Why using string in linked-list gives errors?

Question

The program I have made has few issues.

Main issue is when I try to scan a string entry, program crashes. I can't understand where is the problem and I don't know how to solve it. Integer part works fine but string elements seems to have some problems.

How I can manage and fix that problem? I have checked several topics about it but I still could not understand, can someone show me by changing mistaken lines of my code?

#include <stdio.h>
#include <stdlib.h>
#include <malloc.h>
#pragma warning(disable:4996)

struct flight {
    int number;
    char source[20];
    char destination[20]; 
    struct flight* next;
};

void enter();
void display();
void delete();
int count();

typedef struct flight NODE;

NODE* head_node, * first_node, * temp_node = 0, * prev_node, next_node;
int data;
char data2[20], data3[20];
struct flight f[];

void enter()
{
    printf("\nEnter flight number: \n");
    scanf("%d", &data);
    printf("\nEnter flight source: \n");
    scanf(" %s", &data2);
    printf("\nEnter flight destination: \n");
    scanf(" %s", &data3);


    temp_node = (NODE*)malloc(sizeof(NODE));

    temp_node->number = data;
    temp_node->source = data2[20];
    temp_node->destination = data3[20];


    if (first_node == 0)
    {
        first_node = temp_node;
    }
    else
    {
        head_node->next = temp_node;
    }
    temp_node->next = 0;
    head_node = temp_node;
    fflush(stdin);
}

temp_node->source = data2[20]; is copying only one character (and reading off the end of the array) — jmq, May 24 '20 at 22:17
@jmq yes exactly, and what should i do to use all characters in the word? — R-Gate, May 24 '20 at 22:18
@R-Gate - as was mentioned earlier use strcpy [strcpy(temp_node->source, data2). Assuming the data2 is NULL terminated before the 20th character. — jmq, May 24 '20 at 22:21
@ArdentCoder: `fflush(stdin)` has defined behavior in OP’s C implementation. — Eric Postpischil, May 24 '20 at 22:23
I have changed this part '''temp_node->source = data2[20]; temp_node->destination = data3[20];''' — R-Gate, May 24 '20 at 22:23
to this: strcpy(temp_node->source, data2); strcpy(temp_node->destination, data3); — R-Gate, May 24 '20 at 22:24
if you don't see an error message for `scanf(" %s", &data2);`, adjust your compiler settings — M.M, May 24 '20 at 22:30
If you add a `NODE *tail` that always points to the last node in the list, you can add at the end of your list in 0(1) time and you nodes will be in the order entered instead of in reverse order due to the *forward-chaining* you are currently using. Just something to consider. — David C. Rankin, May 24 '20 at 23:04

Ardent Coder · Accepted Answer · 2020-05-24T22:43:28.997

1

This is not the way to copy strings in C:

temp_node->source = data2[20];
temp_node->destination = data3[20];

You can use strcpy for this purpose. Also make sure that data2 and data3 are properly initialized, your code has a lot of uninitialized variables.

strcpy(temp_node->source, data2);
strcpy(temp_node->destination, data3);

The way you are reading strings:

scanf(" %s", &data2);
scanf(" %s", &data3);

is a bit unsafe. You can specify the length in the format specifier and also avoid & (more details in anastaciu's comment):

scanf(" %19s", data2);
scanf(" %19s", data3);

And note that fflush(stdin) is UB, you might want to think about it unless you're sure of how your compiler handles it.

Aside: struct flight f[]; you did not specify the array size.

edited May 24 '20 at 22:43

answered May 24 '20 at 22:24

Ardent Coder

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Yes that's what exactly I have done, thanks a lot everyone – R-Gate May 24 '20 at 22:26
What should I use instead of fflush(stdin) then? – R-Gate May 24 '20 at 22:27
You can refer this: https://stackoverflow.com/questions/7898215/how-to-clear-input-buffer-in-c – Ardent Coder May 24 '20 at 22:28
1

@R-Gate, `scanf(" %s", &data2);` should be `scanf(" %19s", data2);`, the same with `data3`. No `address-of` operator needed and limit the size of the scanned string to it's container -1 to reserve space for a null terminator. – anastaciu May 24 '20 at 22:31
even with fflush, program works normal. it seems right at the moment, isn't it? – R-Gate May 24 '20 at 22:31
1

@anastaciu See the initial comments, my answer was only intended for that purpose. Feel free to edit it :) – Ardent Coder May 24 '20 at 22:31
@R-Gate Maybe it's fine in your implementation, but it's a good practice to avoid writing non-portable code :) – Ardent Coder May 24 '20 at 22:32
1

@ArdentCoder, I was warning the OP of the problems with `scanf`, I did it in your answer because there are many comments already in the question and the OP might not see it, I would prefer if you add it yourself if you want, because it's not a minor edit and you would now best how to organize your answer. – anastaciu May 24 '20 at 22:39

Why using string in linked-list gives errors?

1 Answers1