Why does this loop generate an error after the first conversion?

Question

When I execute the program and I enter C or F and complete the process, this is the image of what I see:

I want to hide the line in the red rectangle. This is the code I have:

#include <stdio.h>

int main(void){
    float f,c;
    char var;
    int x;

    while(var!='x'){
        printf("x=Ende c=Umrechnung c->f f=Umrechnung f->c:");
        scanf("%c",&var);
        if(var=='c'){
            printf("Grad Celsius =");
            scanf("%f",&c);
            f=(c*1.8)+32;
            printf("%.2f Grad Celsius sind %.2f Grad Fahrenheit\n",c,f);
        }
        else if(var=='f'){
            printf("Grad Fahrenheit =");
            scanf("%f",&f);
            c=(f-32)/1.8;
            printf("%.2f Grad Fahrenheit sind %.2f Grad Celsius\n",f,c);
        }
        else if(var=='x'){
            printf("fertig !! chao\n");
        }
        else {
            printf("Ungiltige Umreechnungsart!!\n");
        }
    }
    return 0;
}

Aside: `while(var!='x')` note that `var` is an *uninitiliased variable* on the first loop. — Weather Vane, Nov 07 '15 at 17:27
you also have to initialize `var`, otherwise `while(var!='x')` would be undefined behaviour. `printf("Ungiltige Umreechnungsart!!\n");` should be `printf("Ungültige Umrechnungsart!\n");`. — mch, Nov 07 '15 at 17:28

PeCosta · Answer 1 · 2015-11-07T19:21:10.157

0

In your case that happens because you are locking for a given character using the scanf() and the newline left from the previous call to scanf().

If you call scanf( %c,&var) with a space before the %c formatting will fix the issue because it will match any white space of any size (including none and the space left from your previous newline at the call of scanf().

The function scanf() provides no buffer overflow protection, this is particularly a problem when you are reading multiple characters, for example scanf("%s",&var) and might become unsafe to use.

A better and safer way to do that is to use fgets():

 fgets (value, MAX_VALUE_SIZE, stdin);

You can then format the input to whatever you'd like using for example sscanf()

edited Nov 07 '15 at 19:21

answered Nov 07 '15 at 17:04

PeCosta

537
4
13

Which buffer are you talking about? And how does the `t` character from `printf` find its way to the input? – Weather Vane Nov 07 '15 at 17:29
The buffer to were the scanf() stores the matching directives to. – PeCosta Nov 07 '15 at 17:43
And how might this buffer overflow? The program has a `char` buffer and a `float` buffer. – Weather Vane Nov 07 '15 at 17:46
The buffer might overflow if you use for example scanf("%s") where there is no well established end for the directive. – PeCosta Nov 07 '15 at 17:49
I was talking about scanf() in general, my bad if that was not clear! I was not stating that this problem had any relation to a buffer overflow. But only that the bad use of scaf() can result in such. – PeCosta Nov 07 '15 at 17:50
@WeatherVane I made a mistake it is not t that finds its way to `scanf()` is the `/n` – PeCosta Nov 07 '15 at 17:54
So how does this answer the question? – Weather Vane Nov 07 '15 at 17:57
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/94496/discussion-between-pecosta-and-weather-vane). – PeCosta Nov 07 '15 at 18:09
As it stands, this answer is wrong (`scanf()` is not reading the output from `printf()`, for example). See also notes in chat. – Jonathan Leffler Nov 07 '15 at 18:46

Why does this loop generate an error after the first conversion?

1 Answers1