strcpy / strncmp segmentation fault in struct fileds

Question

I'm trying to add a new node to a struct with a char* field (word)

Definition of listT:

enum boolean {false, true};
struct list {
    enum boolean sorted;
    union{
        int words;
        char *word;
    };
    struct list* next;
    struct list* previous;
};
typedef struct list listT;

add_word_node function is being called by main as: add_word_node(read_word, list_head) where read_word is given by the user with scanf. Word is being pass as a string but after strncpy there is no terminating byte.

>Debugger:
     add_word_node (word=0xbffff0fa "ally", head=0x804c038) at prog.c    
>Debugger:
     (gdb) p newnode->word


     $2 = 0x804c068 "allyP\224\373\267\377\377\377\377" 




 listT *add_word_node(char word[], listT *head) {
    listT *newnode;
    listT *curr = NULL;//sorted
    //listT *prev;//sorted

    newnode = malloc(sizeof(listT)); /* allocate new node and check */
    newnode->word = malloc(sizeof(char)* WORDLEN);
    strncpy(newnode->word, word, strlen(word));
    //newnode->word = strndup(word, strlen(word));


    if (newnode == NULL) {
        return (NULL);
    }

    if (head->sorted == false){ //eisagwgh sto telos ths listas
        newnode->next = head;
        head->previous->next = newnode;
        newnode->previous = head->previous;
        head->previous = newnode;
    }else {
        if(head->next == head){
            newnode->next = head;
            newnode->previous = head;
            head->next = newnode;
            head->previous = newnode;

        }else{
            for (curr = head->next; curr->next != NULL; curr = curr->next){//for( curr = head->next; ;){
                if(curr == NULL) break;
                if(strncmp(curr->word,newnode->word,strlen(word)) > 0) break;
                //else curr= curr->next;
            }
            if(strncmp(curr->word,newnode->word,strlen(word))== 0){
                return(curr);
            }
            newnode->next = curr;
            newnode->previous = curr->previous;
            newnode->previous->next = newnode;
            newnode->next->previous = newnode;
        }
    }

    return (newnode);

}

I have read some other topics about this problem and I changed the function to use word[] instead of char* but it still doesn't work. Please tell me if you need more info. Also, when I use strndup, it sometimes work without any error.

Answer 1

As you say you have a char *word then this pointer should be allocated memory

newnode->word = malloc(sizeof(char) * (WORDLEN+1)); /* Please access the structure elements accordingly */

Memory should be allocated to the pointer before writing something to it.

Answer 2

To expand @Gopi's answer a bit:

Your statement

newnode = (listT *) malloc(sizeof(listT) + WORDLEN);

allocates memory to the pointer newnode . This means that, for example if WORDLEN is the same as sizeof(listT) , then you will be allocating memory for two listT elements for newnode . This is similar to, say, char *ptr = malloc(2); which will allocate memory for two char elements for ptr .

TL;DR: This WORDLEN memory will NOT be allocated to newnode->word , it is allocated to newnode , and that's that. To fix your problem, you must allocate memory to newnode->word separately. newnode does not need that memory, but newnode->word does.

strndup() works because it allocates the memory for you.

If you still struggle to understand this, you may have some misconceptions on pointers.

newnode->word is a pointer. It has a fixed size on a certain machine (eg 8 bytes). For simplicity assume listT is defined as follows:

typedef struct {
    char *word;
} listT;

Then when you allocate memory, you only need to allocate 8 bytes to newnode so that it can hold one pointer, and then you need to allocate memory to that pointer, which will be used to store some characters.

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Although not related to your problem, I would also like to point out that you shouldn't cast the return value of malloc() . See Do I cast the result of malloc?