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);