

From man clock_gettime:

       The  res  and  tp  arguments  are  timespec structures, as specified in
       <time.h>:

           struct timespec {
               time_t   tv_sec;        /* seconds */
               long     tv_nsec;       /* nanoseconds */
           };

<sys/time.h> has a macro for calculating the difference of two timevals called timersub.
Modifying it for timespecs is straightforward.

// from <sys/time.h>
// used timersub macro, changed timeval to timespec
// kept the order of operands the same, that is a - b = result
# define timespec_diff_macro(a, b, result)                  \
  do {                                                \
    (result)->tv_sec = (a)->tv_sec - (b)->tv_sec;     \
    (result)->tv_nsec = (a)->tv_nsec - (b)->tv_nsec;  \
    if ((result)->tv_nsec < 0) {                      \
      --(result)->tv_sec;                             \
      (result)->tv_nsec += 1000000000;                \
    }                                                 \
  } while (0)

Or, as a function:

/**
 * @fn timespec_diff(struct timespec *, struct timespec *, struct timespec *)
 * @brief Compute the diff of two timespecs, that is a - b = result.
 * @param a the minuend
 * @param b the subtrahend
 * @param result a - b
 */
static inline void timespec_diff(struct timespec *a, struct timespec *b,
    struct timespec *result) {
    result->tv_sec  = a->tv_sec  - b->tv_sec;
    result->tv_nsec = a->tv_nsec - b->tv_nsec;
    if (result->tv_nsec < 0) {
        --result->tv_sec;
        result->tv_nsec += 1000000000L;
    }
}

<sys/time.h> has a macro for calculating the difference of two timevals called timersub.
Modifying it for timespecs is straightforward.

Thanks for that info. I just discovered that <sys/time.h> also has timespecsub() so you don't even need to modify it. It is a BSD function (actually a macro) and not a POSIX one. However, in linux we have libbsd to provide it.

The man pages includes many functions (or macros) which may also be interesting to you: man timespecsub

timerspecsub nor timersub are part of any standard.

timerspecsub nor timersub are part of any standard.

Yet they are useful, and relatively portable (Linux and BSDs).