C++ Writing bitset to binary file

Question

hopefully someone here can help. My problem is as follows:

I am creating files which contain binary data. At the start of each file is a binary header which has information regarding the contents of the file. The file header is a fixed size, 52 bytes. The header has specific pieces of information at specific byte offsets within the header, however some pieces of information only cover portions of a byte, say 3 bits.

For example:

Byte 1-4 = file length

Byte 5-8 = header length

Byte 8-9 = version info

Byte 10-13 = file creation timestamp

bit 1-4 = Month (1-12)

bit 5-9 = Day (1-31)

bit 10-14 = Hour (0-23)

bit 15-20 = Minute (0-59)

bit 21 = UTC offset direction

bit 22-26 = UTC offset hour

bit 27-32 = UTC offset minute

etc...

Some of the values are defined statically, some are determined at runtime. What I've attempted to do is create a 'map' of the header, defining the number of bits an attribute must consume, and the value represented by the bits. These are stored in a vector of int pairs, int_pair.first being the value, and int_pair.second the number of bits. I then convert the supplied values (all integers) to binary format and insert the binary notation into a stringstream. I then create a bitset from the string representation of the binary value, and write that to a file. My problem is that the bytes are not showing up in the output file in the proper order.

I'll omit the method for obtaining the values and just supply integers in my example, and I'll truncate some of the info in the header for brevity (so in this example the header is 14 bytes, not 52), but here is roughly what I'm doing:

#include <iostream>
#include <fstream>
#include <sstream>
#include <string>
#include <bitset>
#include <vector>
#include <algorithm>

int main ()
{
    vector<pair<int,int>> header_vec;

    header_vec.push_back(make_pair(9882719,32)); // file length
    header_vec.push_back(make_pair(52,32)); // header length
    header_vec.push_back(make_pair(6,3)); // high release identifier
    header_vec.push_back(make_pair(4,5)); // high version identifier
    header_vec.push_back(make_pair(6,3)); // low release identifier
    header_vec.push_back(make_pair(4,5)); // low version identifier

    // file open timestamp
    header_vec.push_back(make_pair(9,4));  // month
    header_vec.push_back(make_pair(6,5));  // day
    header_vec.push_back(make_pair(19,5)); // hour
    header_vec.push_back(make_pair(47,6)); // min
    header_vec.push_back(make_pair(0,1));  // utc direction
    header_vec.push_back(make_pair(0,5));  // utc offset hours
    header_vec.push_back(make_pair(0,6));  // utc offset minutes

    ostringstream oss;

    // convert each integer to binary representation
    for ( auto i : header_vec )
    {
        for (unsigned int j(i.second-1); j != -1; --j)
        {
            oss << ((i.first &(1 << j)) ? 1 : 0);
        }
    }

    // copy oss
    string str = oss.str();

    // create bitset
    bitset<112> header_bits(string(str.c_str()));

    // write bitset to file
    ofstream output("header.out", ios::out | ios::binary );
    output.write( reinterpret_cast<char *>(&header_bits), 14);
    output.close();

    return 0;

}

Now, for the most part this method seems to work, except that the bits are reversed. If I look at the output file in fm, I expect to see this:

File: header.out    (0x0e bytes)
Byte: 0x0

00    00 96 cc 5f 00 00 00 34 c4 c4 93 4e f0 00           ..._...4...N...O

      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f      0123456789abcdef

When in fact I see this:

File: header.out    (0x0e bytes)
Byte: 0x0

00    00 f0 4e 93 c4 c4 34 00 00 00 5f cc 96 00           @O...N...4..._..

      0  1  2  3  4  5  6  7  8  9  a  b  c  d  e  f      0123456789abcdef

I tried to reverse str prior to creating the bitset, but that does not yield the desired output either.

I suppose I dont understand enough about the bitset to realize why this is happening. Any and all input is greatly appreciated! Also, if there is a different method for accomplishing this please share!

Thanks in advance... -J

Answer 1

Writing the bitset<> directly as a memory dump is surely non-portable, as shown by the need for the reinterpret_cast<>. In other words, even if the data is laid out in a nice block, you do not know how that is done.

If I were you, I'd write a dumber function to extract chunks of 8-bits from the bitset and write them as bytes to the file using the normal access operator [].

As for another approach, what I usualy do when I want to read/ write a binary file is define a structure or set of structures that map directly to the layout of the file.

For example:

struct Timestamp
{
    int month:4;
    int day:5;
    int hour:5;
    int minute:6;
    int utcOffsetDirection:1;
    int utcOffsetHour:5;
    int utcOffsetMinute:5;

};