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RTEMS device driver ioctl argument?

Hi all,

When we call RTEMS device driver function ioctl(fd, a, b), it gets to
the driver ioctl entry point with 3 arguments: major, minor, arg. With
'arg' a pointer to rtems_libio_ioctl_args_t struct:

typedef struct {
  rtems_libio_t *iop;
  uint32_t command;
  void  *buffer;
  uint32_t ioctl_return;
} rtems_libio_ioctl_args_t;

with 'a' in 'command', 'b' in '*buffer'.

The questions are:

- what's 'iop'?
- is the error code supposed to be returned in 'ioctl_return'? If yes, 
how does the caller retrieve it?