15
#include <stdio.h>

int main(int argc, char** argv) {
    int num = 0;

    printf("Input: ");
    scanf("%d", &num); <<<

    printf("%d\n", num);

    return 0;
}

scanf("%d", &num);

Clang-Tidy: 'scanf' used to convert a string to an integer value, but function will not report conversion errors; consider using 'strtol' instead

I wrote a very simple code with CLion and it recommends me 'strtol' instead of 'scanf'.

But I'm using only integer variable and there is no strings. I can't figure out why the inspection message pops up.

How do I modify this code?

EnDelt64
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    Possible duplicate of [Using fgets and strtol to get a single integer](https://stackoverflow.com/questions/49460307/using-fgets-and-strtol-to-get-a-single-integer) – J...S Jun 11 '18 at 09:59
  • See also [What can I use for input conversion instead of scanf?](https://stackoverflow.com/q/58403537) – zwol Sep 21 '20 at 02:39
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    @EnDelt64 if you understand can you explain it to me in simple words why it's give such warning ? – Abhishek Mane Dec 23 '21 at 16:18

1 Answers1

9

How do I modify this code?

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

enum { INPUT_SIZE = 30 };

int main () {
    char *ptr;
    long ret;
    char str[INPUT_SIZE];

    fgets(str, INPUT_SIZE, stdin);    
    ret = strtol(str, &ptr, 10);

    if( ret == LONG_MAX || ret == LONG_MIN ) {
        perror("!! Problem is -> ");
    }
    else if (ret) {
        printf("The number is %ld\n", ret);
    }
    else {
        printf("No number found input is -> %s\n", ptr);
    }

    return(0);
}

If successful, strtol() returns the converted long int value.

If unsuccessful, strtol() returns 0 if no conversion could be performed. If the correct value is outside the range of representable values, strtol() returns LONG_MAX or LONG_MIN, according to the sign of the value. If the value of base is not supported, strtol() returns 0.

If unsuccessful strtol() sets errno to one of the following values:

Error Codes:

EINVAL The value of base is not supported.

ERANGE The conversion caused an overflow. Source : IBM

Can you check overflow using scanf() for example?

Input:  1234 stackoverflow
Output: The number is 1234

Input:  123nowhitespace
Output: The number is 123

Input:  number is 123
Output: No number found input is -> number is 123

Input:  between123between
Output: No number found input is -> between23between

Input:  9999999999999999999
Output: !! Problem is -> : Result too large

Maybe off-topic but, Jonathan Leffler says in his comments(in another topics) that handle warnings just as errors.

Soner from The Ottoman Empire
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  • Is it okay to use `ptr` without initialization? – EnDelt64 Jun 11 '18 at 10:13
  • @NBlizz < I think `NULL` can be passed there. However, `char *` passed helps to see what input is entered. After entering `123 teststring` as input, `123` is taken as number, `teststring` as ptr. The latter is applied to if input doesn't include number at the beginning. – Soner from The Ottoman Empire Jun 11 '18 at 10:22
  • @NBlizz < Sizes of C datatypes are subject to OS you use. You can run [the code snippet](https://ideone.com/oGZgdP) to see your long size. In my machine it is 8, which means long storage is between `-9,223,372,036,854,775,807` to `9,223,372,036,854,775,807`. You don't need to check LONG_MIN or MAX because they are aliases to comply with every machine. You can try exceeding long limit by aforementioned link, code snippet. – Soner from The Ottoman Empire Jun 11 '18 at 11:21
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    @snr still I am not understanding why it's give such warning ? – Abhishek Mane Dec 23 '21 at 16:14
  • Even when `if( ret == LONG_MAX || ret == LONG_MIN ) {` is true, it is not necessarily an error. It is possible than the user actually entered the exact value of `LONG_MAX` or `LONG_MIN` and that the conversion was successful. It would therefore be better to set `errno` to `0` before the `strtol` function call and to check for `errno == ERANGE` afterwards. – Andreas Wenzel Jun 07 '22 at 16:19