#### 8.1.1 Modern GNU Objective-C runtime API

The GNU Objective-C runtime provides an API which is similar to the one provided by the “Objective-C 2.0” Apple/NeXT Objective-C runtime. The API is documented in the public header files of the GNU Objective-C runtime:

- `objc/objc.h`: this is the basic Objective-C header file, defining the basic Objective-C types such as `id`, `Class` and `BOOL`. You have to include this header to do almost anything with Objective-C.
- `objc/runtime.h`: this header declares most of the public runtime API functions allowing you to inspect and manipulate the Objective-C runtime data structures. These functions are fairly standardized across Objective-C runtimes and are almost identical to the Apple/NeXT Objective-C runtime ones. It does not declare functions in some specialized areas (constructing and forwarding message invocations, threading) which are in the other headers below. You have to include `objc/objc.h` and `objc/runtime.h` to use any of the functions, such as `class_getName()`, declared in `objc/runtime.h`.
- `objc/message.h`: this header declares public functions used to construct, deconstruct and forward message invocations. Because messaging is done in quite a different way on different runtimes, functions in this header are specific to the GNU Objective-C runtime implementation.
- `objc/objc-exception.h`: this header declares some public functions related to Objective-C exceptions. For example functions in this header allow you to throw an Objective-C exception from plain C/C++ code.
- `objc/objc-sync.h`: this header declares some public functions related to the Objective-C `@synchronized()` syntax, allowing you to emulate an Objective-C `@synchronized()` block in plain C/C++ code.
- `objc/thr.h`: this header declares a public runtime API threading layer that is only provided by the GNU Objective-C runtime. It declares functions such as `objc_mutex_lock()`, which provide a platform-independent set of threading functions.

The header files contain detailed documentation for each function in the GNU Objective-C runtime API.