Singly linked list C, printing

Question

I'm a beginner in developing, so my sensei gave me a task to complete in which I need to enter a couple of strings in linked lists and after I enter print, they need to be printed in the correct order, from the first to last.

Here is what I got:

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

    typedef struct Node {
        char data;
        struct Node *next;
    }node;

char createlist(node *pointer, char data[100]) {
    while (pointer->next != NULL) {
        pointer = pointer->next;
    }

    pointer->next = (node*) malloc(sizeof(node));
    pointer = pointer-> next;
    pointer->data = *data;
    pointer->next = NULL;
}

int main() {
    node *first, *temp;
    first = (node*) malloc(sizeof(node));
    temp = first;
    temp->next = NULL;

    printf("Enter the lines\n");
    while (1) {
        char data[100];
        gets(data);
        createlist(first, data);
        if (strcmp(data, "print") == 0)
            printf("%s\n", first->data);
        else if (strcmp(data, "quit") == 0)
            return (0);

    };

}

When I run it I get: Enter the lines: asdfasdf print (null)

Any help would be appreciated since this is my first time using linked lists.

Answer 1

• You should format your code properly.
• first->data is allocated via malloc() and isn't initialized, so using its value invokes undefined behavior .
• In order not to deal the first element specially, you should use pointer to pointer to have createlist() modify first .
• Since createlist() won't return anything, type of its return value should be void .
• I guess you wanted to copy the strings instead of assigning the first character of each strings.
• To print all of what you entered, code to do so have to be written.
• You shouldn't use gets() , which has unavoidable risk of buffer overrun.
• You should free() whatever you allocated via malloc() .

improved code:

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

typedef struct Node
{
    char *data;
    struct Node *next;
} node;

void createlist(node **pointer, char data[100])
{
    while (*pointer != NULL)
    {
        pointer = &(*pointer)->next;
    }

    *pointer = malloc(sizeof(node));
    if (*pointer == NULL)
    {
        perror("malloc 1");
        exit(1);
    }
    (*pointer)->data = malloc(strlen(data) + 1);
    if ((*pointer)->data == NULL)
    {
        perror("malloc 2");
        exit(1);
    }
    strcpy((*pointer)->data, data);
    (*pointer)->next = NULL;
}

int main(void)
{
    node *first = NULL;

    printf("Enter the lines\n");
    while (1)
    { 
        char data[100], *lf;
        if (fgets(data, sizeof(data), stdin) == NULL) strcpy(data, "quit");
        if ((lf = strchr(data, '\n')) != NULL) *lf = '\0'; /* remove newline character */
        createlist(&first, data);
        if (strcmp(data, "print") == 0)
        {
            node *elem = first;
            while (elem != NULL)
            {
                printf("%s\n", elem -> data);
                elem = elem->next;
            }
        }
        else if (strcmp(data, "quit") == 0)
        {
            while (first != NULL)
            {
                node *next = first->next;
                free(first->data);
                free(first);
                first = next;
            }
            return(0);
        }

    }

}

Answer 2

Inside createlist() , you are iterating to the end of the list. There, you are adding a new node and setting a new text entered. By doing so, you are missing that you have already a first node. Because you are iterating to the end in every call of createlist() , you are jumping over your first node every time, so it remains without text and delivers NULL .

In order not to jump over the first initial node, you could alter createlist() like this:

char createlist(node *pointer, char data[100])
{
   while (pointer->data != NULL && pointer->next != NULL)
   {
     pointer = pointer->next;
   }
   ...
   ... 
}

Or you could create the first node not initially, but only after the first line of text was entered.

---

edit : Here are two additional style hints:

• What happens if somebody enters 120 characters? The text will outrun your char[100] array and will fill RAM that is used otherwise. This is a buffer overflow. You could try to grab only the first 100 chars, get the substring . Alternatively, use the length argument of fgets()

• Create a constant for 100, like #define MAX_BUFFER_LENGTH 100 , and use it every time.