The filter() method first checks to see if the list is empty — the critically important exit condition for this method.

filter() 方法首先检查列表是否为空，这是该方法的重要退出条件。

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Then we call the exit() function with an error code; for this utility, we've chosen to return different numbers for every different error condition.

随后我们用错误码调用 exit() 函数；对于本实用程序，我们已经选择了对每个不同错误条件返回不同数字。

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At this point, the while condition loops until an exit condition is reached — ideally, when the capture has completed successfully.

此时，while条件循环，直到达到退出条件—理想情况下是捕获成功完成。

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