How to atomically write a file without giving it a temporary name

tmpfile.cpp

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

// Can't get O_TMPFILE to work for some reason.
#ifndef __O_TMPFILE
# define __O_TMPFILE   (020000000 | __O_DIRECTORY)
#endif

int main() {
  int fd = open(".", __O_TMPFILE | O_RDWR, 0600);
  if (fd == -1) {
    perror("open failed");
    return 1;
  }
  if (12 != write(fd, "hello world\n", 12)) {
    perror("write failed");
    return 1;
  }
  char buf[800];
  sprintf(buf, "/proc/self/fd/%d", fd);
  if (-1 == linkat(AT_FDCWD, buf, AT_FDCWD, "helloworld.txt", AT_SYMLINK_FOLLOW)) {
    perror("linkat failed");
    return 1;
  }
  
  // note that linkat does not allow replacing existing files, so this is less
  // useful than it seems :/
}