Question

I’m having amazing difficulties using the fread and fwrite functions in C++.

The project is writing a rudimentary FAT16 file system and we are restricted to using fread and fwrite.

When I am initially writing the file my code looks like this:

        directoryTable = (directoryEntry *)malloc(clusterSize);

    for (int i = 0; i < bootRecord[0] / 128 ; ++i){

        directoryEntry * newEntry = (directoryEntry *)malloc(sizeof(directoryEntry));
        newEntry->name = (char *)malloc(112);
        newEntry->name[0] = 'a'
        write(1, &(newEntry->name[1]), 1);
        newEntry->size = 0;
        newEntry->type = 0;
        newEntry->creation = 0x0000;
        newEntry->index = 0;

        fwrite(newEntry->name, 112, 1, fp);
        fwrite(&newEntry->size, sizeof(int), 1, fp);
        fwrite(&newEntry->type, sizeof(int), 1, fp);
        fwrite(&newEntry->creation, sizeof(int), 1, fp);
        fwrite(&newEntry->index, sizeof(int), 1, fp);

        directoryTable[i] = *newEntry;
    }

When I’m assigning the first character of directoryEntry->name to ‘a’, my intent is actually to assign it the value 0x00 so I can check later if it’s null. I simply am using a right for debugging purposes. My problem is when I read it, I get nothing back.

And when I’m reading my code looks like this:

        fseek(fp, clusterSize * root, SEEK_SET);

    for(int i = 0; i < clusterSize / 128; ++i){
        directoryEntry * newEntry = (directoryEntry *) malloc(128);
        newEntry->name = (char *) malloc(112);
        fread(newEntry->name, 112, 1, fp);
        write(1, newEntry->name[0],1);
        fread(&newEntry->size, sizeof(int), 1, fp);
        fread(&newEntry->type, sizeof(int), 1, fp);
        fread(&newEntry->creation, sizeof(int), 1, fp);
        fread(&newEntry->index, sizeof(int), 1, fp);
        directoryTable[i] = *newEntry;
    }

It should be noted that the values of clusterSize and root are also read in using similar methods. I’ve already checked and their values are accurate in both situations. Since I was able to read those in without a problem, I have no idea why I’m having such a big problem now. I feel my use of malloc is not quite right, I’ve never worked with it before.

Also, here is a definition of directoryTable if needed:

     typedef struct{
char * name;
unsigned int index;
unsigned int size;
unsigned int type;
unsigned int creation;
    } directoryEntry;

Thank you guys for you time and if you need me to clarify on anything I’d be happy to.

Answer 1

I see a lot of little problems:

1. You do not zero out the memory you malloc’d for name. Try memset(newentry->name, 0, 112);
2. You are writing the second element of the name array when you write it (index 1), and the first element of the name array when you read it (index 0): write(1, &(newEntry->name[1]), 1); vs. write(1, newEntry->name[0],1);
3. Additionally in the first write you are taking the address of it, and in the second write you are not, so it looks likes the second one should read: write(1, &(newEntry->name[0]),1);
4. The call to fseek looks correct, but as a reader of the question I have no way of knowing if clusterSize * root is the correct offset into the file you are reading from. You also do not seek to that offset when you start writing the file (or at least don’t show it), so it becomes murkier.
5. What exactly does the 128 you keep dividing by in your for loops represent? Also in one spot you divide clusterSize by it and in another bootrecord[0]. I assume those are the same size, but…
6. In the read loop you malloc 128 bytes of memory, but in the write loop you malloc (sizeof(directoryEntry). Those are two different sizes, and malloc’ing the 128 bytes is wrong, since the structure contains a pointer to the 112 bytes, and not the actual 112 bytes for name.

I am not sure if any of these are causing the problem you are seeing, which you don’t actually specify by the way, but it may at least get you pointed in the right direction.