client.h: order of inclusions
#include "cl_aliencont.h" // prototypes for functions in cl_aliencont.c
[...]
#include "cl_aircraft.h" // struct aircraft_t is defined here


somewhere in cl_aircraft.c:
#include client.h
void blach (void) {
aircraft_t *aircraft;
[...]
MY_Function(aircraft);
}

// I cannot use void MY_Function(aircraft_t *pointer) here because cl_aliencont.{c|h}
// does not know aircraft_t struct because of order of headers inclusion
somewhere in cl_aliencont.h:
void MY_Function(void *pointer);

somewhere in cl_aliencont.c:
#include "client.h"
void MY_Function (void *pointer)
{
aircraft_t aircraft = (aircraft_t *)pointer;
}