date(1) 速すぎない？

date-iso.cpp

// g++ -std=c++11 -O3 date-iso.cpp -o date-iso

#include <chrono>
#include <cstdio>

using namespace std::chrono;

int main()
{
    auto now = system_clock::now();
    auto timet = system_clock::to_time_t(now);
    std::tm tm{};

    localtime_r(&timet, &tm);

    char buffer[27] = {0};
    std::strftime(buffer, 27, "%FT%T.%z\n", &tm);
    fwrite(buffer, 1, 20, stdout);

    auto nanosec = duration_cast<nanoseconds>(now.time_since_epoch()).count() % 1000000000LL;
    printf("%09lld", nanosec);

    fwrite(buffer + 20, 1, 6, stdout);
}

linux.sh-session

$ uname -a
Linux xxxxxxxx 5.12.8-arch1-1 #1 SMP PREEMPT Fri, 28 May 2021 15:10:20 +0000 x86_64 GNU/Linux


$ g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/11.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --enable-bootstrap --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-werror --with-build-config=bootstrap-lto --enable-link-serialization=1 gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 11.2.0 (GCC)


$ g++ -std=c++11 -O3 date-iso.cpp -o date-iso


$ time (repeat 10000 { ./date-iso > /dev/null })
( repeat 10000; do; ./date-iso > /dev/null; done; )  9.20s user 2.45s system 98% cpu 11.776 total
$ time (repeat 10000 { date +%Y-%m-%dT%H:%M:%S.%N%z > /dev/null })
( repeat 10000; do; date +%Y-%m-%dT%H:%M:%S.%N%z > /dev/null; done; )  4.95s user 1.52s system 98% cpu 6.562 total

macos.sh-session

$ uname -a
Darwin xxxxxxxx 20.5.0 Darwin Kernel Version 20.5.0: Sat May  8 05:10:33 PDT 2021; root:xnu-7195.121.3~9/RELEASE_X86_64 x86_64


$ g++ -v
Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1
Apple clang version 12.0.5 (clang-1205.0.22.11)
Target: x86_64-apple-darwin20.5.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin


$ g++ -std=c++11 -O3 date-iso.cpp -o date-iso


$ time (repeat 10000 { ./date-iso > /dev/null })
( repeat 10000; do; ./date-iso > /dev/null; done; )  7.38s user 9.74s system 70% cpu 24.201 total
$ time (repeat 10000 { gdate +%Y-%m-%dT%H:%M:%S.%N%z > /dev/null })
( repeat 10000; do; gdate +%Y-%m-%dT%H:%M:%S.%N%z > /dev/null; done; )  8.76s user 11.61s system 69% cpu 29.247 total

$ # 参考記録（macOSのdateは秒の小数点以下部分を表示できない）
$ time (repeat 10000 { date +%Y-%m-%dT%H:%M:%S%z > /dev/null })
( repeat 10000; do; date +%Y-%m-%dT%H:%M:%S%z > /dev/null; done; )  5.02s user 9.73s system 82% cpu 17.784 total


$ ./date-iso
2022-03-19T20:20:39.479847000+0900
$ gdate +%Y-%m-%dT%H:%M:%S.%N%z
2022-03-19T20:20:42.207666000+0900
$ date +%Y-%m-%dT%H:%M:%S%z
2022-03-19T20:20:46+0900