#include <unistd.h>
#include <fcntl.h>
#include <err.h>
#include <stdio.h>

static int failures;
static int successes;

#define CHECK(ret)                              \
  do                                            \
    if ((ret) < 0)                              \
      {                                         \
        printf ("error: %s:\n  %m\n", #ret);    \
        ++failures;                             \
      }                                         \
    else                                        \
      {                                         \
        printf ("success: %s\n", #ret);         \
        ++successes;                            \
      }                                         \
  while (0)

static const char *const out_name = "linkat.out";

void
setup_fd (int *pfd)
{
  if (*pfd >= 0)
    close (*pfd);
  *pfd = open (".", O_RDWR | O_TMPFILE, 0);
  if (*pfd < 0)
    err (1, "open");
}

int
main (void)
{
  unlink (out_name);

  int current_fd = open (".", O_RDONLY | O_DIRECTORY);
  if (current_fd < 0)
    err (1, "open (O_DIRECTORY)");
  int proc_fd = open ("/proc/self/fd", O_RDONLY | O_DIRECTORY);
  if (proc_fd < 0)
    err (1, "open (O_DIRECTORY)");

  int fd = -1;
  setup_fd (&fd);
  CHECK (linkat (fd, "", AT_FDCWD, out_name, AT_EMPTY_PATH));
  unlink (out_name);
  setup_fd (&fd);
  CHECK (linkat (fd, "", current_fd, out_name, AT_EMPTY_PATH));
  unlink (out_name);

  char proc_name[100];
  snprintf (proc_name, sizeof (proc_name), "/proc/self/fd/%d", fd);

  setup_fd (&fd);
  CHECK (linkat (AT_FDCWD, proc_name, AT_FDCWD, out_name,
                 AT_SYMLINK_FOLLOW));
  unlink (out_name);
  setup_fd (&fd);
  CHECK (linkat (AT_FDCWD, proc_name, current_fd, out_name,
                 AT_SYMLINK_FOLLOW));
  unlink (out_name);

  snprintf (proc_name, sizeof (proc_name), "%d", fd);
  setup_fd (&fd);
  CHECK (linkat (proc_fd, proc_name, AT_FDCWD, out_name,
                 AT_SYMLINK_FOLLOW));
  unlink (out_name);
  setup_fd (&fd);
  CHECK (linkat (proc_fd, proc_name, current_fd, out_name,
                 AT_SYMLINK_FOLLOW));
  unlink (out_name);

  printf ("successes: %d, failures: %d\n", successes, failures);

  return 0;
}