src/lib/wallclock/approx_time.c

   1 /* Copyright (c) 2003, Roger Dingledine
   2  * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
   3  * Copyright (c) 2007-2021, The Tor Project, Inc. */
   4 /* See LICENSE for licensing information */
   5
   6 /**
   7  * \file approx_time.c
   8  * \brief Cache the last result of time(), for performance and testing.
   9  **/
  10
  11 #include "orconfig.h"
  12 #include "lib/subsys/subsys.h"
  13 #include "lib/wallclock/approx_time.h"
  14 #include "lib/wallclock/wallclock_sys.h"
  15
  16 /* =====
  17  * Cached time
  18  * ===== */
  19
  20 #ifndef TIME_IS_FAST
  21 /** Cached estimate of the current time.  Updated around once per second;
  22  * may be a few seconds off if we are really busy.  This is a hack to avoid
  23  * calling time(NULL) (which not everybody has optimized) on critical paths.
  24  */
  25 static time_t cached_approx_time = 0;
  26
  27 /** Return a cached estimate of the current time from when
  28  * update_approx_time() was last called.  This is a hack to avoid calling
  29  * time(NULL) on critical paths: please do not even think of calling it
  30  * anywhere else. */
  31 time_t
  32 approx_time(void)
  33 {
  34   return cached_approx_time;
  35 }
  36
  37 /** Update the cached estimate of the current time.  This function SHOULD be
  38  * called once per second, and MUST be called before the first call to
  39  * get_approx_time. */
  40 void
  41 update_approx_time(time_t now)
  42 {
  43   cached_approx_time = now;
  44 }
  45 #endif /* !defined(TIME_IS_FAST) */
  46
  47 /**
  48  * Initialize the "wallclock" subsystem by setting the current cached time.
  49  **/
  50 static int
  51 subsys_wallclock_initialize(void)
  52 {
  53   update_approx_time(time(NULL));
  54   return 0;
  55 }
  56
  57 /**
  58  * Subsystem function table describing the "wallclock" subsystem.
  59  **/
  60 const subsys_fns_t sys_wallclock = {
  61   .name = "wallclock",
  62   SUBSYS_DECLARE_LOCATION(),
  63   .supported = true,
  64   /* Approximate time is a diagnostic feature, we want it to init right after
  65    * low-level error handling. */
  66   .level = -98,
  67   .initialize = subsys_wallclock_initialize,
  68 };