Scanf_s keeps returning \0 instead of input

Question

I've become stuck on a, in my mind, pretty simple task. I have written a program that picks a random number, and makes you guess which one it is and exit the program at the end if the characters 'n' or 'N' is entered. The code is as follows:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int input_switcher(int input, int answer)
{
    if (input == answer)    {return 1;} 
    else if(input < answer) {return 2;}
    else if(input > answer) {return 3;}
    else                    {return 0;}
}

int main(void)
{
    int input, answer, tries;
    char keepGoing = 'J';
    srand((int)time(0));

    while(42)
    {
        input = 0, tries = 0;
        answer = 1;//rand()%100+1;

        printf("Jag \"t\x84nker\" p\x86 ett tal mellan 1 och 100, gissa vilket!\n");

            while(42)
            {
                printf("Gissning: ");
                scanf_s("%d", &input);

                switch (input_switcher(input, answer))
                {
                case 1:
                    printf("Grattis, det var r\x84tt!\n");
                    printf("Du gjorde %d f\x94rs\x94k.", tries);
                    break;
                case 2:
                    printf("Du gissade f\x94r l\x86gt, f\x94rs\x94k igen!\n");
                    tries++;
                    break;
                case 3:
                    printf("Du gissade f\x94r h\x94gt, f\x94rs\x94k igen!\n");
                    tries++;
                    break;
                default:
                break;
                }
                if(input == answer)     {break;}
            }
        printf("Ska vi spela en g\x86ng till(J/N)? ");
        scanf_s("%*c%c", &keepGoing);
        if(keepGoing == 'N' || keepGoing == 'n')        {return 0;}
        else if(keepGoing == 'J' || keepGoing == 'j')   {system("cls");}
    }   
    return 0;
}

The problem I'm having is that my last scanf_s gives me \0 instead of the character i enter. Have I identified the problem or am I wrong? What can I change to remedy this problem?

You may want to explain your reasoning for why you chose that specific format specifier, `"%*c%c"` in the one-place you suspect as the source of your problem. — WhozCraig, Sep 16 '13 at 19:04
@WhozCraig The `%*c` is there to consume the leftover \n from previous scanf entry. — Mwigs, Sep 16 '13 at 19:14
@MarcusWigert; Use an space before `%c` specifier to consume `\n`. — haccks, Sep 16 '13 at 19:38

haccks · Accepted Answer · 2013-09-17T13:03:43.430

5

Your last scanf_s receives \n and not \0. To eat up this \n change

printf("Ska vi spela en g\x86ng till(J/N)? ");
scanf_s("%*c%c", &keepGoing);

to

 printf("Ska vi spela en g\x86ng till(J/N)? ");
 scanf_s(" %c", &keepGoing);
          ^
          |
   space before %c specifier

It is working.
Output: On Win7 with GCC 4.7.1

enter image description here

edited Sep 17 '13 at 13:03

answered Sep 16 '13 at 19:21

haccks

104,019
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I've added the space before `%c` and the problem persists. – Mwigs Sep 16 '13 at 19:51
Have you deleted `%*c`? – haccks Sep 16 '13 at 19:57
1

It's working now after I restarted my computer and patched VS2012. – Mwigs Sep 20 '13 at 07:09

Gangadhar · Answer 2 · 2013-09-16T19:56:44.400

1

scanf() is Just enough in this instead of scanf_s ==>

You can modify scanf in the two places

Like this:

scanf("%d", &input);  
scanf(" %c", &keepGoing); 
         ^  //use space here , to consume the leftover \n from previous scanf entry

And if you still face issue You can use fflush(stdin); before last scanf_s() In Windows this would works.

             fflush(stdin);  
             scanf("%c", &keepGoing);

edited Sep 16 '13 at 19:56

answered Sep 16 '13 at 19:23

Gangadhar

10,248
3
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1

why `scanf_s ==>scanf` ? – P0W Sep 16 '13 at 19:28
Actually OP does not require `scanf_s`,if have any security enhancements , need to use scanf_s . but i need to say. you can use only scanf instead of scanf_s. – Gangadhar Sep 16 '13 at 19:33
1

`fflush(stdin)` will invoke undefined behavior. – haccks Sep 16 '13 at 19:57
@haccks In windows usage of fflush(stdin); would not invoke UB. – Gangadhar Sep 16 '13 at 20:13

Scanf_s keeps returning \0 instead of input

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