Question: Say I have a struct defined somewhere deep in low-level code used all over the place in the most crazy and unknown ways:

```
struct T {
    unsigned short name_len;
    char d_name[LENGTH];
}

```

With accompanying functions that fill d\_name with whatever needs to be put there, like

```
struct T* fill( somethingOrOther* X)

```

And I would like to extend the old struct+function to include a new variable:

```
struct T {
    unsigned short name_len;
    char d_name[LENGTH];
    unsigned short type_len;
    char d_type;
}

```

and the new version of the function would also fill the d\_type variable with useful things.

Would this type of change break the API? Couldn't I just use the new T instead of the old T, and additionally access the new members?

Answer: 
If T really is used all over the place in crazy and unknown ways, then a change like that is likely to break something. Somewhere there will be a piece of code that has a local declaration of T instead of using your header file, or casts a 'mystruct \*' to a 'T \*', or something equally repugnant.
Yes- when you use an opaque pointer like that, then it is irrelevant of the contents. As long as your users only ever used the opaque pointers, you can muck around with the struct and it's implementation all you like.