How to set a fixed windows size for a GTK+ app in C?

gtk_widget_set_size_request(mainWindow, 400, 300);
gtk_window_set_resizable (GTK_WINDOW(mainWindow), FALSE);

This must solve the problem.

Use gtk_window_set_resizable function for this purpose

gtk_window_set_default_size(GTK_WINDOW(mainWindow), 400, 300);
gtk_window_set_resizable (GTK_WINDOW(mainWindow), FALSE);