=== base.h ===
enum
{
　　TITLE,
　　PLAY,
};
extern void chg_state(int);
=== base.c ===
#include "base.h"
static int state = TITLE;
static int before_state = g_state;
int main(void)
{
　　if(state == TITLE){ title(); }
　　else if(state == PLAY){ play(); }

　　return 0;
}
void chg_state(int next_state)
{
　　before_state = g_state;
　　state = next_state;
}
=== title.c ===
#include "base.h"
void titile(void)
{
　　if( ... ){
　　　　chg_state(PLAY);
　　}
}