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cscanfstdio

Scanf is mutating my number

I have a program that grabs a user input number and adds the appropriate ending (1st, 11th, 312th, etc) So I gave it to my friend to find bugs/ break it. The code is

    int number;
    printf("\n\nNumber to end?: ");
    scanf("%d",&number);
    getchar();
    numberEnder(number);

when 098856854345712355 was input, scanf passed 115573475 to the rest of the program. Why? 115573475 is less than the max for an int.

Solution

  • 098856854345712355 is too big of a number for int.

    long long number;
printf("\n\nNumber to end?: ");
scanf("%lld",&number);
getchar();
numberEnder(number);

    The answer to why you get a specific garbage answer is GIGO. The standard doesn't specify the result for bad input and it's implementation specific. I would hazard a guess that if you apply a 32 bit mask to the input, that's the number you would get.

    Just out of curiosity, I looked up an implementation of scanf.

    It boils down to

    …
                                res = strtoq(buf, (char **)NULL, base);
…
                                    *va_arg(ap, long *) = res;
…

    Which should truncate off the high-bits in the cast. That would support the bit-mask guess, but it would only be the case for this implementation.