How to specify compiler for "gem install"?

Question

How can I specify which compiler should be used for gem install? In Bash

CC=gcc gem install ...

does not help, because it still uses Solaris Studio on my system.

Answer 1

For sqlite3-ruby your method is supported since Jan 2011 commit. But in general it's not possible to override this because of how rubygems works. It

1. Unpacks the gem.

2. Looks into it spec & discovers that it uses 'extensions' attribute--usually a path to a specially written extconf.rb file.

3. Runs that file to generate a Makefile. Unless author of extconf.rb explicitly said to honor env variables, they are ignored. For example, in latest versions of sqlite3-ruby gem, its extconf.rb has a line

   RbConfig::MAKEFILE_CONFIG['CC'] = ENV['CC'] if ENV['CC']
   

   So if you run in Bash

   CC=gcc gem install sqlite3-ruby
   

   extconf.rb will generate the Makefile with the variable CC in it that is set to gcc or whatever you want.

4. After Makefile is generated, rubygems executes make utility to compile the extension.

Theoretically, if rubygems allowed us to specify additional command line parameters for make, we could use its -e option and then do not depend on authors of extconf.rb at all. (-e option gives variables taken from the environment precedence over variables from makefiles.)