Reading binary data with seekg fails.

Question

I'm trying to read binary data from a specific offset.

I write the data in the following way:

long RecordIO::writeRecord(Data *record)
{
    this->openWrite();

    fstream::pos_type offset = file->tellp();
    file->write(reinterpret_cast<char*>(record), sizeof(Data));
    return (long)offset;
}

The offset returned is stored, and retrieved later. Data is a struct with the data.

Later i try to read that same data again with the following code:

Data* RecordIO::getRecord(long offset)
{
    openRead();
    file->seekg((fstream::pos_type) offset);
    Data data;
    file->read(reinterpret_cast<char *>(&data), sizeof(Data));
    return new Data(data);
}

sizeof(Data) returns 768. Some off the offsets i get back is 768 and 1536. But when I check the contents of the data, i get complete gibberish. Am i doeing something wrong? Edit:

This is the struct:

struct Data{
  long key;
  char postcode[8];
  char info1[251];
  char info2[251];
  char info3[251];
};

And this is how i fill it:

for(int i = 1; i <= numOfRecords; ++i){
    newData.key = i;

    newData.postcode[0] = '1' + (rand() % 8);
    newData.postcode[1] = '0' + (rand() % 9);
    newData.postcode[2] = '0' + (rand() % 9);
    newData.postcode[3] = '0' + (rand() % 9);
    newData.postcode[4] = ' ';
    newData.postcode[5] = 'A' + (rand() % 25);
    newData.postcode[6] = 'Z' - (rand() % 25);
    newData.postcode[7] = '\0';

 for(int j = 0; j < 250; ++j){
        newData.info1[j] = '+';
        newData.info2[j] = '*';
        newData.info3[j] = '-';
    }

 newData.info1[250] = '\0';
    newData.info2[250] = '\0';
    newData.info3[250] = '\0';

    int offset = file->writeRecord(&newData);
 index->setOffset(i, offset);
}

Btw, the data is stored correctly, because i can retreive them one by one, sequentialy

Answer 1

Show us the definition of the Data structure. I suspect that Data is not a POD (plain-old-data) type and requires more specialized serialization.

Edit: Thanks. That's a POD struct, so this is not the problem.

Answer 2

You do this:

file->write(reinterpret_cast<char*>(record), sizeof(Data));

Do you ever close or flush the file? The data will be buffered in memory to be written to disk later unless you force it.