Wrote the Unix clock function. It's primitive, but it works.

test/meson.build

@@ -8,6 +8,8 @@ libui_test_sources = [
 
 if libui_OS == 'darwin'
 	libui_test_sources += ['testing_darwin.c']
+else
+	libui_test_sources += ['testing_unix.c']
 endif
 
 if libui_OS == 'windows'

test/testing_unix.c

@@ -0,0 +1,59 @@
+// 23 april 2019
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include "testing.h"
+
+struct testingTimer {
+	clock_t start;
+	clock_t end;
+};
+
+testingTimer *testingNewTimer(void)
+{
+	testingTimer *t;
+
+	t = (testingTimer *) malloc(sizeof (testingTimer));
+	// TODO handle failure
+	memset(t, 0, sizeof (testingTimer));
+	return t;
+}
+
+void testingFreeTimer(testingTimer *t)
+{
+	free(t);
+}
+
+void testingTimerStart(testingTimer *t)
+{
+	t->start = clock();
+}
+
+void testingTimerEnd(testingTimer *t)
+{
+	t->end = clock();
+}
+
+int64_t testingTimerNsec(testingTimer *t)
+{
+	clock_t c;
+	clock_t sec;
+	int64_t sec64;
+	clock_t subsec;
+	double subsecf;
+	int64_t subsec64;
+
+	c = t->end - t->start;
+
+	sec = c / CLOCKS_PER_SEC;
+	sec64 = (int64_t) sec;
+	sec64 *= testingTimerNsecPerSec;
+
+	subsec = c % CLOCKS_PER_SEC;
+	subsecf = (double) subsec;
+	subsecf /= CLOCKS_PER_SEC;
+	subsecf *= testingTimerNsecPerSec;
+	subsec64 = (int64_t) subsecf;
+
+	return sec64 + subsec64;
+}