change macOS < 10.12 clock to SYSTEM_CLOCK, fixes #2537 (#2538)

* change macOS < 10.12 clock to SYSTEM_CLOCK, fixes #2537

* remove clock_id option from alt_clock_gettime since we always want a monotonic clock.

* update header definition for alt_clock_gettime

* pass clock definition down to host_get_clock_service for macOS < 10.12

* change to monotonic clocks

src/clock.cpp

@@ -60,15 +60,9 @@
 
 int alt_clock_gettime (int clock_id, timespec *ts)
 {
-    // The clock_id specified is not supported on this system.
-    if (clock_id != CLOCK_REALTIME) {
-        errno = EINVAL;
-        return -1;
-    }
-
     clock_serv_t cclock;
     mach_timespec_t mts;
-    host_get_clock_service (mach_host_self (), CALENDAR_CLOCK, &cclock);
+    host_get_clock_service (mach_host_self (), clock_id, &cclock);
     clock_get_time (cclock, &mts);
     mach_port_deallocate (mach_task_self (), cclock);
     ts->tv_sec = mts.tv_sec;
@@ -163,7 +157,7 @@ uint64_t zmq::clock_t::now_us ()
     struct timespec tv;
 
 #if defined ZMQ_HAVE_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED < 101200 // less than macOS 10.12
-    int rc = alt_clock_gettime (CLOCK_MONOTONIC, &tv);
+    int rc = alt_clock_gettime (SYSTEM_CLOCK, &tv);
 #else
     int rc = clock_gettime (CLOCK_MONOTONIC, &tv);
 #endif
@@ -250,7 +244,7 @@ uint64_t zmq::clock_t::rdtsc ()
 #else
     struct timespec ts;
     #if defined ZMQ_HAVE_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED < 101200 // less than macOS 10.12
-        alt_clock_gettime (CLOCK_MONOTONIC, &ts);
+        alt_clock_gettime (SYSTEM_CLOCK, &ts);
     #else
         clock_gettime (CLOCK_MONOTONIC, &ts);
     #endif

src/condition_variable.hpp

@@ -214,9 +214,9 @@ namespace zmq
                 struct timespec timeout;
 
 #if defined ZMQ_HAVE_OSX && __MAC_OS_X_VERSION_MIN_REQUIRED < 101200 // less than macOS 10.12
-                alt_clock_gettime(CLOCK_REALTIME, &timeout);
+                alt_clock_gettime(SYSTEM_CLOCK, &timeout);
 #else
-                clock_gettime(CLOCK_REALTIME, &timeout);
+                clock_gettime(CLOCK_MONOTONIC, &timeout);
 #endif
 
                 timeout.tv_sec += timeout_ / 1000;