GitHub上检出代码，跳转到第一个commit，那是2005年4月提交的 `e83c5163316f89bfbde7d9ab23ca2e25604af290`。 需要认真复习我的C语言知识了，不过重点不是在于语言，而是这个工具本身的设计。 CACHE_H这个头文件就挺精彩，数据结构直接是基于硬件结构来设计的，注释写的很清楚“根本不考虑可移植性，因为这仅仅是个cache，一切以效率为优先考量”。 ``` /* * Basic data structures for the directory cache * * NOTE NOTE NOTE! This is all in the native CPU byte format. It's * not even trying to be portable. It's trying to be efficient. It's * just a cache, after all. */ #define CACHE_SIGNATURE 0x44495243 /* "DIRC" */ struct cache_header { unsigned int signature; unsigned int version; unsigned int entries; unsigned char sha1[20]; }; /* * The "cache_time" is just the low 32 bits of the * time. It doesn't matter if it overflows - we only * check it for equality in the 32 bits we save. */ struct cache_time { unsigned int sec; unsigned int nsec; }; ``` 此外，作为最基础的数据结构，CACHE_H也定义了一组接口函数。 ``` /* Initialize the cache information */ extern int read_cache(void); /* Return a statically all...