I try to read a binary file which is part of a zip archive. Therefore I opened the zip archive with Poco::Zip. Then I read the binary file chunkwise

struct Header
{
    uint64_t tag;
    uint32_t payloadLength;
} __attribute__((packed));

std::ifstream inp("my.zip", std::ios::binary);
Poco::Zip::ZipArchive arch(inp);
Poco::Zip::ZipArchive::FileHeaders::const_iterator it = arch.findHeader("file.dat");
if (it != arch.headerEnd()) {
    Poco::Zip::ZipInputStream inStream(inp, it ->second);
    Header hdr;
    inStream.read(reinterpret_cast<char*>(&hdr), sizeof(hdr));
    
    char buffer[hdr.payloadLength];
    inStream.read(reinterpret_cast<char*>(buffer), sizeof(hdr.payloadLength));
    
    ...
}

I got garbage data when reading from the ZipInputStream.

When I use

std::string str((std::istreambuf_iterator<char>(inStream)), std::istreambuf_iterator<char>());

it works, but I want to avoid this solution because it buffers the complete file in one possible large buffer. What is goning wrong here with read()?