overflow when change from string to int in C

Question

In the C programming language, a string such as "2147483649" overflows as an int since the largest unsigned int is 2147483647. When I want to convert strings of integers to ints, how do I go about checking for these overflow cases?

I can't just check if it is >=-(2^32-1) and <= (2^32-1) since the process of converting this string to an int (eg atoi()) already changes this value. Is there an easy way instead of checking the number of digits of the string before converting and also each digit at a time to make sure it's within the range of ints?

Use `strtol`, `atoi()` has no error checking. Check limits with `LONG_MAX` and `LONG_MIN` from `limits.h` — RoadRunner, Jan 26 '17 at 08:38
May be this will help you: http://stackoverflow.com/questions/7456902/long-vs-int-c-c-whats-the-point — ctrl-shift-esc, Jan 26 '17 at 08:39
If you need convert larger values how about using `atoll` (if you can use C99)? — Yuriy Ivaskevych, Jan 26 '17 at 08:45

score 7 · Answer 1 · answered Jan 26 '17 at 08:42

7

You can convert the resulting integer back to string and compare it to the source string. Although it doesn't look elegant to me. :-)

answered Jan 26 '17 at 08:42

D. Mika

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I was about to say the same thing. – selbie Jan 26 '17 at 08:43
1

That still invokes undefined behaviour. – kamikaze Jan 26 '17 at 09:12

score 5 · Accepted Answer · answered Jan 26 '17 at 08:46

Using atoi() on a value that cannot be represented as an integer invokes undefined behaviour. I.e. you should not use atoi().

Instead you can use strtol():

#include <stdlib.h>
#include <errno.h>

long val = strtol("2147483649", 0, 10);
if (val == LONG_MIN && errno == ERANGE) {
    …underflow…
} else if (val == LONG_MAX && errno == ERANGE) {
    …overflow…
}

score 0 · Answer 3 · answered Jan 26 '17 at 08:39

0

Use strtol(). It gives you a long so if on your platform that's larger than an int you can check that for overflow. And if the number in the string doesn't even fit in a long, the function will return a special value LONG_MIN or LONG_MAX and set errno.

answered Jan 26 '17 at 08:39

John Zwinck

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score -1 · Answer 4 · answered Jan 26 '17 at 08:40

-1

If you're implementing your own atoi function, you can check for overflow as you accumulate the result:

int value = 0;
for-each digit-character in string
    int newValue = value + ( 10 * place * digit-character )
    if newValue < value then die("overflow detected")
    value = newValue
next

answered Jan 26 '17 at 08:40

Dai

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One problem you've got there. I know because I wrote the same thing and found the bug, ahah. Try it with 6147483648 for example. On the second last iteration, when you multiply 614748364 by 10, it overflows twice and it still ends up at x mod 2^31 = 1852516352, larger than the previous iteration. – NathanTempelman Jan 03 '19 at 00:50

overflow when change from string to int in C

4 Answers4