Let's use the existing Rect and Color types in the GUI API.

Any type that doesn't depend on indirect data can probably be used here.

Kernel/GUITypes.h

@@ -1,13 +1,9 @@
 #pragma once
 
-// GUI system call API types.
+#include <Widgets/Color.h>
+#include <Widgets/Rect.h>
 
-struct GUI_Rect {
-    int x;
-    int y;
-    int width;
-    int height;
-};
+// GUI system call API types.
 
 struct GUI_WindowFlags { enum {
     Visible = 1 << 0,
@@ -16,9 +12,9 @@ struct GUI_WindowFlags { enum {
 typedef unsigned GUI_Color;
 
 struct GUI_CreateWindowParameters {
-    GUI_Rect rect;
-    GUI_Color background_color;
-    unsigned flags;
+    Rect rect;
+    Color background_color;
+    unsigned flags { 0 };
     char title[128];
 };
 
@@ -29,9 +25,9 @@ enum class GUI_WidgetType : unsigned {
 
 struct GUI_CreateWidgetParameters {
     GUI_WidgetType type;
-    GUI_Rect rect;
-    GUI_Color background_color;
-    bool opaque;
-    unsigned flags;
+    Rect rect;
+    Color background_color;
+    bool opaque { true };
+    unsigned flags { 0 };
     char text[256];
 };

Kernel/ProcessGUI.cpp

@@ -37,9 +37,8 @@ int Process::gui$create_window(const GUI_CreateWindowParameters* user_params)
         return -EFAULT;
 
     auto params = *user_params;
-    Rect rect { params.rect.x, params.rect.y, params.rect.width, params.rect.height };
 
-    if (rect.is_empty())
+    if (params.rect.is_empty())
         return -EINVAL;
 
     ProcessPagingScope scope(EventLoop::main().server_process());
@@ -52,14 +51,14 @@ int Process::gui$create_window(const GUI_CreateWindowParameters* user_params)
     m_windows.append(window->makeWeakPtr());
 
     window->setTitle(params.title);
-    window->setRect(rect);
+    window->setRect(params.rect);
 
     auto* main_widget = new Widget;
     window->setMainWidget(main_widget);
-    main_widget->setWindowRelativeRect({ 0, 0, rect.width(), rect.height() });
+    main_widget->setWindowRelativeRect({ 0, 0, params.rect.width(), params.rect.height() });
     main_widget->setBackgroundColor(params.background_color);
     main_widget->setFillWithBackgroundColor(true);
-    dbgprintf("%s<%u> gui$create_window: %d with rect {%d,%d %dx%d}\n", name().characters(), pid(), window_id, rect.x(), rect.y(), rect.width(), rect.height());
+    dbgprintf("%s<%u> gui$create_window: %d with rect {%d,%d %dx%d}\n", name().characters(), pid(), window_id, params.rect.x(), params.rect.y(), params.rect.width(), params.rect.height());
 
     return window_id;
 }
@@ -92,9 +91,8 @@ int Process::gui$create_widget(int window_id, const GUI_CreateWidgetParameters*
     auto& window = *m_windows[window_id];
 
     auto params = *user_params;
-    Rect rect { params.rect.x, params.rect.y, params.rect.width, params.rect.height };
 
-    if (rect.is_empty())
+    if (params.rect.is_empty())
         return -EINVAL;
 
     Widget* widget = nullptr;
@@ -112,10 +110,10 @@ int Process::gui$create_widget(int window_id, const GUI_CreateWidgetParameters*
     int widget_id = m_widgets.size();
     m_widgets.append(widget->makeWeakPtr());
 
-    widget->setWindowRelativeRect(rect);
+    widget->setWindowRelativeRect(params.rect);
     widget->setBackgroundColor(params.background_color);
     widget->setFillWithBackgroundColor(params.opaque);
-    dbgprintf("%s<%u> gui$create_widget: %d with rect {%d,%d %dx%d}\n", name().characters(), pid(), widget_id, rect.x(), rect.y(), rect.width(), rect.height());
+    dbgprintf("%s<%u> gui$create_widget: %d with rect {%d,%d %dx%d}\n", name().characters(), pid(), widget_id, params.rect.x(), params.rect.y(), params.rect.width(), params.rect.height());
 
     return window_id;
 }

Userland/.gitignore

@@ -21,3 +21,4 @@ mkdir
 touch
 sync
 more
+guitest

Userland/guitest.cpp

@@ -0,0 +1,50 @@
+#include <stdio.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+#include <stdlib.h>
+#include <fcntl.h>
+#include <assert.h>
+#include <Kernel/GUITypes.h>
+#include <Kernel/Syscall.h>
+
+int main(int argc, char** argv)
+{
+    GUI_CreateWindowParameters wparams;
+    wparams.rect = { 200, 200, 300, 200 };
+    wparams.background_color = 0xffc0c0;
+    strcpy(wparams.title, "GUI test app");
+    int window_id = syscall(SC_gui_create_window, &wparams);
+    if (window_id < 0) {
+        perror("gui_create_window");
+        return 1;
+    }
+
+    GUI_CreateWidgetParameters label_params;
+    label_params.type = GUI_WidgetType::Label;
+    label_params.rect = { 20, 20, 260, 20 };
+    label_params.background_color = 0xffffff;
+    label_params.opaque = true;
+    strcpy(label_params.text, "Hello World!");
+    int label_id = syscall(SC_gui_create_widget, window_id, &label_params);
+    if (label_id < 0) {
+        perror("gui_create_widget");
+        return 1;
+    }
+
+    GUI_CreateWidgetParameters button_params;
+    button_params.type = GUI_WidgetType::Button;
+    button_params.rect = { 60, 60, 120, 20 };
+    button_params.background_color = 0xffffff;
+    button_params.opaque = true;
+    strcpy(button_params.text, "I'm a button!");
+    int button_id = syscall(SC_gui_create_widget, window_id, &button_params);
+    if (button_id < 0) {
+        perror("gui_create_widget");
+        return 1;
+    }
+
+    for (;;) {
+    }
+    return 0;
+}