使用的任何符号都必须在外部进行定义。
函数s1没有在当前文件中定义，因为编译器实际上并不知道这个外部函数在哪儿，我们现在把这个符号表项保留为空白。
The function S1 is defined outside of the current file, and because the compiler doesn't actually know where the external function is, we leave this part of the symbol table blank for now.
函数s1没有在当前文件中定义，因为编译器实际上并不知道这个外部函数在哪儿，我们现在把这个符号表项保留为空白。
The function S1 is defined outside of the current file, and because the compiler doesn't actually know where the external function is, we leave this part of the symbol table blank for now.
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