System: Add srs duration api

trunk/src/core/srs_core_time.cpp

@@ -23,3 +23,12 @@
 
 #include <srs_core_time.hpp>
 
+srs_utime_t srs_duration(srs_utime_t start, srs_utime_t end)
+{
+    if (start == 0 || end == 0) {
+        return 0;
+    }
+
+    return end - start;
+}
+

trunk/src/core/srs_core_time.hpp

@@ -36,6 +36,9 @@ typedef int64_t srs_utime_t;
 #define srsu2ms(us) ((us) / SRS_UTIME_MILLISECONDS)
 #define srsu2msi(us) int((us) / SRS_UTIME_MILLISECONDS)
 
+// Them time duration = end - start. return 0, if start or end is 0.
+srs_utime_t srs_duration(srs_utime_t start, srs_utime_t end);
+
 // The time unit in ms, for example 120 * SRS_UTIME_SECONDS means 120s.
 #define SRS_UTIME_SECONDS 1000000LL