org-howto/articles/2024/01/zstream-isnt-moveable.org

[2024] zlib z_stream isn't moveable

• TL;DR
• Setup
• Problem + Diagnosis
• Solution

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TL;DR

Be aware that zlib's z_stream struct cannot be moved. Presume it contains internal pointers to itself.

Setup

• Using zstreambuf (provided as byproduct of my cmake-examples project) I ran into errors after move assignment.
• Making zstreambuf.open() work with a zstreambuf that's already been used for i/o.
• Attempted to cleanup state within zstreambuf.close(), using move assignment on buffered_inflate_zstream and buffered_deflate_zstream.

Problem + Diagnosis

• Unit testing revealed that uncompress failed after second zstreambuf.open().
• It turns out that although zlib's z_stream struct is fully exposed (to work with zlib, application code should declare a variable with type z_stream), the z_stream struct can't be moved to another location. Presumably at least one of (::deflateInit2(), ::inflateInit2()) does some address-dependent computation involving one or more z_stream members.

Solution

• Replace z_stream member with a pointer:

  Instead of:

    // compression/include/compression/base_zstream.hpp
  
    #include <zlib.h>
  
    struct base_zstream {
        // ...omitted...
  
        z_stream zstream_;
    };

  write:

    // compression/include/compression/base_zstream.hpp
  
    #include <utility>
  
    struct base_zstream {
        // ...omitted...
  
        std::unique_ptr<z_stream> p_native_zs_;
    };

  Then implement ::swap() etc by swapping pointers.