C - How to Read in a Linked List from a Binary File?

Question

I need some assistance with my code. I've created the function for writing the linked list to a binary file. Now I'm trying to read in a linked list from a binary file that my write function has outputted. My attempt at trying to read in the binary file and creating the link segfaults. This is my attempt below.

What am I doing wrong?

void readlist(struct link **headptr) {                                                                                                                      
    FILE *text = fopen("numbers.bin", "rb");                                                                                                                 
    struct link *head = *rootptr;                                                                                                                                                                                                                                                           


   while (head->next != NULL) {                                                                                                                             
       struct link *newlink = (struct link*) malloc(sizeof(struct link));                                                                                    
       fread(&newlink->val, sizeof(int), 1, text);                                                                                                       
       head->next = newlink;                                                                                                                                 
       head = newlink;                                                                                                                                       
    }                                                                                                                                                        
       fclose(text);   
}                                                                                                                                                                                                                                                                              

Answer 1

I hope this code is helpful to you. if you use a global variable headptr, it will be more simple.

#include <stdio.h>
#include <stdlib.h>
#include <memory.h>
#include <unistd.h>
#include <fcntl.h>

#define LISTSAVE "numbers.bin"

struct link {
    int val ;
    struct link *next ;
} ;

void print_list(struct link *headptr) {
    struct link *tmp = headptr->next ;
    int cnt=0 ;
    printf("---list start----\n") ;
    while(tmp) {
        printf("%d ", tmp->val) ;
        cnt++ ;
        tmp=tmp->next ;
    }
    printf("\n%d items\n", cnt) ;
    printf("---list end----\n") ;
}


void add_first(struct link *headptr, int val) {
    struct link *tmp = malloc(sizeof(struct link)) ;
    tmp->val = val ;
    tmp->next = headptr->next ;
    headptr->next = tmp ;
}

void add_tail(struct link *headptr, int val) {
    struct link *tmp = headptr;
    struct link *tmp2 = malloc(sizeof(struct link)) ;
    tmp2->val = val ;
    tmp2->next=NULL ;

    while(tmp->next) {
        tmp=tmp->next ;
    }
    tmp->next=tmp2 ;
}

void del_list(struct link *headptr) {
    struct link *tmp = headptr->next ;
    struct link *tmp2 = NULL ;
    while (tmp) {
        tmp2=tmp->next ;
        free(tmp);
        tmp=tmp2 ;
    }
    headptr->next=NULL ;
}

void save_list(struct link *headptr) {
    FILE *text = fopen(LISTSAVE, "wb+") ;
    struct link *tmp=headptr->next ;
    int cnt=0 ;

    if ( text==NULL || headptr==NULL ) {
        printf("filed to save.\n") ;
        return ;
    }
    while (tmp!=NULL ) {
        cnt++ ;
        fwrite(&tmp->val, sizeof(int), 1, text) ;
        tmp = tmp->next ;
    }
    fclose(text) ;
    printf("write %d items ok\n", cnt) ;
}

void read_list(struct link *headptr) {
    FILE *text = fopen(LISTSAVE, "rb") ;
    int val ;
    int cnt=0 ;
    while( fread(&val, sizeof(int), 1, text) > 0 ) {
        add_tail(headptr, val) ;
        cnt++ ;
    }
    fclose(text);
    printf("read %d items ok\n", cnt) ;
}


int main() {
    struct link head ;
    head.val=0 ;
    head.next=NULL ;

    add_first(&head, 40) ;
    add_first(&head, 30) ;
    add_first(&head, 20) ;
    add_first(&head, 10) ;
    add_tail(&head, 50) ;
    add_tail(&head, 60) ;

    print_list(&head) ;
    printf("--save list\n") ;
    save_list(&head) ;
    del_list(&head) ;

    printf("--read list\n") ;
    read_list(&head) ;
    print_list(&head) ;
    del_list(&head) ;

    return 0 ;
}

the output is this

---list start----
10 20 30 40 50 60 
6 items
---list end----
--save list
write 6 items ok
--read list
read 6 items ok
---list start----
10 20 30 40 50 60 
6 items
---list end----

Answer 2

You can't save a generic linked list to a file. For it to work, you'd need to be in full control of memory allocations including the addresses you get. Generally, you won't be.

If you overlay the linked list over a single buffer, then you can save it by translating pointers to indices and back (or just by using indices instead of pointers), but generically, you cannot save a linked list to disk, free the memory (including by exiting), reload, and then expect the saved pointers to work.