DragonFlyBSD

fopen(".", "w") fails with errno = EEXIST.
Per POSIX https://pubs.opengroup.org/onlinepubs/9699919799/functions/fopen.html
one would expect one of EISDIR, EINVAL, EACCES.

How to reproduce: Run this program.

#include <errno.h>
#include <stdio.h>

int main ()
{
  FILE *fp = fopen (".", "w");
  if (fp == NULL)
    {
      perror ("fopen");
    }
}