Strtol function returning wrong value

Question

I need to get a credit card number as input on an exercise, to dos so I'm ussing the function fgets (getting it as a string to check if it is realy a number) and then strtol to make it a long integer. But the integer value return by the function does not match the one on the string.

while (*endptr != '\n') {
        char card_number_string[20];
        char *endptr;
        long int card_number;

        printf("Number: ");
        fgets(card_number_string, 20, stdin);
        card_number = strtol(card_number_string, &endptr, 10);
        printf("Result: %ld", card_number);
    }

For example, when I input the value 4003600000000014 the output of the strtol shown in the printf is 2147483647.

Is it a bug on my code? Or the function does return something different like this??

Note: All the variables are declared on top of the main function, they are in there just to know how they were declared

I think it is better to use string (`char` array) for card number instead of numeric variable. Imagine that first 2 digits are 00. — i486, Aug 02 '20 at 13:56
I am using integer because in this particular problem the input never starts with 0 — Luís Otávio, Aug 02 '20 at 14:09
In some cases the data looks like numeric but in fact is string even if all characters are digits. — i486, Aug 02 '20 at 14:44

chux - Reinstate Monica · Accepted Answer · 2020-08-02T14:30:45.210

2

Value is outside long range. 2147483647 is the max value of long in your case.

Use long long. (or unsigned long long if negative CC# are not needed)

    //long int card_number;
    long long card_number;

    printf("Number: ");
    fgets(card_number_string, 20, stdin);
    
    //card_number = strtol(card_number_string, &endptr, 10);
    card_number = strtoll(card_number_string, &endptr, 10);

    //printf("Result: %ld", card_number);
    printf("Result: %lld", card_number);

Note: char *endptr; shadows the endptr in while (*endptr != '\n')

edited Aug 02 '20 at 14:30

answered Aug 02 '20 at 13:54

chux - Reinstate Monica

143,097
13
135
256

I checked, via the limits.h the max long value is 9804467591446527. And using a long long variable the output of the function is still wrong. About the char what does it mean that it shadows? – Luís Otávio Aug 02 '20 at 14:08
"using a long long variable the output of the function is still wrong." --> and what was that wrong output? Did code use `strtoll()` (2 ells) and `"%lld"`? – chux - Reinstate Monica Aug 02 '20 at 14:13
@LuísOtávio When `while (*endptr != '\n') {` is executed, the `endptr` used is not the new `endptr` defined inside the loop: `char *endptr;`. – chux - Reinstate Monica Aug 02 '20 at 14:15
It was outputing the same value, but i was still using `strtol()` and not `strtoll()`. Now it works! Thanks! – Luís Otávio Aug 02 '20 at 14:15
Ohh aboout the endpt, i put it in the wrong place on the snippet, it is originally on top of the main function, i wanted only to show my variables, I'll edit it, thanks! – Luís Otávio Aug 02 '20 at 14:19
Tip: avoid "just the right size" buffers for user input. User input is evil. Suggest at least 2x what you think a sane user would need. – chux - Reinstate Monica Aug 02 '20 at 14:19
Editing the code after answers arrive is poor SO etiquette. It this case, roll-back and append a note at the end of the post explaining `endptr`. – chux - Reinstate Monica Aug 02 '20 at 14:22
I'm using low buffer because it is for automated online test and 19 is the maximum input size. For a real case I would be using a bigger buffer for sure. – Luís Otávio Aug 02 '20 at 14:23
1

`char *endptr; ... while (*endptr != '\n') {` is poor code. What is the value of `endptr` and `*endptr` the first time `while (*endptr != '\n')` is executed? Good luck. – chux - Reinstate Monica Aug 02 '20 at 14:24
@ReinstateMonica thank you again! I started using the site recently and don't nknow much about etiquette rules – Luís Otávio Aug 02 '20 at 14:27
@LuísOtávio You could take the [tour](https://stackoverflow.com/tour). – chux - Reinstate Monica Aug 02 '20 at 14:29
Thanks! I'll look at it – Luís Otávio Aug 02 '20 at 14:33

Strtol function returning wrong value

1 Answers1