franklinexpress - 1 year ago 186
C Question

# itoa() c implementation int min underflow

I'm running some test cases against my itoa() function but keep getting

did not allocate memory for the int min value

I'm doing the check but it's something I'm missing here, what is it?

char *ft_itoa(int x) {
char *s;
size_t len;
long int n;

n = x;
if (x == -2147483648)
return (ft_strdup("-2147483648"));

len = ft_intlen(n) + 1;
if (!(s = (char*)malloc(sizeof(char) * len)))
return (NULL);

if (n == 0)
s[0] = '0';

if (n < 0) {
s[0] = '-';
n = -n;
}
s[len - 1] = '\0';
while (n) {
len--;
s[len - 1] = (n % 10) + '0';
n /= 10;
}
return (s);
}

This line:

if (x == -2147483648)

does not do what you think it does. C does not have negative integer constants. This is an unsigned int constant with the value 2^31, that you apply the unary minus operator on. This means that the expression x == -21... will depend on the C standard your compiler uses.

If you use C99 or C11, you'll be fine. There is a signed type that is big enough - long long is guaranteed to be big enough for this number, so both x and -21... will be converted into long long and then compared. But if you're using a C89 compiler and your machine doesn't have a long enough type, you're hitting implementation-defined behavior here:

When an integer is demoted to a signed integer with smaller size, or an unsigned integer is converted to its corresponding signed integer, if the value cannot be represented the result is implementation-defined.

This is why people are saying to use limits.h. Not because they are being pedantic, but because this is dangerous territory. If you look closely at what limits.h contains, you'll most likely find a line like this:

#define INT_MIN (- INT_MAX - 1)

This expression actually has the correct type and value.

Other than that I can't see any errors in the code you posted. If this is not the problem either ft_intlen or ft_strdup are wrong. Or you're calling your function in testing wrong (the same problems apply to -21... when calling tests).

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