shortreads.cc

/*
 * Copyright 2025 Marcus Müller
 * SPDX-license-identifier: EUPL-1.2
 *
 * Needs C++17 or later, and fmt, which you should easily be able to install from your
 * package manager.
 */

#include <fcntl.h>
#include <fmt/chrono.h>
#include <fmt/format.h>
#include <unistd.h>
#include <chrono>

constexpr unsigned int HOWMANY = 1'000'000;
constexpr unsigned int MAX_READ_SIZE = 16;

int main(int argc, char* argv[])
{
    std::string fname = "/dev/zero";
    if (argc > 1) {
        fname = argv[1];
    }
    int fd = open(fname.c_str(), O_RDONLY);
    auto start = std::chrono::high_resolution_clock::now();
    char readbuffer[MAX_READ_SIZE];
    int readlength = 0;
    for (auto counter = HOWMANY; counter; --counter) {
        readlength = read(fd, &readbuffer, MAX_READ_SIZE);
        if (readlength < 1) {
            return -1;
        }
        lseek(fd, 0, SEEK_SET);
    }
    auto end = std::chrono::high_resolution_clock::now();
    auto diff = std::chrono::duration_cast<std::chrono::nanoseconds>(end - start);
    fmt::print("{} per read()\n", diff / HOWMANY);
    fmt::print("{} last output\n", std::string(&readbuffer[0], readlength));
}

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