A numeric base, or radix, determines how positional digits in a numeric string are interpreted. C conversion functions such as strtoull and strtoul receive the base as an integer parameter. section 14.1 section 16.4
One string, different bases
Calling strtoull("0789a", &next, 10) reads 0789 and stops at 'a'. Calling it with base 0 applies the special prefix rules: the leading '0' selects octal, so parsing stops at '8'. Thus the same characters produce different results depending on the base. section 14.1
The base parameter supports two ordinary modes and one automatic mode:
- Use a positive base, such as 10, to interpret the string in that specified base. section 14.1
- Use base 0 to infer the effective base from the prefix: hexadecimal for a leading 0x, octal for a leading 0, and decimal otherwise. section 16.4
- Track the first character that cannot belong to the selected base through strtoull's end-pointer argument. section 14.1
97 static
98 size_t numberline_inner(char const*restrict act,
99 size_t numb[restrict], int base){
100 size_t n = 0;
101 for (char* next = 0; act[0]; act = next) {
102 numb[n] = strtoull(act, &next, base);
103 if (act == next) break;
104 ++n;
105 }
106 return n;
107 }
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Inspect the parsing loop that repeatedly passes the chosen base to strtoull and advances through the input using the returned end pointer. section 14.1
