Is there a stricter strtoull() in any ubiquitous C library?
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Is there a stricter strtoull() in any ubiquitous C library?
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I want a function that will interpret a string as a strictly unsigned integer, failing when the string overflows, represents a negative number, or does not represent a number.
strtoull() does set errno to ERANGE on overflow or EINVAL for a string that does not begin with a number, but accepts negative numbers as valid.
strtoumax() is the same as strtoull()
atoll() and strtonum() are ruled out because they are strictly for signed integers.
sscanf() sets errno to ERANGE on overflow and stops processing the string if it does not begin with a number (reflected in sscanf's return value), but also accepts negative numbers as valid.
Is there another option that I'm missing?
2 Answers
There are no out-of-the-box solutions that fit your requirements, but it is easy enough to write such a function yourself, using strtoull or strtoll as basis.
For negative numbers, either you can check for the minus sign beforehand, or you can convert to a signed number first and report an error for negative values.
For numbers followed by other characters, you can check that the endp pointer that you pass to strto(u)ll points to the end of the string.
You are describing two separate problems. One is that the string should be in a specific form, namely all digits (but you didn't specify whether to permit leading and trailing spaces). The second is that the range should not overflow. There is no readily available function that provides exactly this combination.
The first requirement is best satisfied by testing (or parsing) the string before decoding it. In most languages I would use a regular expression. In C++ you can use library functions. In C you write loops and test characters.
The second requirement is actually quite tricky. You can include overflow testing in your decode function but testing against MAXUINT or something similar needs some careful coding. A simple solution is to decode as a double and then test the range before casting to uint.
In summary, when you have requirements as specific as this you normally finish up writing your own functions, perhaps basing it on available "free" source code. Over the 30 years I've been writing C code I've probably written this function or similar about 5 or 6 times. It gets easier with practice!
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