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1//
2// time.cpp
3//
4// Copyright (c) Microsoft Corporation. All rights reserved.
5//
6// The timespec_get() and time() families of functions, which get the current
7// system time as a timespec and a time_t, respectively.
8//
9#include <corecrt_internal_time.h>
10
11
12using get_system_time_function_type = void(WINAPI *)(LPFILETIME);
13
14template <get_system_time_function_type GetSystemTimeFunction, typename TimeSpecType>
15_Success_(return != 0)
16static int __cdecl common_timespec_get(TimeSpecType* const ts, int const base) throw()
17{
18 typedef decltype(ts->tv_sec) time_type;
19 typedef __crt_time_time_t_traits<time_type> time_traits;
20
21 _VALIDATE_RETURN(ts != nullptr, EINVAL, 0);
22
23 if (base != TIME_UTC)
24 {
25 return 0;
26 }
27
28 __crt_filetime_union system_time{};
29 GetSystemTimeFunction(&system_time._filetime);
30
31 __time64_t const filetime_scale{10 * 1000 * 1000}; // 100ns units
32
33 __time64_t const epoch_time{static_cast<__time64_t>(system_time._scalar) - _EPOCH_BIAS};
34
35 __time64_t const seconds {epoch_time / filetime_scale};
36 __time64_t const nanoseconds{epoch_time % filetime_scale * 100};
37
38 if (seconds > static_cast<__time64_t>(time_traits::max_time_t))
39 {
40 return 0;
41 }
42
43 ts->tv_sec = static_cast<time_type>(seconds);
44 ts->tv_nsec = static_cast<long>(nanoseconds);
45 return base;
46}
47
48extern "C" int __cdecl _timespec32_get(_timespec32* const ts, int const base)
49{
50 return common_timespec_get<__acrt_GetSystemTimePreciseAsFileTime>(ts, base);
51}
52
53extern "C" int __cdecl _timespec64_get(_timespec64* const ts, int const base)
54{
55 return common_timespec_get<__acrt_GetSystemTimePreciseAsFileTime>(ts, base);
56}
57
58
59
60// Gets the current system time and converts it to a time_t value. If 'result'
61// is non-null, the time is stored in '*result'. The time is also returned
62// (even if 'result' is null).
63template <typename TimeType>
64static TimeType __cdecl common_time(TimeType* const result) throw()
65{
66 typedef __crt_time_time_t_traits<TimeType> time_traits;
67
68 typename time_traits::timespec_type ts{};
69
70 // The resolution of time() is in seconds, so we can afford to use
71 // a less precise, but faster method of getting the time.
72 // GetSystemTimePreciseAsFileTime uses QueryPerformanceCounter which
73 // needs to be virtualized on some systems in order to be accurate, which can
74 // be significantly slower than calling GetSystemTimeAsFileTime.
75 // For example, hypervisors will virtualize QPC calls in order to take into
76 // account virtual machine live migration.
77 // Note that calling the less precise API is still observable because the tick
78 // count may update +/- 15ms, but this should not affect common use of time().
79 // See https://docs.microsoft.com/en-us/windows/desktop/sysinfo/acquiring-high-resolution-time-stamps
80 // for more details.
81 if (common_timespec_get<GetSystemTimeAsFileTime>(&ts, TIME_UTC) != TIME_UTC)
82 {
83 ts.tv_sec = static_cast<TimeType>(-1);
84 }
85
86 if (result)
87 {
88 *result = ts.tv_sec;
89 }
90
91 return ts.tv_sec;
92}
93
94extern "C" __time32_t __cdecl _time32(__time32_t* const result)
95{
96 return common_time(result);
97}
98
99extern "C" __time64_t __cdecl _time64(__time64_t* const result)
100{
101 return common_time(result);
102}