C - user decision based do/while loop not working correctly

Question

I have a problem which after many tests I think it's due to me not understanding how the input buffer works.

I have a while cycle which should continue to iterate until the user types "no" to stop the iteration.

I have two problems.

The while never stops to iterate regardless the user enters "no" or whatever is not equal to "yes"
As you can see the output at the second cycle has a problem. The program doesn't ask the user to input a string and skip that step, like the user just types ENTER.

CODE:

int foo = 0;

do{

  int i, cycles;
  char array[MAX_LENGTH+1];



  for(cycles=0; cycles < MAX_READ_CYCLES; cycles++){

    i=0;

    printf("\n\nEnter a string: ");

    char ch;
    while ((ch = getchar()) != '\n' && ch != EOF) {
      array[i] = ch;
      i++;
    }

    array[i] = '\0'; //string terminator


    printf("String you entered: %s\n", array);

    printf("\nDo you want to continue? 1: yes / 0: no \n");
    scanf("%d", &foo);

  }

} while( foo == 1);

OUTPUT

Enter a string: test
String you entered: test

Do you want to continue? 1: yes / 0: no
0

Enter a string: String you entered: 

Do you want to continue? 1: yes / 0: no
3

Enter a string: String you entered: 

Do you want to continue?

Try this : `fflush(stdin)` after the output of the string entered. — Mxsky, Dec 24 '15 at 17:46
Unfortunately it's still not working correctly even if I put a `fflush(stdin)`. Plus i was told to avoid using fflush(stdin) because it has an undefined behaviour. — fbid, Dec 24 '15 at 17:49
Here is your answer : http://stackoverflow.com/questions/16908008/scanf-getchar-working-correctly-only-first-time-through-loop — Mxsky, Dec 24 '15 at 17:53

score 5 · Accepted Answer · edited May 23 '17 at 12:31

5

Your program doesn't terminate if the user enters "yes" because of the inner for loop:

#include <stdio.h>
#include <string.h>
#define MAX_LENGTH 100
#define MAX_READ_CYCLES 100

int main() {
    int cycles = 0;
    char foo[4];
    do {
        char array[MAX_LENGTH + 1];

        printf("\n\nEnter a string: ");

        char ch;
        int i = 0;
        while ((ch = getchar()) != '\n' && ch != EOF) {
            array[i] = ch;
            i++;
        }

        array[i] = '\0'; //string terminator


        printf("String you entered: %s\n", array);

        printf("\nDo you want to continue?");
        scanf("%s", foo);

        cycles++;

        while ((ch = getchar()) != '\n' && ch != EOF); // force drop stdin

    } while (strcmp(foo, "yes") == 0 && cycles < MAX_READ_CYCLES);
}

Also see I am not able to flush stdin and http://c-faq.com/stdio/stdinflush2.html

edited May 23 '17 at 12:31

Community

1
1

answered Dec 24 '15 at 17:51

orestisf

1,396
1
15
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You are right, the 1. problem was due to my for loop. Thank you – fbid Dec 24 '15 at 17:58
Bad, bad, bad! The use of `foo` can cause a buffer overflow in the `scanf` and, potentially, a buffer overrun in `strcmp`. – Nik Bougalis Dec 24 '15 at 18:01
1

`fflush`ing an input stream is undefined behaviour! – too honest for this site Dec 24 '15 at 18:11
@Olaf ok, removed the fflush() call – orestisf Dec 24 '15 at 18:12
Technically you cannout flush input anyway (the reason it is UB - think about the meaning o the word). The correct term would be "drop". – too honest for this site Dec 24 '15 at 18:14

David Hoelzer · Answer 2 · 2015-12-24T17:43:22.120

0

You are creating a character array of 3 bytes and then storing more than three bytes into it. Don't forget that there will be a null tacked onto the end of that. Since you are not allocating enough space you are overwriting other memory locations which will always create undefined behavior.

Note, also, that scanf here is very unsafe. It's also not valid to initialize a character array like this: char foo[3]="";

edited Dec 24 '15 at 17:43

answered Dec 24 '15 at 17:32

David Hoelzer

15,862
4
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You are right about the `foo[3]` array, it was just quick example to reproduce a simple test case. Anyways, even if I use an integer variable, `int foo = 0' and then replace the while condition to `while( foo == 1)' the problem persist. – fbid Dec 24 '15 at 17:41

C - user decision based do/while loop not working correctly

2 Answers2