Why is scanf causing infinite forlool

Question

im just starting C and i dont know why this is happening. When i excuted the program it only stored 1 value in the service_code array. For some reason, the scanf() is keeping the loop counter ias 0. I couldn't find a solution. The scanf() is causing the forloop to to run infinitely. Does anyone know how to fix this?

#include <stdio.h>
#include <string.h>

int main() {
    
char name [100];
char str [10];
int discount = 0 ;
int age;
char service_codes[4][6];


printf("Welcome to Nelson Lanka Hospital \n");
printf("Our Services : \n SV105 - Doctor Channeling \n SV156 - Pharmacy \n SV128 - Laboratory \n SV100 - OPD \n \n");

printf("Enter your Details \n Name : ");
scanf("%[^\n]",name);
printf(" Enter age : ");
scanf("%d",&age);



printf(" Enter the Sevice code for the service you need : ");
scanf("%s", str);
strcpy(service_codes[0], str);


for(int i = 1; i<4; i++){

    char yn [2] = "y";
    printf("Do you need any other sevices? (y/n) : ");
    gets(yn);
    if (strcmp(yn, "n")==0){
        break;
    }
    printf(" Enter the Sevice code for the service you need : ");
    scanf("%s", str);
    strcpy(service_codes[i], str);
    printf("%s \t %s \t %d \n",service_codes[i],str,i);

}


for (int x = 0; x<4; x++){
    printf("%s \n",service_codes[x]);
}

}

Does this answer your question? [scanf causing infinite loop in C](https://stackoverflow.com/questions/32849647/scanf-causing-infinite-loop-in-c) — schoche, Sep 23 '20 at 07:00
Always check `scanf()`'s return value for errors. And never use `gets()`, the only function so bad it was removed from the standard. — Shawn, Sep 23 '20 at 07:02

Claudi · Accepted Answer · 2020-09-23T07:22:13.260

For some reason, the scanf() is keeping the loop counter i as 0.

You're probably having a buffer overflow that is altering the variables in the stack (such as the variable i). I see at least two points in your program where a buffer overflow may occur:

scanf("%s", str);: the array str only has room for 9 characters (plus 1 end null character used as a string terminator). If you type in a string longer than 9 characters (including the newline and carriage return characters that are appended when you hit ENTER) then scanf will corrupt the stack.
strcpy(service_codes[i], str);: each element in the array service_codes is defined to have 6 bytes each (room for 5 characters plus 1 end null terminator). By copying a string like this, in which str may be longer than service_code, you'll run into a buffer overflow.

C is a powerful language that allows you to do anything, even shooting at your own feet. You must be careful at every line you write in your code!

Why is scanf causing infinite forlool

1 Answers1