Question

I’ve finally figured out how to write some specifically formatted information to a binary file, but now my problem is reading it back and building it back the way it originally was.

Here is my function to write the data:

void save_disk(disk aDisk)
{
    ofstream myfile("disk01", ios::out | ios::binary);
    int32_t entries;
    entries = (int32_t) aDisk.current_file.size();
    char buffer[10];
    sprintf(buffer, "%d",entries);

    myfile.write(buffer, sizeof(int32_t));

    std::for_each(aDisk.current_file.begin(), aDisk.current_file.end(), [&] (const file_node& aFile)
    {
        myfile.write(aFile.name, MAX_FILE_NAME);
        myfile.write(aFile.data, BLOCK_SIZE - MAX_FILE_NAME);
    });

}

and my structure that it originally was created with and what I want to load it back into is composed as follows.

struct file_node
{
    char  name[MAX_FILE_NAME];
    char  data[BLOCK_SIZE - MAX_FILE_NAME];

    file_node(){};
};

struct disk
{
    vector<file_node> current_file;
};

I don’t really know how to read it back in so that it is arranged the same way, but here is my pathetic attempt anyway (I just tried to reverse what I did for saving):

void load_disk(disk aDisk)
{
    ifstream myFile("disk01", ios::in | ios::binary);
    char buffer[10];

    myFile.read(buffer, sizeof(int32_t));

    std::for_each(aDisk.current_file.begin(), aDisk.current_file.end(), [&] (file_node& aFile)
    {
        myFile.read(aFile.name, MAX_FILE_NAME);
        myFile.read(aFile.data, BLOCK_SIZE - MAX_FILE_NAME);
    });

}

^^ This is absolutely wrong. ^^

I understand the basic operations of the ifstream, but really all I know how to do with it is read in a file of text, anything more complicated than that I’m kind of lost.

Any suggestions on how I can read this in?

Answer 1

You’re very close. You need to write and read the length as binary.

This part of your length-write is wrong:

char buffer[10];
sprintf(buffer, "%d",entries);
myfile.write(buffer, sizeof(int32_t));

It only writes the first four bytes of whatever the length is, but the length is character data from a sprintf() call. You need to write this as a binary-value of entries (the integer):

// writing your entry count.
uint32_t entries = (uint32_t)aDisk.current_file.size();
entries = htonl(entries);
myfile.write((char*)&entries, sizeof(entries));

Then on the read:

// reading the entry count
uint32_t entries = 0;
myFile.read((char*)&entries, sizeof(entries));
entries = ntohl(entries);

// Use this to resize your vector; for_each has places to stuff data now.
aDisk.current_file.resize(entries);
std::for_each(aDisk.current_file.begin(), aDisk.current_file.end(), [&] (file_node& aFile)
{
    myFile.read(aFile.name, MAX_FILE_NAME);
    myFile.read(aFile.data, BLOCK_SIZE - MAX_FILE_NAME);
});

Or something like that.

Note 1: this does NO error checking nor does it account for portability for potentially different endian-ness on different host machines (a big-endian machine writing the file, a little endian machine reading it). Thats probably ok for your needs, but you should at least be aware of it.

Note 2: Pass your input disk parameter to load_disk() by reference:

void load_disk(disk& aDisk)

EDIT Cleaning file_node content on construction

struct file_node
{
    char  name[MAX_FILE_NAME];
    char  data[BLOCK_SIZE - MAX_FILE_NAME];

    file_node()
    { 
        memset(name, 0, sizeof(name));
        memset(data, 0, sizeof(data));
    }
};

If you are using a compliant C++11 compiler:

struct file_node
{
    char  name[MAX_FILE_NAME];
    char  data[BLOCK_SIZE - MAX_FILE_NAME];

    file_node() : name(), data() {}
};