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: 
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.
Crazy and unknown is not good. The best bet is to scan through the codebase for struct T and examine how it is used, with the alternative approach of changing it and see if something breaks... If the code only uses opaque pointers, you should be on the safe side. If the code accesses the members, but doesn't do something weird, you should be safe too, with a complete recompilation. 

If it does something weird, like the examples of rettops, even Jukka's tip may not help. The code could use a hardcoded value for the sizeof to do pointer arithmetic within an array of these structs...