"package gcc-4.9 is not available, but is referred to by another package"， 安装gcc时出错的解决方法

https://stackoverflow.com/questions/48398475/fail-to-install-gcc-4-9-in-ubuntu17-04

sudo apt-get update --fix-missing
sudo apt-get dist-upgrade
sudo apt-get install gcc-4.9 g++-4.9 gcc-4.9-multilib

sudo apt-add-repository -yu 'deb http://ppa.launchpad.net/ubuntu-toolchain-r/test/ubuntu xenial main'
sudo apt update
sudo apt-get install gcc-4.9 g++-4.9 gcc-4.9-multilib

https://www.ubuntuupdates.org/package/core/xenial/main/base/libmpfr4

下载

libmpfr4_3.1.4-1_amd64.deb

sudo dpkg -i libmpfr4_3.1.4-1_amd64.deb