void*_functionPoint(int id,ENTITY* e){
   //...
}
typedef struct{
   int _beep;
   void* functionPoint(int id,ENTITY* e);
};

typedef struct 
{
    void *fnPtr;
} MYSTRUCT;

void MYSTRUCT_fnPtr(ENTITY* ent);

void assign_func(ENTITY* ent)
{
ent.x = 256;
}

MYSTRUCT str;

[...]

str.fnPtr = assign_func;

[...]
MYSTRUCT_fnPtr = str.fnPtr;
MYSTRUCT_fnPtr(you);

typedef struct 
{
	void *fnPtr(ENTITY* ent);
} MYSTRUCT;

void assign_func(ENTITY* ent)
{
	printf("test");
}

MYSTRUCT str;

function main()
{
	MYSTRUCT* temp = &str;
	temp->fnPtr = assign_func;
	temp->fnPtr(NULL);
}