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 ```#include "scan.h" ``` ``` ``` ```static const unsigned int maxint = ((unsigned int)-1)>>1; ``` ``` ``` ```size_t scan_int(const char* src,int* dest) { ``` ``` register const char *tmp; ``` ``` register int l; ``` ``` register unsigned char c; ``` ``` int neg; ``` ``` int ok; ``` ``` tmp=src; l=0; ok=neg=0; ``` ``` switch (*tmp) { ``` ``` case '-': neg=1; ``` ``` case '+': ++tmp; ``` ``` } ``` ``` while ((c=(unsigned char)(*tmp-'0'))<10) { ``` ``` unsigned long int n; ``` ``` /* we want to do: l=l*10+c ``` ``` * but we need to check for integer overflow. ``` ``` * to check whether l*10 overflows, we could do ``` ``` * if ((l*10)/10 != l) ``` ``` * however, multiplication and division are expensive. ``` ``` * so instead of *10 we do (l<<3) (i.e. *8) + (l<<1) (i.e. *2) ``` ``` * and check for overflow on all the intermediate steps */ ``` ``` n=(unsigned int)l<<3; if ((n>>3)!=(unsigned int)l) break; ``` ``` if (n+(l<<1) < n) break; ``` ``` n+=l<<1; ``` ``` if (n+c < n) break; ``` ``` n+=c; ``` ``` if (n > maxint+neg) break; ``` ``` l=n; ``` ``` ++tmp; ``` ``` ok=1; ``` ``` } ``` ``` if (!ok) return 0; ``` ``` *dest=(neg?-l:l); ``` ``` return (size_t)(tmp-src); ``` ```} ```