为什么我的代码链表不返回 NULL 和 SEG FAULT

Question

I try to code a pattern to copy, duplicate a linked list in a struct pointer, but when my last element is print that is cause a seg fault and the last element return an address of type 0x0 or 0xF00000000000 . I don't find where I miss something in my code, may be when I duplicate int dup(t_child **ref, t_child *src) the linked list in my struct, but my knowledge in the C is limited.

below I try to make a simple code to reproduce my problem.

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

typedef struct s_child t_child;
struct s_child {
    int id;
    t_child *next;
};

typedef struct s_mother t_mother;
struct s_mother {
    t_child *child;
    t_mother *next;
};

int add_child(t_child **ref, int rank) {
    t_child *temp;
    temp = NULL;
    if(!(temp = (t_child*)malloc(sizeof(t_child))))
        return (0);
    temp->id = rank;
    temp->next = (*ref);
    (*ref) = temp;
    return(1);
}

int dup(t_child **ref, t_child *src) {
    int rank = 0;
    int ret = 0;
  while(src) {
        ret = add_child(ref, rank);
        if(!ret)
            break;
        rank++;
        src = src->next;
    }
    return(ret);
}

int add_mother(t_mother **ref, t_child *c) {
    t_mother *temp;
    temp = NULL;
    if(!(temp = (t_mother*)malloc(sizeof(t_mother))))
        return (0);
    dup(&temp->child, c);
    temp->next = (*ref);
  (*ref) = temp;
    return(1);
}

int main() {
    t_child *c;
    c = NULL;
    for(int i = 0 ; i < 4 ; i++) {
    add_child(&c, i);
  }

  t_mother *m;
  m = NULL;
  add_mother(&m, c);

    while(m->child) {
        printf("id: %i\n",m->child->id);
        m->child = m->child->next;
        printf("m->child %p\n", m->child);
        if(m->child == NULL) 
            printf("m->child NULL\n");
    }
    return(0);
}

terminal output

id: 3
m->child 0x7fb302402b60
id: 2
m->child 0x7fb302402b50
id: 1
m->child 0x7fb302402b40
id: 0
m->child 0xf000000000000000
[1]    16550 segmentation fault  ./a.out

Answer 1

Hey did you check that your linked list ended with NULL or even setting the first pointer with NULL ?

When you "move" inside your linked list you must have NULL as last element, then in case like :

while(*ptr_on_list* != NULL)

you can't segfault. So also check you stop "moving" in your linked list BEFORE you reach the NULL .

Hope i helped you, feel free to notice me if not