Packed field reference compiler error

[bgottula]

While working on upgrading nix to Mint 21.1 (based on Ubuntu Jammy) I got the following compiler error for capture:

[ 12%] Building CXX object src/CMakeFiles/capture.dir/SERFile.cpp.o
/home/rgottula/dev/zwo/capture/src/SERFile.cpp: In destructor ‘SERFile::~SERFile()’:
/home/rgottula/dev/zwo/capture/src/SERFile.cpp:119:26: error: cannot bind packed field ‘((SERFile*)this)->SERFile::header_->SERHeader_t::FrameCount’ to ‘int&’
  119 |                 header_->FrameCount,
      |                 ~~~~~~~~~^~~~~~~~~~
make[2]: *** [src/CMakeFiles/capture.dir/build.make:160: src/CMakeFiles/capture.dir/SERFile.cpp.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:98: src/CMakeFiles/capture.dir/all] Error 2

Some discussion of this:

• https://github.com/gabime/spdlog/issues/1926
• https://stackoverflow.com/questions/27491432/why-cant-i-return-a-reference-to-a-packed-field
• https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36566

Seems like gcc got stricter about some edge case for references to packed struct members. I don't fully understand the details, or exactly how to fix / work-around it.

@jgottula any suggestions?

[jgottula]

Having a look.

[jgottula]

The obvious real problem here is that the SER format was designed by an idiot

One possible workaround that might work, maybe: using #pragma pack instead of packed attribute https://gcc.gnu.org/bugzilla/show_bug.cgi?id=36566#c11

[jgottula]

Another workaround would be to just declare an int32_t in that block scope, assign it the value of header_->FrameCount and then pass that in to the logging function.

Or just declare some inline by-value getters in struct SERHeader_t and use those; e.g.:

int32_t GetFrameCount() { return FrameCount; }
int32_t GetFrameCount() const { return FrameCount; }

[jgottula]

(And actually you’d only need the const-qualified getter. I put a const and non-const getter because I forgot what I was doing for a bit.)