fread and fwrite a linked list in C - c

Question

Here is my struct:
struct Car{
char plateNum[10];
char returnDate[7];
int milage;
float income;
struct Car * next;
};
typedef struct Car Car;
I need to use fwrite and fread to store the value and load back in after. Is there an easy way?

Answer 1

To write a LL to a file
// Be sure to have opened the file in binary mode
Car *x = head;
// Walk the list and write each node.
// No need to write the next field - which happens to be the last one.
// v-----------------v size of data before the `next` field
while (x && fwrite(x, offsetof(Car, next), 1, out_stream) == 1) {
x = x->next;
}
To read records from a file into a LL and return the head node:
#include <stddef.h>
// Be sure to have opened the file in binary mode
Car *ReadCars(FILE *in_stream) {
Car Top;
Top.next = NULL; // code only uses the `next` field of Top
Car *previous = &Top;
Car x;
// While another record was successfully read ...
while (fread(&x, offsetof(Car, next), 1, in_stream) == 1) {
// Fill the next field
x.next = NULL;
// Allocate space and copy
previous->next = malloc(sizeof *(previous->next));
assert(previous->next);
*(previous->next) = x;
// Advance to the next
previous = previous->next;
}
return Top.next;
}

Answer 2

The following was written off the cuff by me and has not been tested, so it might need tweaking. Please also note; for the sake of time, I have not tested the return value of fwrite and fread or checked for read errors. YOU SHOULD DO THIS.
Writing the file
int length = lengthOfList(bar); // Assuming you've already created bar as a linked list of Cars
Car foo[length];
putLinkedListIntoArray(&bar, foo);
FILE* fh = NULL;
if((fh = fopen("filename", "wb")) == NULL) {
// Error and die
}
fwrite(&length, sizeof(int), 1, fh);
fwrite(bar, sizeof(Car), length, fh);
fclose(fh);
Reading the file
FILE* fh = NULL;
if((fh = fopen("filename", "rb")) == NULL) {
// Error and die
}
int length;
fread(&length, sizeof(int), 1, fh);
Car foo[length];
fread(foo, sizeof(Car), length, fh);
fclose(fh);
relinkCarList(foo, length);
Functions
int lengthOfList(Car* start) {
int length;
for(length = 0; start->next != NULL; length++) {
start = start->next;
}
return length;
}
void putLinkedListIntoArray(Car* start, Car* array) {
for(int i = 0; start->next != NULL; i++) {
array[i] = *start;
start = start->next;
}
}
void relinkCarList(Car* array, int length) {
for(int i = 0; i < length; i++) {
if(i < length - 1) {
array[i].next = array[i + 1].next;
}
}
}