Adding a char array to a linked list

Question

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

typedef struct node{
    char word[20];
    struct node * next;
}node;

int main(){
    FILE *ifp;
    char newword[20];
    node * head;

    ifp = fopen("para.txt","r");
    head = (node * )malloc(sizeof(node));

    while(fscanf(ifp,"%s",newword) != EOF){
         head -> next = NULL;
         head -> word = newword;
     }

    return 0;
}

I want to add the words which is read by the text file to a link list. I tried to do with this code but I couldn't. How can I fix this.

1. You should not cast malloc 2. Should check the return value from fopen — Ed Heal
– Ed Heal, Commented Jun 6, 2018 at 18:23
Possible duplicate of Assigning a char pointer to char array in C — n. m. could be an AI
– n. m. could be an AI, Commented Jun 6, 2018 at 18:24

Ed Heal · Accepted Answer · 2018-06-06 18:38:33Z

1

You only allocate one node (head) and then change its contents each iteration of the loop. To create a linked list, you need to allocate a new node for each word (each iteration of the loop). Something like this should do it:

int main(){
    FILE *ifp;
    char newword[20];
    node * head = NULL;
    node  *last = NULL;
    node  *current;

    ifp = fopen("para.txt","r");
    if (ifp == NULL) {
        fprintf(stderr, "Unable to open file para.txt\n");
        return EXIT_FAILURE;
    }
    while(fscanf(ifp,"%19s",newword) != EOF){
         current = malloc(sizeof(node));
         strcpy(current -> word,newword);
         if(last) {
             last->next = current;
         }
         else {
             head = current;
         }
         last = current;
    }

    return EXIT_SUCCESS;
}

edited Jun 6, 2018 at 18:38

Ed Heal

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answered Jun 6, 2018 at 18:23

Linuxios

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4 Comments

Ed Heal Over a year ago

Shouldn't current -> word = newword; use strcpy?

Linuxios Over a year ago

@EdHeal: Yes, thanks; edit was made by krpra (thank you).

ChuckCottrill Over a year ago

Works until strlen(word)>20, and since strcpy used, overwrites buffer

Ed Heal Over a year ago

I have added a check to ensure no buffer overrun and fopen check.

ChuckCottrill · Accepted Answer · 2018-06-06 18:35:57Z

Keep track of a head and tail, or just push at head. The following appends to end, efficiently.

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

typedef struct node{
    struct node * next;
    char word[1];
    //or, char word[20];//fixed length word
}node;

node*
nodeNew(char* word) {
    if(!word) return NULL;
    //either dynamically allocate for strlen(word)
    node* pnode = malloc( sizeof(node)+strlen(word) );
    //or fixed len word 
    if( pnode ) {
        pnode->next = NULL;
        strcpy(pnode->word, word);
        //or, strncpy(pnode->word, word, 20-1); //fixed length
    }
    return pnode;
}

int main()
{
    FILE *ifp;
    char newword[200]; //since using fscanf, need to avoid buffer overflow, should use fgets and strtok instead
    node * head;

    if( !(ifp = fopen("para.txt","r") ) { printf("error\n"); exit(0); }
    head = NULL;

    while(fscanf(ifp,"%s",newword) != EOF){
        if( !head ) { tail = head = nodeNew(newword); }
        else { tail->next = nodeNew(newword);
    }
    //head points to first element, head->next points to next element
    //tail points to last element

    return 0;
}

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