Merge remote-tracking branch 'tor-github/pr/280'

2 files changed, 24 insertions, 0 deletions

diff --git a/src/lib/time/tvdiff.c b/src/lib/time/tvdiff.c
index 8617110e52..bc8a1166e7 100644
--- a/src/lib/time/tvdiff.c
+++ b/src/lib/time/tvdiff.c
@@ -165,3 +165,25 @@ tv_to_msec(const struct timeval *tv)
   conv += ((int64_t)tv->tv_usec+500)/1000L;
   return conv;
 }
+
+/**
+ * Return duration in seconds between time_t values
+ * <b>t1</b> and <b>t2</b> iff <b>t1</b> is numerically
+ * less or equal than <b>t2</b>. Otherwise, return TIME_MAX.
+ *
+ * This provides a safe way to compute difference between
+ * two UNIX timestamps (<b>t2</b> can be assumed by calling
+ * code to be later than <b>t1</b>) or two durations measured
+ * in seconds (<b>t2</b> can be assumed to be longer than
+ * <b>t1</b>). Calling code is expected to check for TIME_MAX
+ * return value and interpret that as error condition.
+ */
+time_t
+time_diff(const time_t t1, const time_t t2)
+{
+  if (t1 <= t2)
+    return t2 - t1;
+
+  return TIME_MAX;
+}
+
diff --git a/src/lib/time/tvdiff.h b/src/lib/time/tvdiff.h
index d78330d7d8..a15ce52ad6 100644
--- a/src/lib/time/tvdiff.h
+++ b/src/lib/time/tvdiff.h
@@ -18,4 +18,6 @@ long tv_udiff(const struct timeval *start, const struct timeval *end);
 long tv_mdiff(const struct timeval *start, const struct timeval *end);
 int64_t tv_to_msec(const struct timeval *tv);
 
+time_t time_diff(const time_t from, const time_t to);
+
 #endif