Dear ImGui from C

C has no namespaces, no overloads and no default arguments, and Dear ImGui uses all three. cimgui.h gives you every one of ImGui’s 399 functions anyway, plus every struct with its real C++ layout, so io->DisplaySize and style->Colors[ImGuiCol_Text] work just as they do in C++.

It follows the conventions of cimgui and dear_bindings, so if you’ve used either, you already know the names. It’s generated from imgui.h, which is why nothing is missing.

Hello from C

#include "cimgui.h"
#include "cimgui_impl_bui.h"

static void frame(void* user)
{
    float* value = (float*)user;
    ImGui_ShowDemoWindow();
    ImGui_Begin("Hello");
    ImGui_SliderFloat("Value", value, 0.0f, 1.0f);
    ImGui_Text("%.2f", *value);
    ImGui_End();
}

int main(void)
{
    float value = 0.5f;
    ImGui_CreateContext();
    cImGui_ImplBUI_App* app = cImGui_ImplBUI_CreateApp("Hello", 1280, 800, frame, &value, NULL);
    cImGui_ImplBUI_RunApp(app);
    cImGui_ImplBUI_DestroyApp(app);
    ImGui_DestroyContext();
}

Link bui::cimgui in CMake:

add_subdirectory(BUI)
add_executable(myapp main.c)
target_link_libraries(myapp PRIVATE bui::cimgui)

The backend calls ImGui_NewFrame(), your frame(user) and ImGui_Render() itself, and Blender’s style and font are on by default. BUI’s own C demo is the bui_imgui_demo_c target (examples/imgui_demo_c.c).

How the names work

Six rules turn any C++ call into its C spelling:

A few more details:

• printf-style functions keep their ...: ImGui_Text("%d items", n). The ...V variants take a va_list.
• Every struct has its C++ layout, checked by static_asserts when BUI builds. Vectors are spelled out: ImVector_ImDrawCmd has Size, Capacity and Data.
• Vectors in literals: (ImVec2){ 20, 20 } works anywhere an ImVec2 goes.

Here’s a draw-list snippet from the C demo, to show the rules together:

ImDrawList* dl = ImGui_GetWindowDrawList();
const ImVec2 p = ImGui_GetCursorScreenPos();
ImDrawList_AddRectFilled(dl, p, (ImVec2){ p.x + 60, p.y + 20 }, IM_COL32(71, 114, 179, 255));
ImDrawList_AddCircle(dl, (ImVec2){ p.x + 90, p.y + 10 }, 9, IM_COL32_WHITE);
ImGui_Dummy((ImVec2){ 100, 22 });

The backend in C: cimgui_impl_bui.h

The C backend’s functions use a cImGui_ImplBUI_ prefix:

config may be NULL for the defaults. To change one field, start from the defaults:

cImGui_ImplBUI_Config cfg = cImGui_ImplBUI_DefaultConfig();
cfg.FontSize = 15.0f;   /* pixels at scale 1; 0 is Blender's UI text size */
cImGui_ImplBUI_App* app = cImGui_ImplBUI_CreateApp("Tool", 1280, 800, frame, &state, &cfg);

The fields are BlenderStyle, BlenderFont, FontSize, FollowScale and SettleFrames.

Headless rendering and tests

The C backend can run ImGui without a window: frames render into an off-screen raster, and you feed input the way a window would. It’s how BUI tests its own bindings.

cImGui_ImplBUI_App* app = cImGui_ImplBUI_CreateHeadlessApp(800, 600, frame, &state, NULL);
cImGui_ImplBUI_Render(app, 3);                 /* run 3 frames */
cImGui_ImplBUI_Click(app, 100, 40);            /* a left click */
cImGui_ImplBUI_TypeText(app, "hello");
cImGui_ImplBUI_Key(app, "Return", 0);          /* X11 key names */
cImGui_ImplBUI_SavePNG(app, "shot.png");
cImGui_ImplBUI_DestroyApp(app);

• Input: MouseMove, MouseDown, MouseUp (button 0 left, 1 right, 2 middle), Click, Scroll, KeyDown, KeyUp, Key and TypeText. Key names are X11 keysyms ("Return", "BackSpace", "Left", "a"). Modifiers are BK_SHIFT 1, BK_CTRL 2 and BK_ALT 4.
• Output: cImGui_ImplBUI_Pixels(app, &w, &h) returns the last frame as 0xAARRGGBB values, row by row. SavePNG writes it to a file.

The C demo uses exactly this for bui_imgui_demo_c --screenshot out.png.

Next steps