Adding a string or char array to a byte vector

Question

I'm currently working on a class to create and read out packets send through the network, so far I have it working with 16bit and 8bit integers (Well unsigned but still).

Now the problem is I've tried numerous ways of copying it over but somehow the _buffer got mangled, it segfaulted, or the result was wrong.

I'd appreciate if someone could show me a working example.

My current code can be seen below.

Thanks, Xeross

Main

#include <iostream>
#include <stdio.h>
#include "Packet.h"

using namespace std;

int main(int argc, char** argv)
{
    cout << "#################################" << endl;
    cout << "#       Internal Use Only       #" << endl;
    cout << "#     Codename PACKETSTORM      #" << endl;
    cout << "#################################" << endl;
    cout << endl;

    Packet packet = Packet();
    packet.SetOpcode(0x1f4d);

    cout << "Current opcode is: " << packet.GetOpcode() << endl << endl;

    packet.add(uint8_t(5))
          .add(uint16_t(4000))
          .add(uint8_t(5));

    for(uint8_t i=0; i<10;i++)
        printf("Byte %u = %x\n", i, packet._buffer[i]);

    printf("\nReading them out: \n1 = %u\n2 = %u\n3 = %u\n4 = %s",
        packet.readUint8(),
        packet.readUint16(),
        packet.readUint8());

    return 0;
}

Packet.h

#ifndef _PACKET_H_
#define _PACKET_H_

#include <iostream>
#include <vector>

#include <stdio.h>
#include <stdint.h>
#include <string.h>

using namespace std;

class Packet
{
    public:
        Packet() : m_opcode(0), _buffer(0), _wpos(0), _rpos(0) {}
        Packet(uint16_t opcode) : m_opcode(opcode), _buffer(0), _wpos(0), _rpos(0) {}

        uint16_t GetOpcode() { return m_opcode; }
        void SetOpcode(uint16_t opcode) { m_opcode = opcode; }

        Packet& add(uint8_t value)
        {
            if(_buffer.size() < _wpos + 1)
                _buffer.resize(_wpos + 1);

            memcpy(&_buffer[_wpos], &value, 1);
            _wpos += 1;

            return *this;
        }
        Packet& add(uint16_t value)
        {
            if(_buffer.size() < _wpos + 2)
                _buffer.resize(_wpos + 2);

            memcpy(&_buffer[_wpos], &value, 2);
            _wpos += 2;

            return *this;
        }

        uint8_t readUint8()
        {
            uint8_t result = _buffer[_rpos];
            _rpos += sizeof(uint8_t);
            return result;
        }
        uint16_t readUint16()
        {
            uint16_t result;
            memcpy(&result, &_buffer[_rpos], sizeof(uint16_t));

            _rpos += sizeof(uint16_t);
            return result;

        }

        uint16_t m_opcode;
        std::vector<uint8_t> _buffer;
    protected:

        size_t _wpos; // Write position
        size_t _rpos; // Read position
};

#endif // _PACKET_H_

Your question indicates a problem with appending a string or char array, but I don't see the code that attempts to do this. — Marcelo Cantos
– Marcelo Cantos, Commented Apr 28, 2010 at 12:21
Don't mix printf and cout. use cout only instead. Also stdio.h and it's likes are deprecated. Use <cstdio> for example. For each C header just type c before it and remove the .h — the_drow
– the_drow, Commented Apr 28, 2010 at 12:25
Just to mention: you could also use a template method for add() and use sizeof instead of hard-coded type lengths. To avoid code repetition. — ereOn
– ereOn, Commented Apr 28, 2010 at 12:30
@xeross: It will work with any type your code was designed for ;) In other words, if you only intend to add basic integers, keep it that way, and just replace the 1 and 2 with sizeof(T), where T is the template type. Check this link to get started: cplusplus.com/doc/tutorial/templates — ereOn
– ereOn, Commented Apr 28, 2010 at 12:44

