Read/Write to file using void pointers in C - c

Question

As part of a college assignment, I'm trying to do a simple C app, using Win32 for GUI programming and writing my own dynamic linked list for storing data. Since i could use it for other things later, I'm trying to write a generic list, with "built in" functions for reading and writing data to a file. Now here's my problem
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef struct tagA{
int t_int;
float t_float;
char t_char;
char t_vchar[30];
} A;
typedef struct tagB{
void *data;
struct tagB *prox;
} B;
int main(){
A dA = {1, 1.5, 'a', "teste123"};
B dB = {&dA, NULL};
dB.data = &dA;
B dB2 = {0};
FILE *fp;
fp = fopen("Data.txt", "w");
size_t dSize = sizeof(dA);
printf("Struct size: %d", dSize);
if(fp == NULL){
printf("nope");
}else{
fwrite(&dB.data, dSize, 1, fp);
}
fclose(fp);
fp = fopen("Data.txt", "r");
dB2.data = malloc(dSize);
fread(&dB2.data, dSize, 1, fp);
printf("\n\n%s", ((A*)dB2.data)->t_vchar);
}
This is the code I'm trying to work out.
The writing works just fine, and I just have to make it to a separate function that receives the pointer to the data struct and its size.
But the reading is off... Once read and printed to the console, the t_int shows a really large number, the t_float has nothing, and so the char and string...
Also, I know its not the best written, most efficient and safe code, but it's just a prototype, something that came into my mind...

Answer 1

Your problem is you are writing the address of &dB.data with fwrite instead of dB.data itself (it is already a pointer). For example:
fwrite(dB.data, dSize, 1, fp);
will resolve your problem (with the like corresponding change to fread).

Answer 2

void *data;
It's not useful to declare data as void*. Declare it as A *data; instead.
fwrite(&dB.data, dSize, 1, fp);
...
fread(&dB2.data, dSize, 1, fp);
dB2.data is a pointer, just use dB2.data. See example below. For better readability you can declare ptr_data instead of data
typedef struct tagA {
int t_int;
float t_float;
char t_char;
char t_vchar[30];
} A;
typedef struct tagB {
A *data;
struct tagB *prox;
} B;
int main() {
A dA = { 1, 1.5, 'a', "teste123" };
B dB = { 0 };
dB.data = &dA;
B dB2 = { 0 };
FILE *fp;
fp = fopen("Data.txt", "w");
size_t dSize = sizeof(dA);
printf("Struct size: %d", dSize);
if(fp == NULL) {
printf("nope");
}
else {
fwrite(dB.data, dSize, 1, fp);
}
fclose(fp);
fp = fopen("Data.txt", "r");
dB2.data = malloc(dSize);
fread(dB2.data, dSize, 1, fp);
printf("\n\n%s\n", dB2.data->t_vchar);
return 0;
}

Answer 3

Your error is at fwrite(), fread(). The first param of them is the pointer which points at the real data you want to write/read.
Since you want to read/write a struct A, the pointer must actually point at struct A, not struct A*.
Just replace
fwrite(&dB.data, dSize, 1, fp);
fread(&dB2.data, dSize, 1, fp);
with
fwrite(dB.data, dSize, 1, fp);
fread(dB2.data, dSize, 1, fp);