（从文件中读取）C 中的链表

Question

I'm new to C programming, and I'm having some difficulties. I'm trying to re-code to use a linked list to store the structs you read from the file, but I can't understand how to save the text line for line and add it to the linked list.

This is my code:

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


void write_to_file() {
    FILE* file = NULL;
    file = fopen("Cars.dat", "wb");
    Car c = { "Toyota", "Civic", "black", 5, 50000 };
    size_t ret = fwrite(&c, sizeof c, 1, file);
    Car d = { "Toyota1", "Civic1", "Red", 2, 55000 };
    ret = fwrite(&d, sizeof d, 1, file);
    fclose(file);

}

void print_cars(Car *c, int count) {
    FILE* f = fopen("output.txt", "w");
    for (int i = 0; i < count; i++) {
        fprintf(f, "%s %s %s %d %f\n", c[i].model, c[i].manufacturer, c[i].color, c[i].seatCapacity, c[i].price);
    }
    fclose(f);
}


void read_from_file() {
    Car c;
    size_t SIZE = sizeof c;
    size_t ret = 1;
    FILE* file = NULL;
    file = fopen("Cars.dat", "rb");
    int count = 0;
    do{
        ret = fread(&c,  sizeof c, 1, file);
        if (ret != 1)
            break;
        count++;
        //printf("%s %s %s %d %f\n", c.model, c.manufacturer, c.color, c.seatCapacity, c.price);
    }while(!feof(file));
    fclose(file);
    if (count == 0) {
        printf("No cars in the file provided\n");
        return;
    }
    file = fopen("Cars.dat", "rb");
    Car* cars_array = NULL;
    cars_array = (Car *)malloc(count * SIZE);
    ret = fread(cars_array, SIZE*count, 1, file);
    //printf("%d\n", ret);
    print_cars(cars_array, count);

}

int main()
{
    //write_to_file();
    read_from_file();
    return 0;
}

Struct code:

#ifndef Cars_h
#define Cars_h

typedef struct cars {
    char manufacturer[35]; //toyota, honda
    char model[35]; //camry, civic
    char color[20]; //black, red
    int seatCapacity; //4,5
    float price; //20k, 25k 
}
Car;

#endif

If anyone could give me some pointers on what I need to do next to make it work, I would highly appreciate it. Thank you

Answer 1

You could implement a linked list of ' Car ' objects by adding a next pointer to the Car structure. For example:

typedef struct Car {
    /* existing fields */
    char model[LLL];
    char manufacturer[MMM];
    char color[CCC];
    int seatCapacity;
    float price;

    struct Car *next; /* new member for linked list */
} Car;

(An assumption is made about Car as source code for Cars.h was not provided.)

You would have to account for this extra field in Cars.dat (where it could be stored with a NULL value assuming that the file has just an array and no notion of a linked list).

When reading, you could malloc space for each Car separately, fread it into the malloc -ed buffer, and update the previous ' Car 's next pointer to point to the newly loaded Car . In this manner you can build up a simple linked list of Car s.

ps I've never heard of a Toyota Civic:)

Answer 2

You can create a new struct as below which includes your Car struct as a field. And a pointer to the next node.

typedef struct {
    Car payload;
    CarNode* nextNode;
}CarNode

And you don't need to calculate the count of records in the file. Just read one and malloc a CarNode and use the CarNode.payload field as the buffer to hold the content read from the file. Something like below snippet:

CarNode head, tail; // Suppose you are using single linked list.
/*
Initialize the head and tail pointer. At first, they should both point to the first node.
*/
...

/*
Allocate a new node while reading a record from the file.
*/
CarNode *p = (CarNode *)malloc(sizeof(CarNode));
ret = fread(&(p->payload), sizeof(Car), 1, file);

/*
Insert the newly read node into the link list.
*/
tail->nextNode = p;
tail = p;