text: mkdirat − create a directory relative to a directory file descriptor
  #include <sys/stat.h>
  int mkdirat(int dirfd, const char *pathname, mode_t mode); the pathname given in pathname is relative and dirfd is the special value AT_FDCWD, then pathname is interpreted relative to the current working directory of the calling process (like mkdir(2)).
  If the pathname given in pathname is absolute, then dirfd is ignored.
  On success, mkdirat() returns 0. On error, −1 is returned and errno is set to indicate the error.
  The same errors that occur for mkdir(2) can also occur for mkdirat(). The following additional errors can occur for mkdirat():
  EBADF
  dirfd is not a valid file descriptor.
  ENOTDIR
  pathname is a relative path and dirfd is a file descriptor referring to a file other than a directory.
  See openat(2) for an explanation of the need for mkdirat().
  This system call is non-standard but is proposed for inclusion in a future revision of POSIX.1.
  mkdirat() was added to Linux in kernel 2.6.16.
  mkdir(2), openat(2), path_resolution(2)