C / C++

#include <stdio.h>

int main(){
  char greeting[13] = "Hello world!";
  printf("%s \n", greeting);

  return 0;
}

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer user_data)
{
  GtkWidget *window;

  window = gtk_application_window_new (app);
  gtk_window_set_title (GTK_WINDOW (window), "Window");
  gtk_window_set_default_size (GTK_WINDOW (window), 200, 200);
  gtk_widget_show_all (window);
}

int main(int argc, char **argv)
{
  GtkApplication *app;
  int status;

  app = gtk_application_new ("org.gtk.example", G_APPLICATION_FLAGS_NONE);
  g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);
  status = g_application_run (G_APPLICATION (app), argc, argv);
  g_object_unref (app);

  return status;
}

#include <gtk/gtk.h>

void hello(void)
{
  g_print ("Hello World\n");
}

void destroy(void)
{
  gtk_main_quit ();
}

int main(int argc, char *argv[])
{
  GtkWidget *window;
  GtkWidget *button;

  gtk_init (&argc, &argv);

  window = gtk_window_new (GTK_WINDOW_TOPLEVEL);
  gtk_signal_connect (GTK_OBJECT (window), "destroy",
    GTK_SIGNAL_FUNC (destroy), NULL);
  gtk_container_border_width (GTK_CONTAINER (window), 10);

  button = gtk_button_new_with_label ("Hello World");

  gtk_signal_connect (GTK_OBJECT (button), "clicked",
    GTK_SIGNAL_FUNC (hello), NULL);
  gtk_signal_connect_object (GTK_OBJECT (button), "clicked",
    GTK_SIGNAL_FUNC (gtk_widget_destroy),
    GTK_OBJECT (window));
  gtk_container_add (GTK_CONTAINER (window), button);
  gtk_widget_show (button);

  gtk_widget_show (window);

  gtk_main ();

  return 0;
}

#include <X11/Xlib.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void){
  Display *d;
  Window w;
  XEvent e;
  char *msg = "Hello, World!";
  int s;

  d = XOpenDisplay(NULL);
  if (d == NULL){
    fprintf(stderr, "Cannot open display\n");
    exit(1);
  }

  s = DefaultScreen(d);
  w = XCreateSimpleWindow(d, RootWindow(d, s), 10, 10, 100, 100, 1,
  BlackPixel(d, s), WhitePixel(d, s));
  XSelectInput(d, w, ExposureMask | KeyPressMask);
  XMapWindow(d, w);

  while (1) {
    XNextEvent(d, &e);
    if (e.type == Expose) {
      XFillRectangle(d, w, DefaultGC(d, s), 20, 20, 10, 10);
      XDrawString(d, w, DefaultGC(d, s), 10, 50, msg, strlen(msg));
    }

    if (e.type == KeyPress)
    break;
  }

  XCloseDisplay(d);
  return 0;
}