[c++] What is Linux’s native GUI API?

Both Windows (Win32 API) and OS X (Cocoa) have their own APIs to handle windows, events and other OS stuff. I have never really got a clear answer as to what Linux’s equivalent is?

I have heard some people say GTK+, but GTK+ being cross platform. How can it be native?

GUI is a high level abstraction of capability, so almost everything from XOrg server to OpenGL is ported cross-platform, including for Windows platform. But if by GUI API you mean *nix graphics API then you might be wandering around "Direct Rendering Infrastructure".

Wayland is also worth mentioning as it is mostly referred as a "future X11 killer".

Also note that Android and some other mobile operating systems don't include X11 although they have a Linux kernel, so in that sense X11 is not native to all Linux systems.

Being cross-platform has nothing to do with being native. Cocoa has also been ported to other platforms via GNUStep but it is still native to OS X / macOS.

Strictly speaking, the API of Linux consists of its system calls. These are all of the kernel functions that can be called by a user-mode (non-kernel) program. This is a very low-level interface that allows programs to do things like open and read files. See http://en.wikipedia.org/wiki/System_call for a general introduction.

A real Linux system will also have an entire "stack" of other software running on it, in order to provide a graphical user interface and other features. Each element of this stack will offer its own API.

The closest thing to Win32 in linux would be the libc, as you mention not only the UI but events and "other os stuff"

To aid in what has already been mentioned there is a very good overview of the Linux graphics stack at this blog: http://blog.mecheye.net/2012/06/the-linux-graphics-stack/

This explains X11/Wayland etc and how it all fits together. In addition to what has already been mentioned I think it's worth adding a bit about the following API's you can use for graphics in Linux:

Mesa - "Mesa is many things, but one of the major things it provides that it is most famous for is its OpenGL implementation. It is an open-source implementation of the OpenGL API."

Cairo - "cairo is a drawing library used either by applications like Firefox directly, or through libraries like GTK+, to draw vector shapes."

DRM (Direct Rendering Manager) - I understand this the least but its basically the kernel drivers that let you write graphics directly to framebuffer without going through X

The linux kernel graphical operations are in /include/linux/fb.h as struct fb_ops. Eventually this is what add-ons like X11, Wayland, or DRM appear to reference. As these operations are only for video cards, not vector or raster hardcopy or tty oriented terminal devices, their usefulness as a GUI is limited; it's just not entirely true you need those add-ons to get graphical output if you don't mind using some assembler to bypass syscall as necessary.

I suppose the question is more like "What is linux's native GUI API".

In most cases X (aka X11) will be used for that: http://en.wikipedia.org/wiki/X_Window_System.

You can find the API documentation here

Linux is a kernel, not a full operating system. There are different windowing systems and gui's that run on top of Linux to provide windowing. Typically X11 is the windowing system used by Linux distros.

Wayland

As you might hear, wayland is the featured choice of many distros these days, because of its protocol is simpler than the X.

Toolkits of wayland

Toolkits or gui libraries that wayland suggests are:

• QT 5
• GTK+
• LSD
• Clutter
• EFL

XWindows is probably the closest to what could be called 'native' :)