simple-copy.cpp

#include <fcntl.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <cstring>
#include <iostream>

// 128 MiB buffer
constexpr size_t BUF_SIZE = 128 * 1024;
char buf[BUF_SIZE];

int main(int argc, char* argv[]) {
    if (argc != 3) {
        std::cerr << "Provide source and destination filenames\n";
        exit(1);
    }

    struct stat src_stat = {};
    if (stat(argv[1], &src_stat) < 0) {
        perror(argv[0]);
        exit(1);
    }

    int src_fd = open(argv[1], O_RDONLY);
    if (src_fd < 0) {
        perror(argv[0]);
        exit(1);
    }
    int dst_fd = open(argv[2], O_WRONLY | O_CREAT, src_stat.st_mode);
    if (dst_fd < 0) {
        perror(argv[0]);
        exit(1);
    }
    if (ftruncate(dst_fd, 0) < 0) {
        perror(argv[0]);
        exit(1);
    }

    while (true) {
        ssize_t bytes_read = read(src_fd, buf, BUF_SIZE);

        if (bytes_read < 0) {
            perror(argv[0]);
            exit(1);
        }

        ssize_t bytes_written = write(dst_fd, buf, bytes_read);

        if (bytes_written < 0) {
            perror(argv[0]);
            exit(1);
        }

        if (bytes_read < ssize_t(BUF_SIZE)) {
            break;
        }
    }

    // ignore the error if it happens
    close(src_fd);
    close(dst_fd);
}