copy.cpp

#include <fcntl.h>
#include <unistd.h>
#include <cerrno>
#include <cstddef>
#include <cstdio>
#include <cstdlib>
#include <cstring>

#define ERROR(fmt, ...)                                         \
    do {                                                        \
        fprintf(stderr, "%s:%d: " fmt "\n", __FILE__, __LINE__, \
                ##__VA_ARGS__);                                 \
        exit(1);                                                \
    } while (false)

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

void copy_data(int src_fd, int dst_fd, size_t count) {
    size_t total_copied = 0;

    while (total_copied < count) {
        size_t to_copy = count > BUF_SIZE ? BUF_SIZE : count;

        ssize_t read_n = read(src_fd, buf, to_copy);
        if (read_n < 0) {
            ERROR("%s", strerror(errno));
        }

        ssize_t wrote_n = write(dst_fd, buf, read_n);
        if (wrote_n < 0) {
            ERROR("%s", strerror(errno));
        }

        if (size_t(read_n) < to_copy) {
            break;
        }

        total_copied += read_n;
    }
}

void copy_with_holes(int src_fd, int dst_fd) {
    off_t end = lseek(src_fd, 0, SEEK_END);
    if (end < 0) {
        ERROR("%s", strerror(errno));
    }

    if (end == 0) {
        return;
    }

    off_t next_data = 0;
    while (true) {
        off_t next_hole = lseek(src_fd, next_data, SEEK_HOLE);
        if (next_hole < 0) {
            ERROR("%s", strerror(errno));
        }

        if (lseek(src_fd, next_data, SEEK_SET) < 0) {
            ERROR("%s", strerror(errno));
        }

        copy_data(src_fd, dst_fd, next_data - next_hole);

        if (next_hole >= end) {
            break;
        }

        next_data = lseek(src_fd, next_hole, SEEK_DATA);
        if (next_data < 0) {
            ERROR("%s", strerror(errno));
        }

        if (lseek(dst_fd, next_data, SEEK_SET) < 0) {
            ERROR("%s", strerror(errno));
        }
    }
}

int main(int argc, char** argv) {
    if (argc != 3) {
        ERROR("Provide source and destionation files");
    }

    int src_fd = open(argv[1], O_RDONLY);
    if (src_fd < 0) {
        ERROR("%s: %s", argv[1], strerror(errno));
    }

    int dst_fd =
        open(argv[2], O_RDWR | O_CREAT, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH);
    if (dst_fd < 0) {
        ERROR("%s: %s", argv[2], strerror(errno));
    }
    if (ftruncate(dst_fd, 0) < 0) {
        ERROR("%s", strerror(errno));
    }

    copy_with_holes(src_fd, dst_fd);

    close(src_fd);  // ignore errors
    close(dst_fd);
}