What to do in my case when pyenv install is not possible after upgrading to macOS Big Sur

What I tried & the error I encountered

1. An environment where multiple versions can be installed and switched with pyenv on mac was working (python 3.6 3.8 series)
2. Upgraded to macOS Big sir
3. 3.6 3.8 When I try to install another version of the series with `pyenv install`, the installation does not complete with the following error.

BUILD FAILED ...
...
Last 10 log lines:
./Modules/posixmodule.c:8210:15: error: implicit declaration of function 'sendfile' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
        ret = sendfile(in, out, offset, &sbytes, &sf, flags);
...

Many people seemed to have encountered this, and the solution was discussed in pyenv's GitHub issue. While referring to this page, make a note of the operation when it was solved in my environment.

environment

• macOS Big Sur 11.1 --Xcode installed before Big Sur upgrade --pyenv 1.2.21 (using python 3.6 3.8 series) --The shell is zsh

Solved operation

Reinstall xcode tools

sudo rm -rf /Library/Developer/CommandLineTools
xcode-select --install

Reinstall 2 packages with brew

brew reinstall zlib bzip2

Add the following to ~/.zshrc

export PATH="$HOME/.pyenv/bin:$PATH"
export PATH="/usr/local/bin:$PATH"

eval "$(pyenv init -)"
eval "$(pyenv virtualenv-init -)"
export LDFLAGS="-L/usr/local/opt/zlib/lib -L/usr/local/opt/bzip2/lib"
export CPPFLAGS="-I/usr/local/opt/zlib/include -I/usr/local/opt/bzip2/include"

I reopened the terminal for the changes I made to take effect. At this point, version 3.8 series could be installed with `pyenv install` in my environment.

Version 3.6 series installation

I tried `` `pyenv install 3.6.12``` asking if this would include 3.6 series, but I got the first error. Then, when I typed the following command (one line) according to the Reference Page, the installation of version 3.6.0 was successful.

CFLAGS="-I$(brew --prefix openssl)/include -I$(brew --prefix bzip2)/include -I$(brew --prefix readline)/include -I$(xcrun --show-sdk-path)/usr/include" LDFLAGS="-L$(brew --prefix openssl)/lib -L$(brew --prefix readline)/lib -L$(brew --prefix zlib)/lib -L$(brew --prefix bzip2)/lib" pyenv install --patch 3.6.0 < <(curl -sSL https://github.com/python/cpython/commit/8ea6353.patch\?full_index\=1)

Then, changing only the version number and typing the following command succeeded in installing version 3.6.12.

CFLAGS="-I$(brew --prefix openssl)/include -I$(brew --prefix bzip2)/include -I$(brew --prefix readline)/include -I$(xcrun --show-sdk-path)/usr/include" LDFLAGS="-L$(brew --prefix openssl)/lib -L$(brew --prefix readline)/lib -L$(brew --prefix zlib)/lib -L$(brew --prefix bzip2)/lib" pyenv install --patch 3.6.12 < <(curl -sSL https://github.com/python/cpython/commit/8ea6353.patch\?full_index\=1)

References

https://github.com/pyenv/pyenv/issues/1737