为什么在此链表创建中出现分段错误？

Question

I am very new to C and I am not understanding where the segmentation fault is occurring in my code. I am just trying to create a linked list which will be the type unit for a hash table. I understand that I probably didn't use a malloc where I was supposed to. The main function will run but as soon as I add one item I get the segmentation fault.

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


typedef struct word_link
{
    char* val;
    struct word_link * next;
} word_link;

void add_to_list(char* word, word_link *head);

void add_to_list(char* word, word_link *head){

  int i;
  word_link * temp = NULL;
  word_link * p = NULL;

  temp = (word_link*)malloc(sizeof(word_link));
  temp->val = word;
  temp->next = NULL;

  if(head == NULL){
    head = temp;
  } else {
    p = head;
    while(p->next != NULL){
      p = p->next;
    }
    p->next = temp;
  }
}
void main() {

  int i = 0;
  struct word_link *lst = malloc(sizeof(word_link));
  char* word = "";
  while(i == 0){
  printf("what to add?  ");
  scanf("%s",word);
  add_to_list(word, lst);
  printf("continue?  ");
  scanf("%d", i);
  }
  printf("%s", lst->val);
  printf("%s", "asdf;kl");
}

Answer 1

Your head is local to add_to_list function and will be destroyed once control exits add_to_list function.

Also any changes done to head inside add_to_list will not affect the original head lst .

Solution:

You can pass the reference of original head to insert in order to retain changes made in add_to_list as below.

void add_to_list(char* word, word_link **head);

void add_to_list(char* word, word_link **head){    
  int i; 
  word_link * temp = NULL;
  word_link * p = NULL;

  temp = (word_link*)malloc(sizeof(word_link));
  if (temp == NULL) return;

  temp->val = word;
  temp->next = NULL;

  if(*head == NULL){
    *head = temp;
  } else {
    p = *head;
    while(p->next != NULL){
      p = p->next;
   }
    p->next = temp;
  }
}

And you call add_to_list as below.

add_to_list(word, &lst);

Another Issue:

char* word = ""; this will create word as pointer to immutable string literal. Modifying the word content scanf("%s",word); will result in UB and since every time you are passing the same pointer to add_to_list every node in the list will point to same word .

You might want to declare it as below.

char *word = malloc(256); Inside the while loop and pass it to add_to_list .