可以快速的在两个文件中拷贝数据，不需要将数据拷贝到用户态，然后再从用户态写入到另外一个文件中，典型的应用场景像Copy-On-Write(多个文件共享底层相同的数据块)、NFS等

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <unistd.h>

int
main(int argc, char **argv)
{
    int fd_in, fd_out;
    struct stat stat;
    off64_t len, ret;

    if (argc != 3) {
        fprintf(stderr, "Usage: %s <source> <destination>\\n", argv[0]);
        exit(EXIT_FAILURE);
    }

    fd_in = open(argv[1], O_RDONLY);
    if (fd_in == -1) {
        perror("open (argv[1])");
        exit(EXIT_FAILURE);
    }

    if (fstat(fd_in, &stat) == -1) {
        perror("fstat");
        exit(EXIT_FAILURE);
    }

    len = stat.st_size;

    fd_out = open(argv[2], O_CREAT | O_WRONLY | O_TRUNC, 0644);
    if (fd_out == -1) {
        perror("open (argv[2])");
        exit(EXIT_FAILURE);
    }

    do {
        ret = copy_file_range(fd_in, NULL, fd_out, NULL, len, 0);
        if (ret == -1) {
            perror("copy_file_range");
            exit(EXIT_FAILURE);
        }

        len -= ret;
    } while (len > 0 && ret > 0);

    close(fd_in);
    close(fd_out);
    exit(EXIT_SUCCESS);
}