I’ve been using with rbenv to manage installation and switching of Ruby versions for the pass year and have been very happy with it. I recently took the plunge and upgraded my main Apple MacBook Pro from macOS Catalina to Big Sur. Everything seemed to work well after the upgrade. Until I tried to install a new version of Ruby.

Problem

When performing the usual command to install Ruby with rbenv, I started getting this message:

~ $ rbenv install 2.6.7
Downloading ruby-2.6.7.tar.bz2...
-> https://cache.ruby-lang.org/pub/ruby/2.6/ruby-2.6.7.tar.bz2
Installing ruby-2.6.7...
ruby-build: using readline from homebrew
BUILD FAILED (macOS 11.2.3 using ruby-build 20210423)
Inspect or clean up the working tree at /var/folders/mq/tlm78wy92v54ygbzfykqc8640000gn/T/ruby-build.20210424214159.42314.u6mGui
Results logged to /var/folders/mq/tlm78wy92v54ygbzfykqc8640000gn/T/ruby-build.20210424214159.42314.log
Last 10 log lines:
        rb_native_mutex_destroy(&vm->waitpid_lock);
        ^
vm.c:2489:34: warning: expression does not compute the number of elements in this array; element type is 'const int', not 'VALUE' (aka 'unsigned long') [-Wsizeof-array-div]
                             sizeof(ec->machine.regs) / sizeof(VALUE));
                                    ~~~~~~~~~~~~~~~~  ^
vm.c:2489:34: note: place parentheses around the 'sizeof(VALUE)' expression to silence this warning
compiling dmyenc.c
1 warning and 1 error generated.
make: *** [vm.o] Error 1
make: *** Waiting for unfinished jobs....
~ $

Trying to figure out the problem by looking at the message, it didn’t seem like something I could fix. Searching the rbenv Github issues didn’t give many clues. Knowing that rbenv uses ruby-build to automate the Ruby build process, I looked at the issues reported. It looks like I was not the only one having similar problems.

I tried several of the suggestions found from those issues and none of the solutions worked.

Solution

I decided to turn to my friends on Twitter to see if anyone had faced this issue. Twitter never lets me down and Robby Russell of Planet Argon came through,  suggesting installing Ruby with these CFLAGS:

CFLAGS="-Wno-error=implicit-function-declaration" rbenv install 2.6.7

It worked perfectly and I was able to get additional versions of Ruby installed. This should also work if you’re having problems with asdf Ruby version manager too. Asdf uses ruby-build behind the scenes.

I wondered why I hadn’t stumbled on this solution in the ruby-build issues on Github. It turned out I saw the issue but ignored it because it referenced installing older versions of Ruby when Xcode 12 was installed. I have Xcode 12 but was installing new versions of Ruby. The ticket was a little deceiving as it worked with new versions as well.

For those interested in the details, they can be found in the ticket – Installing older Ruby versions on OSX after Xcode 12.