输入 12 位数字并保存为 int。 C

Question

I am new to C and memory allocation. I am trying to take a 12 digit input and save as int so later on I can do some calculations. So far I have:

#include <stdio.h>

void main(void){
    char number[12];
    do
    {
        printf("Credit Card Number: ");
        scanf("%lli", number);
    } while (number == 1);
    
    printf("%lli", number);
}

Right now I use long long int since I can use 64 bits, but when I run it and type in 123 I get:

27583791809822484

Could someone explain what I am doing wrong, why the output is 27583791809822484 and if I have any styling errors.

Answer 1

scanf %lli expects a pointer to a long long int . You provided a pointer to an array of 12 char.

printf %lli expects a long long int . You provide a pointer.

long long int card_num;
scanf("%lli", &card_num);
printf("%lli", card_num);

---

If you wanted to portable code, you'd use

#include <stdint.h>
#include <inttypes.h>

uint64_t card_num;
scanf("%" SCNu64, &card_num);
printf("%" PRIu64, card_num);

---

If you wanted to store the number as a string (which is quite reasonable for a credit card number), then char number[12] makes sense, but you'd use %s . Actually, it would have to be char number[13] to be large enough to store 12 digits and the trailing NUL.

char card_num[13];
scanf("%12s", card_num);
printf("%s", card_num);

Answer 2

#include <stdio.h>

void main(void){
    
    long long int creditcard;

    printf("Credit Card Number: ");
    scanf("%lli", &creditcard);


    printf("%lli", creditcard);
}

this will work.

EDIT: As stated in the comments, if you have leading zeros, this won't work. Instead taking it as string like above is better.