我在Linux內核2.6.43中的handle.c中找到了兩個struct irq_desc * irq_to_desc（unsigned int irq）函數聲明。struct irq_desc *irq_to_desc(unsigned int irq)   {       return radix_tree_lookup(&irq_desc_tree, irq);   }和struct irq_desc *irq_to_desc(unsigned int irq)  {    return radix_tree_lookup(&irq_desc_tree, irq);  }它們在一個文件中。但是，為什么該文件可以成功編譯？