[Resolved]ZLib compression from UE4 to Boost ?


I am sending compressed json data from the UE4 client to a C++ server made with boost.
I am using ZLib to compress and decompress all json but it doesn’t work. I am now encoding it in base64 to avoid some issues but that doesn’t change a thing.

I currently stopped trying to send the data and I am writing it in a file from the client and trying to read the file and decompress on the server side.
When the server is trying to decompress it I get an error from ZLib : zlib error: iostream error

My question is the following : Did anyone manage to compress and decompress data between a UE4 client and a C++ server ?
I cannot really configure anything on the server side (because boost has its ZLib compressor) and I don’t know what is wrong with the decompression.

Any idea ?


Well I got it to work by linking and using the raw zlib library on the UE4 client, I also had to encode into ANSI char for my server to be able to convert it into std::string (I will need to look into utf8 on the server but not for now since I don’t send any text message yet).

For those interested I will give my solution for the compression decompression here and then you can tweak it.

Client UE4 :

First of all build the zlib library and I choose the same version as the one in my current version of UE4 (4.18), so I used zlib 1.2.8.

Then I built it manually on windows, but since it’s cmake it wont be much different on linux or osx :

mkdir C:\Builds\zlib;
cd C:\Builds\zlib cmake -G "Visual Studio 15 2017" -A x64 C:\local\zlib-x.x.x
cmake --build .  

Then you need to tell UE4 you want to link it into your project (*.build.cs) :


(if anyone knows how to have the path to the project dynamically it would be nice)

Compress on the client :

 void MessageManager::compressString(const FString &json, TArray<uint8> &compressedData)
    //convert into ANSI to be able to cast it into std::string for now
    auto jsonANSI = StringCast<ANSICHAR>(*json);
    TArray<uint8> UncompressedBinaryArray((uint8*)jsonANSI.Get(), jsonANSI.Length());

    compressedData.SetNum(UncompressedBinaryArray.Num() * 1023, true);

    //int ret;
    z_stream strm;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;

    strm.avail_in = UncompressedBinaryArray.Num();
    strm.next_in = (Bytef *)UncompressedBinaryArray.GetData();
    strm.avail_out = compressedData.Num();
    strm.next_out = (Bytef *)compressedData.GetData();

    // the actual compression work.
    deflateInit(&strm, Z_DEFAULT_COMPRESSION);
    deflate(&strm, Z_FINISH);

    // Shrink the array to minimum size
    compressedData.RemoveAt(strm.total_out, compressedData.Num() - strm.total_out, true);

Decompress on the client :

FString MessageManager::decompressString(TArray<uint8> &compressedData)
    TArray<uint8> UncompressedBinaryArray;
    UncompressedBinaryArray.SetNum(compressedData.Num() * 1032);

    //int ret;
    z_stream strm;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;
    strm.opaque = Z_NULL;

    strm.avail_in = compressedData.Num();
    strm.next_in = (Bytef *)compressedData.GetData();
    strm.avail_out = UncompressedBinaryArray.Num();
    strm.next_out = (Bytef *)UncompressedBinaryArray.GetData();

    // the actual DE-compression work.
    inflate(&strm, Z_FINISH);

    return FString((ANSICHAR*)UncompressedBinaryArray.GetData());

On the server (C++ using boost and STD) this is how to compress :

void MessageManager::compressString(const std::string &data, std::vector<char> &compressedData)
    std::stringstream compressed;
    std::stringstream decompressed(data);

    boost::iostreams::filtering_streambuf<boost::iostreams::input> in;
    boost::iostreams::copy(in, compressed);

    std::string str = compressed.str();

And decompress :

std::string MessageManager::decompressString(std::vector<char> &compressedData)
    std::stringstream compressed;
    std::stringstream decompressed;

    boost::iostreams::filtering_streambuf<boost::iostreams::input> in;
    boost::iostreams::copy(in, decompressed);

    std::string str(decompressed.str());
    return str;

I hope it will help someone out there

To skip the zlib build step you can use the engine one by adding this to your .Build.cs:

AddEngineThirdPartyPrivateStaticDependencies(Target, "zlib");

and include this header in your cpp:

#include "ThirdParty/zlib/zlib-1.2.5/Inc/zlib.h"

(that was from 4.16, version numbers might have gone up since then)

Nice I didn’t know that :slight_smile:
Yes right now it is the 1.2.8 version