Read "varint" from linux sockets

Question

I need to read a VarInts from linux sockets in C/C++. Any library, idea or something?

I tried reading and casting char to bool[8] to try without success to read a VarInt...

Also, this is for compatibility with new Minecraft 1.7.2 communication protocol, so, the documentation of the protocol may also help.

Let me explain my project: I'm making a Minecraft server software to run in my VPS (because java is too slow...) and I got stuck with the protocol. One thread waits for the connections and when it has a new connection, it creates a new Client object and starts the Client thread that starts communicating with the client.

I think that there is no need to show code. In case I'm wrong, tell me and I'll edit with some code.

Answer 1

First off, note that varints are sent as actual bytes, not strings of the characters 1 and 0.

For an unsigned varint, I believe the following will decode it for you, assuming you've got the varint data in a buffer pointed to by data. This example function returns the number of bytes decoded in the reference argument int decoded_bytes.

uint64_t decode_unsigned_varint( const uint8_t *const data, int &decoded_bytes )
{
    int i = 0;
    uint64_t decoded_value = 0;
    int shift_amount = 0;

    do 
    {
        decoded_value |= (uint64_t)(data[i] & 0x7F) << shift_amount;     
        shift_amount += 7;
    } while ( (data[i++] & 0x80) != 0 );

    decoded_bytes = i;
    return decoded_value;
}

To decode a signed varint, you can use this second function that calls the first:

int64_t decode_signed_varint( const uint8_t *const data, int &decoded_bytes )
{
    uint64_t unsigned_value = decode_unsigned_varint(data, decoded_bytes);
    return (int64_t)( unsigned_value & 1 ? ~(unsigned_value >> 1) 
                                         :  (unsigned_value >> 1) );
}

I believe both of these functions are correct. I did some basic testing with the code below to verify a couple datapoints from the Google page. The output is correct.

#include <stdint.h>
#include <iostream>


uint64_t decode_unsigned_varint( const uint8_t *const data, int &decoded_bytes )
{
    int i = 0;
    uint64_t decoded_value = 0;
    int shift_amount = 0;

    do 
    {
        decoded_value |= (uint64_t)(data[i] & 0x7F) << shift_amount;     
        shift_amount += 7;
    } while ( (data[i++] & 0x80) != 0 );

    decoded_bytes = i;
    return decoded_value;
}

int64_t decode_signed_varint( const uint8_t *const data, int &decoded_bytes )
{
    uint64_t unsigned_value = decode_unsigned_varint(data, decoded_bytes);
    return (int64_t)( unsigned_value & 1 ? ~(unsigned_value >> 1) 
                                         :  (unsigned_value >> 1) );
}



uint8_t ex_p300[] = { 0xAC, 0x02 };
uint8_t ex_n1  [] = { 0x01 };

using namespace std;

int main()
{
    int decoded_bytes_p300;
    uint64_t p300;

    p300 = decode_unsigned_varint( ex_p300, decoded_bytes_p300 );

    int decoded_bytes_n1;
    int64_t  n1;

    n1 = decode_signed_varint( ex_n1, decoded_bytes_n1 );

    cout << "p300 = " << p300 
         << "   decoded_bytes_p300 = " << decoded_bytes_p300 << endl;

    cout << "n1 = " << n1 
         << "   decoded_bytes_n1 = " << decoded_bytes_n1 << endl;

    return 0;
}

To encode varints, you could use the following functions. Note that the buffer uint8_t *const data should have room for at least 10 bytes, as the largest varint is 10 bytes long.
#include

// Encode an unsigned 64-bit varint.  Returns number of encoded bytes.
// 'buffer' must have room for up to 10 bytes.
int encode_unsigned_varint(uint8_t *const buffer, uint64_t value)
{
    int encoded = 0;

    do
    {
        uint8_t next_byte = value & 0x7F;
        value >>= 7;

        if (value)
            next_byte |= 0x80;

        buffer[encoded++] = next_byte;

    } while (value);


    return encoded;
}

// Encode a signed 64-bit varint.  Works by first zig-zag transforming
// signed value into an unsigned value, and then reusing the unsigned
// encoder.  'buffer' must have room for up to 10 bytes.
int encode_signed_varint(uint8_t *const buffer, int64_t value)
{
    uint64_t uvalue;

    uvalue = uint64_t( value < 0 ? ~(value << 1) : (value << 1) );

    return encode_unsigned_varint( buffer, uvalue );
}