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