This is a bloat free minimal state immediate mode graphical user interface toolkit written in ANSI C. It was designed as a simple embeddable user interface for application and does not have any direct dependencies. It does not have a default renderbackend, os window and input handling but instead provides a very modular library approach by providing a simple input state storage for input and draw commands describing primitive shapes as output. So instead of providing a layered library that tries to abstract over a number of platform and render backends it only focuses on the actual GUI.

• Immediate mode graphical user interface toolkit
• Written in C89 (ANSI C)
• Small codebase (~8kLOC)
• Focus on portability, efficiency, simplicity and minimal internal state
• No global or hidden state
• No direct dependencies
• Configurable style and colors
• UTF-8 support
• Optional vertex buffer output and font handling

The library is self-contained within four different files that only have to be copied and compiled into your application. Files zahnrad.c and zahnrad.h make up the core of the library, while stb_rect_pack.h and stb_truetype.h are for a optional font handling implementation and can be removed if not needed.

• zahnrad.c
• zahnrad.h
• stb_rect_pack.h
• stb_truetype.h

There are no dependencies or a particular building process required. You just have to compile the .c file and #include zahnrad.h into your project. To actually run you have to provide the input state, configuration style and memory for draw commands to the library. After the GUI was executed all draw commands have to be either executed or optionally converted into a vertex buffer to draw the GUI.

/* allocate memory to hold draw commands */
struct zr_command_queue queue;
zr_command_queue_init_fixed(&queue, malloc(MEMORY_SIZE), MEMORY_SIZE);

/* setup configuration */
struct zr_style style;
struct zr_user_font font = {...};
zr_style_default(&style, ZR_DEFAULT_ALL, &font);

/* initialize window */
struct zr_window window;
zr_window_init(&window, zr_rect(50, 50, 220, 180),
    ZR_WINDOW_BORDER|ZR_WINDOW_MOVEABLE|ZR_WINDOW_SCALEABLE,
    &queue, &style, &input);

/* setup widget data */
enum {EASY, HARD};
zr_size option = EASY;
zr_float value = 0.6f;

struct zr_input input = {0};
while (1) {
    zr_input_begin(&input);
    /* record input */
    zr_input_end(&input);

    /* GUI */
    struct zr_context context;
    zr_begin(&context, &window);
    {
        zr_header(&context, "Show", ZR_CLOSEABLE, 0, ZR_HEADER_LEFT);
        zr_layout_row_static(&context, 30, 80, 1);
        if (zr_button_text(&context, "button", ZR_BUTTON_DEFAULT)) {
            /* event handling */
        }
        zr_layout_row_dynamic(&context, 30, 2);
        if (zr_option(&context, "easy", option == EASY)) option = EASY;
        if (zr_option(&context, "hard", option == HARD)) option = HARD;
        zr_layout_row_begin(&context, ZR_STATIC, 30, 2);
        {
            zr_layout_row_push(&context, 50);
            zr_label(&context, "Volume:", ZR_TEXT_LEFT);
            zr_layout_row_push(&context, 110);
            value = zr_slider(&context, 0, value, 1.0f, 0.1f);
        }
        zr_layout_row_end(&context);
    }
    zr_end(&context, &window);

    /* draw */
    const struct zr_command *cmd;
    zr_foreach_command(cmd, &queue) {
        /* execute draw call command */
    }
    zr_command_queue_clear(&queue);
}

Immediate mode in contrast to classical retained mode GUIs store as little state as possible by using procedural function calls as "widgets" instead of storing objects. Each "widget" function call takes hereby all its necessary data and immediately returns the user modified state back to the caller. Immediate mode graphical user interfaces therefore combine drawing and input handling into one unit instead of separating them like retain mode GUIs.

Since there is no to minimal internal state in immediate mode user interfaces, updates have to occur every frame, on every user input update or program state change which on one hand is more drawing expensive than classic retained GUI implementations but on the other hand grants a lot more flexibility and support for overall layout changes. In addition without any state there is no duplicated state between your program, the gui and the user which greatly simplifies code. Further traits of immediate mode graphic user interfaces are a code driven style, centralized flow control, easy extensibility and understandability.

The demo and example code can be found in the demo folder. There is demo code for Linux(X11), nanovg and OpenGL with SDL. The documentation can be found in gui.h. I documented each submodule and function with description and parameter explanation.

Personally I stay out of all "discussions" about C vs C++ since they are totally worthless and never brought anything good with it. The simple answer is I personally love C and have nothing against people using C++ especially the new iterations with C++11 and C++14. While this hopefully settles my view on C vs C++ there is still ANSI C vs C99. While for personal projects I only use C99 with all its niceties, libraries are a little bit different. Libraries are designed to reach the highest number of users possible which brings me to ANSI C as the most portable version. In addition not all C compiler like the MSVC compiler fully support C99, which finalized my decision to use ANSI C.

This Project uses ANSI C which does not have the header file <stdint.h> and therefore does not provide the fixed sized types that I need. Therefore I defined my own types which need to be set to the correct size for each platform. But if your development environment provides the header file you can define ZR_USE_FIXED_SIZE_TYPES to directly use the correct types.

• stb_rect_pack.h and stb_truetype.h by Sean Barret (public domain)
• Tutorial from Jari Komppa about imgui libraries
• Johannes 'johno' Norneby's article
• ImGui: The inspiration for this project from ocornut
• Nvidia's imgui toolkit

(The zlib License)