build_needs_escape函数目的是要建立一个位图bitmap,表示哪些字符需要转意。此函数在escape.c中,首先到escape.h中看看。
#include "config.h" /* Highest character number that can possibly be passed through un-escaped */ #define NEEDS_ESCAPE_BITS 128 //表示128位就行 #ifndef NEEDS_ESCAPE_SHIFT #define NEEDS_ESCAPE_SHIFT 5 /* 1 << 5 is 32 bits */ #endif #define NEEDS_ESCAPE_WORD_LENGTH (1<<NEEDS_ESCAPE_SHIFT) // #define NEEDS_ESCAPE_INDEX(c) ((c)>>NEEDS_ESCAPE_SHIFT) //index嘛,表示在_needs_escape中的第几个数上表示 /* Assume variable shift is fast, otherwise this could be a table lookup */ #define NEEDS_ESCAPE_MASK(c) (1<<((c)&(NEEDS_ESCAPE_WORD_LENGTH - 1))) //NEEDS_ESCAPE_WORD_LENGTH - 1相当于一个掩码,取c最右边的五bit,然后在把1左移这么多位,就可以通过它获得相应位的状态了 /* Newer compilers could use an inline function. * This macro works great, as long as you pass unsigned int or unsigned char. */ #define needs_escape(c) ((c)>=NEEDS_ESCAPE_BITS || _needs_escape[NEEDS_ESCAPE_INDEX(c)]&NEEDS_ESCAPE_MASK(c)) //实现很简洁。。。 extern unsigned long _needs_escape[(NEEDS_ESCAPE_BITS+NEEDS_ESCAPE_WORD_LENGTH-1)/NEEDS_ESCAPE_WORD_LENGTH]; void build_needs_escape(void);