#### 40 lines 1.0 KiB C Raw Blame History

 ```#include "scan.h" ``` ``` ``` ```static const unsigned long maxlong = ((unsigned long)-1)>>1; ``` ``` ``` ```size_t scan_longn(const char *src,size_t n,long *dest) { ``` ``` register const char *tmp; ``` ``` register long int l; ``` ``` register unsigned char c; ``` ``` int neg; ``` ``` int ok; ``` ``` if (!n--) return 0; ``` ``` tmp=src; l=0; ok=neg=0; ``` ``` switch (*tmp) { ``` ``` case '-': neg=1; ``` ``` case '+': ++tmp; ``` ``` } ``` ``` while (n-->0 && (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 long)l<<3; if ((n>>3)!=(unsigned long)l) break; ``` ``` if (n+(l<<1) < n) break; ``` ``` n+=l<<1; ``` ``` if (n+c < n) break; ``` ``` n+=c; ``` ``` if (n > maxlong+neg) break; ``` ``` l=n; ``` ``` ++tmp; ``` ``` ok=1; ``` ``` } ``` ``` if (!ok) return 0; ``` ``` *dest=(neg?-l:l); ``` ``` return (size_t)(tmp-src); ``` ```} ```