I would recommend vart's method. Although you could do it the messy way

screenman.h
Code:
#ifndef INCLUDED_SCREENMAN_H
#define INCLUDED_SCREENMAN_H

struct screenMan_s
{
   int test;
};

extern struct screenMan_s * smp;

#endif /* INCLUDED_SCREENMAN_H */
screenman.c
Code:
#include "screenman.h"

static struct screenMan_s sm = {0};
struct screenMan_s * smp = &sm;

void screenman_init(void)
{
   /* blah */
   smp->test = 0;   /* not required, but this is a demo :D */
}
mouse.c
Code:
#include "screenman.h"

void mousePassive(int x, int y)
{
   smp->test = 5;           /* just make sure you don't dereference NULL :) */
   /* etc */
}