fc.c

#define _GNU_SOURCE
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <limits.h>

int
main(int argc, char *argv[])
{
   int in_fd, out_fd, len;

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

   in_fd = open(argv[1], O_RDONLY);
   out_fd = open(argv[2], O_WRONLY);

   if (in_fd == -1) {
       perror("open");
       exit(EXIT_FAILURE);
   }
   if (out_fd == -1) {
       perror("open");
       exit(EXIT_FAILURE);
   }

   do {
       len = copy_file_range(in_fd, NULL, out_fd, NULL, INT_MAX, 0);

       if (len < 0) {
           if (errno == EAGAIN) {
               continue;
           }
           perror("copy_file_range");
           exit(EXIT_FAILURE);
       } else if (len == 0) {
           break;
       }
   } while (1);

   close(in_fd);
   close(out_fd);

   exit(EXIT_SUCCESS);
}