Ferruccio · Accepted Answer · 2010-04-28 14:46:06Z

5

Since you're using an std::vector for your buffer, you may as well let it keep track of the write position itself and avoid having to keep manually resizing it. You can also avoid writing multiple overloads of the add function by using a function template:

template <class T>
Packet& add(T value) {
    std::copy((uint8_t*) &value, ((uint8_t*) &value) + sizeof(T), std::back_inserter(_buffer));
    return *this;
}

Now you can write any POD type to your buffer.

implicitly:

int i = 5;
o.write(i);

or explictly:

o.write<int>(5);

To read from the buffer, you will need to keep track of a read position:

template <class T>
T read() {
    T result;
    uint8_t *p = &_buffer[_rpos];
    std::copy(p, p + sizeof(T), (uint8_t*) &result);
    _rpos += sizeof(T);
    return result;
}

You will need to explicitly pass a type parameter to read. i.e.

int i = o.read<int>();

Caveat: I have used this pattern often, but since I am typing this off the top of my head, there may be a few errors in the code.

Edit: I just noticed that you want to be able to add strings or other non-POD types to your buffer. You can do that via template specialization:

template <>
Packet& add(std::string s) {
    add(string.length());
    for (size_t i = 0; i < string.length(); ++i)
        add(string[i]);
    return *this;
}

This tells the compiler: if add is called with a string type, use this function instead of the generic add() function.

and to read a string:

template <>
std::string read<>() {
    size_t len = read<size_t>();
    std::string s;
    while (len--)
        s += read<char>();
    return s;
}

edited Apr 28, 2010 at 14:46

answered Apr 28, 2010 at 12:36

Ferruccio

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5 Comments

Not Available Over a year ago

However I would need a seperate function for reading strings as those can be of variable length. Thanks for the code I'll try it out now.

Not Available Over a year ago

The second value should've been &value in copy((uint8_t*) &value, ((uint8_t*) &value) + sizeof(Type), back_inserter(_storage));

Not Available Over a year ago

Hmm okay all the writes are working, normal reads are working, but it seems I can't add specialization with an argument like template <> string read<string>(size_t length) {} any idea what I could use for this ?

Not Available Over a year ago

@Ferruccio that way I can't specify a length for the string

Ferruccio Over a year ago

You don't need to. The add method writes a length and string contents. The read method should read the length and build a string appropriately. I've added a string read method to my answer.

Dmitry Yudakov · Accepted Answer · 2010-04-28 13:40:01Z

1

You could use std::string as internal buffer and use append() when adding new elements.

Thus adding strings or const char* would be trivial.

Adding/writing uint8 can be done with casting it to char, writing uint16 - to char* with length sizeof(uint16_t).

void write_uint16( uint16_t val )
{
    m_strBuffer.append( (char*)(&var), sizeof(val) );
}

Reading uint16:

uint16_t read_int16()
{
    return ( *(uint16_t*)(m_strBuffer.c_str() + m_nOffset) );
}

answered Apr 28, 2010 at 13:40

Dmitry Yudakov

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Comments

Mark B · Accepted Answer · 2010-04-28 13:40:37Z

0

You appear to be attempting to print ten bytes out of the buffer when you've only added four, and thus you're running off the end of the vector. This could be causing your seg fault.

Also your printf is trying to print a character as an unsigned int with %x. You need to use static_cast<unsigned>(packet._buffer[i]) as the parameter.

Stylistically: Packet packet = Packet(); could potentially result in two objects being constructed. Just use Packet packet;

Generally try to avoid protected attributes (protected methods are fine) as they reduce encapsulation of your class.

answered Apr 28, 2010 at 13:40

Mark B

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1 Comment

Not Available Over a year ago

nope that wasn't causing the segfault, it printed just fine for some odd reason. I've now switched to using the template stuff Ferruccio proposed and that works pretty good, all that remains is the read function for strings.

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