highres_time_stamp.h File Reference

#include <stdint.h>

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Functions
uint64_t	get_highres_time_stamp (void)

Function Documentation

get_highres_time_stamp()

uint64_t get_highres_time_stamp

(

void

)

This function returns a high-resolution clock counter to be used to seed a random generating function (see split_mix_64.cpp). The purpose of high-resolution counter is that if the random-generating function is called subsequently in very short intervals (about nano-second), the seed value should be distinct.

In UNIX, such distinct seed values can be generated by std::clock() function which gives the processor counter. This function has enough resolution to generate distinct values if called subsequently. In MacOS, this function is available only after version 10.12.

In Windows, however, std::clock() is not a process counter, rather, it measures the wall clock with resoluton of one second! Thus, this function is not suitable at all on Windows. Instead, the QueryPerformanceCounter is used.

In Case if machine is neither POSIX compilant nor Windows, this function falls back to std::time and std::clock.

Note that this function works without C++11.

An alternative approach is to use rdtsc(), but its use is discouraged, since it is far less portable and many processors (like ARM64 archetecture) do not support it. Unlike rdtsc, this function works on both X86_64 and ARM64.

Definition at line 88 of file highres_time_stamp.cpp.

{
    // Zero means no proper function found for this OS and processor to
    // support high-precision time counting.
    uint64_t time_stamp = 0;
 
    #if defined(USE_CLOCK_GETTIME)
 
        // Using POSIX clock_gettime
        struct timespec clock;
        if (clock_gettime(CLOCK_REALTIME, &clock) == -1)
        {
            // One means there is an error, but the program continues without
            // raising an error.
            time_stamp = 1;
        }
 
        time_stamp = clock.tv_sec * 1e9 + clock.tv_nsec;
 
    #elif defined(USE_QUERY_PERFORMANCE_COUNTER)
 
        // Using Windows API for query performance counter
        LARGE_INTEGER ticks;
        if (!QueryPerformanceCounter(&ticks))
        {
            time_stamp = 1;
        }
        else
        {
            time_stamp = static_cast<uint64_t>(ticks.QuadPart);
        }
 
    #else
 
        // Use std::time and std::clock.
        // Note that std::time has low resolution second accuracy. To improve
        // it, we add std::clock, which has high-resolution (in UNIX only).
        // In UNIX, std::clock is the process time with high resolution. In
        // Windows, note well that std::clock is the wall time with "second"
        // resolution, and should be avoided.
        time_stamp = static_cast<uint64_t>(std::time(0)) +
                     static_cast<uint64_t>(std::clock());
 
    #endif
 
    return time_stamp;
}

Referenced by SplitMix64::SplitMix64().

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