#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/mman.h>
#include <string.h>
#include <sys/stat.h>

int main (int argc, char *argv[]) {
  struct stat st, s2t;

  int s = open(argv[1], O_RDONLY);
  if (s == -1) { perror("open source"); exit(1); }

  int s2 = open(argv[2], O_RDONLY);
  if (s2 == -1) { perror("open source2"); exit(1); }

  int d = open(argv[3], O_RDWR | O_CREAT | O_EXCL, 0644);
  if (d == -1) { perror("open destination"); exit(1); }

  if (fstat(s, &st)) { perror("stat source"); exit(1); }
  if (fstat(s2, &s2t)) { perror("stat source2"); exit(1); }
  if (ftruncate(d, st.st_size + s2t.st_size)) { perror("truncate destination"); exit(1); }

  void *sp = mmap(NULL, st.st_size, PROT_READ, MAP_SHARED, s, 0);
  if (sp == MAP_FAILED) { perror("map source"); exit(1); }
  madvise(sp, st.st_size, MADV_SEQUENTIAL);

  void *s2p = mmap(NULL, s2t.st_size, PROT_READ, MAP_SHARED, s2, 0);
  if (s2p == MAP_FAILED) { perror("map source2"); exit(1); }
  madvise(s2p, s2t.st_size, MADV_SEQUENTIAL);

  void *dp = mmap(NULL, st.st_size + s2t.st_size, PROT_WRITE | PROT_READ, MAP_SHARED, d, 0);
  if (dp == MAP_FAILED) { perror("map destination"); exit(1); }
  madvise(dp, st.st_size + s2t.st_size, MADV_SEQUENTIAL);

  memcpy(mempcpy(dp, sp, st.st_size), s2p, s2t.st_size);
  return 0;
}