可以查看当前磁盘块的使用情况：哪些块空闲，哪些块被哪些文件占用。
You can view the current usage of disk blocks: which blocks free, which blocks are occupied by which files.
在大部分系统中，malloc和free都不是可重入的，因为它们使用静态数据结构来记录哪些内存块是空闲的。
On most systems, malloc and free are not reentrant, because they use a static data structure that records which memory blocks are free.
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