sscanf and %i / radix detection of large unsigned integers

Question

I'm working with files containing large numbers (disk offsets) and have run into a problem using scanf for %lli - the numbers are of the format 0x... but scanf doesn't read all the bits.

Here's example code (I tested on https://www.onlinegdb.com/online_c_compiler).

#include <stdio.h>
#include <stdlib.h>

int main()
{
    char* str = malloc(30);
    unsigned long long l;
    sprintf(str, "0x%llx\n", 0xffffffffffffffffULL);
    printf("%s\n", str);
    sscanf(str, "%lli", &l);
    printf("%llx\n", l);

    return 0;
}

It outputs:

0xffffffffffffffff

7fffffffffffffff

whereas I expect both numbers to be equal.

If I use the %llx specifier, it works properly, but I don't want to limit myself to hex-formatted numbers.

Answer 1

Unfortunately i stands for signed integer, and the corresponding expected argument is a pointer to int; with ll the width will be long long, but it would still be signed. There is no scanf format character for unsigned integer with base detection.

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There is no perfect solution. You can use strtoull(str, &endptr, 0) (or even strtoumax), which will detect the format correctly, but it is much more tedious to use.

But scanf is not perfect either - if the number is too big, then you would always get the maximum value, just like you got LLONG_MAX, without error indication.