Question about how emerge manage updates after gcc update

[Fulgurance]

Hello, actually I'm reading the LFS book, and I have a question. When gentoo performs updates, how emerge do to avoid to rebuild the entire system ? Because normally, I read in the LFS documentation, if you have any glibc (or gcc if I remember) update, you need to rebuild the entire system.

I guess as well if I understood properly, if a new update is available for a library, I have to rebuild all packages using this library. If I would like to keep different versions of libraries, I have to make separate dirs for each versions I guess ?

Because actually I'm coding a package manager, and I realize now maybe, the updates are maybe more difficult to manage than calculating the dependencies . Because I did this part yet. I think updates are more complicated to manage. I like to do that because following the LFS book is very interesting ! I can learn a lot. With a project, it's super captivating

[gentoo_ram]

As long as library interfaces are kept you wouldn't necessarily have to "rebuild the entire system". For example I haven't had to do a rebuild if there's a minor update to glibc. But my understanding is that some libraries have a more "sensitive" ABI where things can break with older compiled code on newer versions. I've heard some boost libraries (C++ libraries) can have issues with compatibility. And OpenSSL has had issues with this in the past.
What might be a larger issue if with some packages that have inter-dependencies where the different parts are compiled with different versions of gcc. Or another big example is the kernel and kernel modules. It's important that your kernel and kernel modules are compiled with the same gcc. So let's say you build and are running a kernel. Then you update gcc. Then you update a package which makes a kernel module. That new kernel module might not work with your existing kernel until you re-build another kernel with the newer gcc. Something else to watch out for.

[sam_]

Indeed, glibc and gcc's libstdc++ both have stable ABI and use symbol versioning to add new bits. glibc last broke ABI in like 2000? 2001? GCC last broke it maybe 10 years ago and it was well-publicised. Rebuilding is only necessary if they announce ABI has been broken, there's an accidental break (which is a serious bug), or if you want to benefit from new features in glibc and/or gcc (like improved optimisations).