Provide cgtimer_sub helper functions.

util.c

@@ -930,6 +930,17 @@ int cgtimer_to_ms(cgtimer_t *cgt)
 {
 	return timespec_to_ms(cgt);
 }
+
+/* Subtracts b from a and stores it in res. */
+void cgtimer_sub(cgtimer_t *a, cgtimer_t *b, cgtimer_t *res)
+{
+	res->tv_sec = a->tv_sec - b->tv_sec;
+	res->tv_nsec = a->tv_nsec - b->tv_nsec;
+	if (res->tv_nsec < 0) {
+		res->tv_nsec += 1000000000;
+		res->tv_sec--;
+	}
+}
 #else
 void cgtimer_time(cgtimer_t *ts_start)
 {
@@ -975,6 +986,11 @@ int cgtimer_to_ms(cgtimer_t *cgt)
 {
 	return timeval_to_ms(cgt);
 }
+
+void cgtimer_sub(cgtimer_t *a, cgtimer_t *b, cgtimer_t *res)
+{
+	timersub(a, b, res);
+}
 #endif
 
 void cgsleep_ms(int ms)

util.h

@@ -91,6 +91,7 @@ void cgtimer_time(cgtimer_t *ts_start);
 void cgsleep_ms_r(cgtimer_t *ts_start, int ms);
 void cgsleep_us_r(cgtimer_t *ts_start, int64_t us);
 int cgtimer_to_ms(cgtimer_t *cgt);
+void cgtimer_sub(cgtimer_t *a, cgtimer_t *b, cgtimer_t *res);
 double us_tdiff(struct timeval *end, struct timeval *start);
 double tdiff(struct timeval *end, struct timeval *start);
 bool stratum_send(struct pool *pool, char *s, ssize_t len);