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: 
As long as the code using this API only obtains `T` objects as pointers returned by the library, and does not declare them itself, `malloc` them itself (using `sizeof(struct T)`), or do anything else that depends on the size of the struct, then it should be fine. If the calling code accessed the contents of the struct, you need to make sure you put new members at the end of the struct.

One possible additional consideration is whether any code depends on `d_name` being at the end of the structure in order to allocate space for and store larger names if the declared size does not fit. I only bring this up because the names of the members suggest the struct is something like `dirent` and that's traditional practice for `dirent`.
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.