Writing a linked list to binary file(C)

Question

So I have set up a linked list and read data from a file, done some calculation and manipulation and now I want to store the new list into a binary file.

Here is my struct setup:

typedef struct Comp{
    char name[5];
    char node1[5], node2[5];
    float value;    //value
}ComponentType;

typedef struct ListNodeT{

    ComponentType Component;
    float voltage, power, current;
    struct ListNodeT *nextPtr;
}ListNodeType;

In a separate function I am trying to write to a a file:

FILE *filePtr;

char fileName[13] = "SaveData.bin";
filePtr = fopen(fileName, "wb");
int index = 0;

while (CircuitData != NULL)
{
    fwrite(CircuitData, sizeof(ListNodeType), 1, filePtr);
    CircuitData = CircuitData->nextPtr;
    index++;
}

The above code is not working so now my question is, can I write to the file using a single fwrite(CircuitData, sizeof(ListNodeType), 1, filePtr) or should I write each component separately as follows:

fwrite(CircuitData->Component.name, sizeof(CircuitData->Component.name), 1, filePtr);

How should I be doing this? My second question is, how would I read this binary file again back into the Struct?

Answer 1

Well, you won't be able to use the pointer-values from a file in a new invocation of the program, so you'll need to do something about that. You could make a copy before printing and zero-out the pointer; then when reading, you'll need to assign the correct pointer values for the newly allocated structs.

I'd probably read them with a recursive function. Something like this:

ListNodeType *read_list_from_file (FILE *fileptr) {
    ListNodeType node;
    ListNodeType *nodeptr;
    if (fread(&node, sizeof(node), 1, fileptr) == sizeof(node)) {
        nodeptr = malloc(sizeof(node));
        //TODO: handle malloc failure
        memcpy(nodeptr, &node, sizeof(node);
        nodeptr->nextPtr = read_list_from_file(fileptr);
        return nodeptr;
    } else {
        return NULL;
    }
}