Basic usage of ( now deprecated, but still incredibly useful, in GTK 4 ) GtkInfoBar widget as a "toast".

infobar.c

/** infobar.c
 *
 * COMPILE
 *
 * gcc `pkg-config --cflags gtk4` -o infobar infobar.c `pkg-config --libs gtk4`
 *
 * RUN
 *
 * ./infobar
 *
 * SOURCE
 *
 * https://docs.gtk.org/gtk4/class.InfoBar.html
 */

#include <gtk/gtk.h>
#include <libintl.h>

static void
activate( GtkApplication *app, gpointer user_data )
{
	GtkWidget *window, *message_label, *widget, *grid;
	GtkInfoBar *bar;

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

	widget = gtk_info_bar_new();
	bar = GTK_INFO_BAR( widget );
	grid = gtk_grid_new();

	gtk_window_set_child( GTK_WINDOW( window ), grid );

	message_label = gtk_label_new( "" );

	gtk_info_bar_add_child( bar, message_label );

	//gtk_info_bar_add_button( bar, gettext("_OK"), GTK_RESPONSE_OK );

	gtk_info_bar_set_show_close_button( GTK_INFO_BAR( bar ),
		TRUE );

	g_signal_connect( bar, "response",
		G_CALLBACK( gtk_widget_hide ), NULL );

	gtk_grid_attach( GTK_GRID( grid ), widget, 0, 2, 1, 1 );

	gtk_label_set_text( GTK_LABEL( message_label ),
		"An error occurred!" );
	gtk_info_bar_set_message_type( bar, GTK_MESSAGE_ERROR );
	gtk_widget_show( GTK_WIDGET( bar ) );

	gtk_widget_show( 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;
}