通过链接列表获取段错误

Question

Apologies for the very basic question, but I can't figure this out. I am trying to build a simple linked list and append some values to it in C.

Below is the list.c file

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

#include "./list.h"

int main(int argc, char *argv[]) {
    int arr[] = {1,2,3,4};
    List *list = createList();
    int i = 0;
    for(i = 0; i < 4; i++) {
        appendList(list, arr[i]);
    }
    return 0;
}

List *createList() {
    List *list = malloc(sizeof(List));
    if(list == NULL) {
        return NULL;
    }

    list->head = malloc(sizeof(Node));
    list->tail = malloc(sizeof(Node));
    if(list->head == NULL || list->tail == NULL) {
        free(list);
        return NULL;
    }

    list->size = 0;
    return list;
}

void appendList(List *list, int num) {
    if(list->head->value == 0) {
        list->head->value = num;
        list->tail->value = num;
        return;
    }
    Node *current = calloc(1, sizeof(Node));
    current = list->head;
    while(current->next != NULL) {
        current = current->next;
    }
    current->next = calloc(1, sizeof(Node));
    if(current->next == NULL) {
        free(current->next);
        printf("Failed to allocate memory");
        exit(1);
    }
    current->next->value = num;
    list->size += 1;
    list->tail = current->next;
}

And below is the header file

#ifndef List_h
#define List_h

#include <stdlib.h>

typedef struct node {
    int value;
    struct node *next;
} Node;

typedef struct {
    Node *head;
    Node *tail;
    int size;
} List;

List *createList();
void appendList(List *, int num);
Node *removeList(List *);
void printList(List *);

#endif

While running through a debugger, my code seems to be working fine, which makes even less sense.

I assume my issue is in the while loop inside of appendList , where I am trying to access some unallocated piece of memory. Is the issue then with the check I am making, current->next != NULL ? Does accessing an unallocated piece of memory necessary return NULL ?

Answer 1

Hmm, well my thoughts are that you've created the initial head and tail Node s and you didn't set its value. Later you use value to determine whether or not you need to add another node or set head and tail to the value passed:

void appendList(List *list, int num) {
    if(list->head->value == 0) {
        list->head->value = num;
        list->tail->value = num;
        return;
    }
    ...

The memory returned from malloc will not be necessarily zero, so your algorithm should ensure that all values are set before proceeding.

You then proceed to reach the end of your list:

Node *current = calloc(1, sizeof(Node));
current = list->head;
while(current->next != NULL) {
    current = current->next;
}

However, again, while list->head exists, you never set the value of list->head->next ! Following an unassigned pointer is not going to end nicely for you in the best of cases.

Consider creating a method to create a new node:

Node* createNode() {
    Node* node = malloc(sizeof(Node));
    if(node == NULL) {
        return NULL;
    }
    node->value = 0;
    node->next = NULL;
    return node;
}

Also please note that there's a minor correction to the code here (unrelated to your segmentation fault, but could still create memory leak):

list->head = malloc(sizeof(Node));
list->tail = malloc(sizeof(Node));
if(list->head == NULL || list->tail == NULL) {
    free(list);
    return NULL;
}

Note that it is possible for list->head to correctly be assigned memory and list->tail to not be correctly assigned memory. In that case, you risk having a memory leak for list->head . Please take the necessary precautions.

Answer 2

Especially in embedded systems, the code compiled for debug mode and the one for release mode can differ. So, for me, there is no surprise that your code works in debug and won't in release.

When creating linked lists using malloc, it is possible that the compiler sets the address of your "struct node * next" element, a non-accessible location in memory. So if you try to access it, you'll get a segfault. (or BAD_EXC in MacOS)

If you suspect that malloc is your problem, try creating a small list with no malloc and see if you have segfault, ie use:

struct node myNode;
struct node* pmyNode = &myNode;

In your while loop, I suppose, you are trying to go to the last element of your list. So, instead of:

while(current->next != NULL) {
    current = current->next;
}

Try to do this:

last_linked_list_element->next = last_linked_list_element;

current = first_linked_list_element;

while(current != current->next) {
    current = current->next;
}

You will break out of the loop when you are at the last element of your list.

Another solution would be to try:

last_linked_list_element->next = NULL;

or

last_linked_list_element->next = &random_identifier;

This will make sure that the pointer locates to an accesible location in memory. Does this solve your problem?

Answer 3

In addition to the previous post, in the following code:

Node *current = calloc(1, sizeof(Node));
current = list->head;
while(current->next != NULL) {
    current = current->next;
}

you should to delete the line Node *current = calloc(1, sizeof(Node)); because in this way you allocate memory and than don't use it (subsequently you assign currect pointer to another value).