Query Windows clock frequency and use reported max

[fio.git] / os / windows / dlls.c

diff --git a/os/windows/dlls.c b/os/windows/dlls.c
new file mode 100644 (file)
index 0000000..774b1c6
--- /dev/null
+++ b/os/windows/dlls.c
@@ -0,0 +1,33 @@
+#include "os/os.h"
+
+#include <windows.h>
+
+void os_clk_tck(long *clk_tck)
+{
+       /*
+        * The timer resolution is variable on Windows. Try to query it 
+        * or use 64 Hz, the clock frequency lower bound. See also
+        * https://carpediemsystems.co.uk/2019/07/18/windows-system-timer-granularity/.
+        */
+       unsigned long minRes, maxRes, curRes;
+       HMODULE lib;
+       FARPROC queryTimer;
+       FARPROC setTimer;
+
+       if (!(lib = LoadLibrary(TEXT("ntdll.dll"))) ||
+               !(queryTimer = GetProcAddress(lib, "NtQueryTimerResolution")) ||
+               !(setTimer = GetProcAddress(lib, "NtSetTimerResolution"))) {
+               dprint(FD_HELPERTHREAD, 
+                       "Failed to load ntdll library, set to lower bound 64 Hz\n");
+               *clk_tck = 64;
+       } else {
+               queryTimer(&minRes, &maxRes, &curRes);
+               dprint(FD_HELPERTHREAD, 
+                       "minRes = %lu, maxRes = %lu, curRes = %lu\n",
+                       minRes, maxRes, curRes);
+
+               /* Use maximum resolution for most accurate timestamps */
+               setTimer(maxRes, 1, &curRes);
+               *clk_tck = (long) (10000000L / maxRes);
+       }
+}
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