環境

% uname -a
Darwin sakuramochi.local 24.6.0 Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:34 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T8103 x86_64
% chruby --version
chruby: 0.3.9
% ruby-install --version
ruby-install: 0.10.1
% ruby --version
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-darwin24]

現象

gemを使ってjekyllをインストールしようとしたところ、以下のエラーが発生した。

% gem install jekyll
(omitted)
Building native extensions. This could take a while...
ERROR:  Error installing jekyll:
	ERROR: Failed to build gem native extension.

    current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
/Users/sakuramochi/.rubies/ruby-3.4.8/bin/ruby extconf.rb
checking for pkg-config for openssl... ["-I/usr/local/Cellar/openssl@3/3.6.0/include ", "-L/usr/local/Cellar/openssl@3/3.6.0/lib", "-lssl -lcrypto"]
-----
Using OpenSSL from pkg-config -I/usr/local/Cellar/openssl@3/3.6.0/include  && -L/usr/local/Cellar/openssl@3/3.6.0/lib && -lssl -lcrypto
-----
checking for -lcrypto... yes
checking for -lssl... yes
checking for openssl/ssl.h... yes
checking for openssl/err.h... yes
checking for rb_trap_immediate in ruby.h,rubysig.h... no
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for rb_thread_fd_select()... yes
checking for rb_fdset_t in ruby/intern.h... yes
checking for rb_wait_for_single_fd()... yes
checking for rb_enable_interrupt()... no
checking for rb_time_new()... yes
checking for inotify_init() in sys/inotify.h... no
checking for __NR_inotify_init in sys/syscall.h... no
checking for writev() in sys/uio.h... yes
checking for pipe2() in unistd.h... no
checking for accept4() in sys/socket.h... no
checking for SOCK_CLOEXEC in sys/socket.h... no
checking for sys/event.h... yes
checking for sys/queue.h... yes
checking for clock_gettime()... yes
checking for CLOCK_MONOTONIC_RAW in time.h... yes
checking for CLOCK_MONOTONIC in time.h... yes
CXXFLAGS=-fdeclspec
creating Makefile

current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
make DESTDIR\= sitearchdir\=./.gem.20260124-2254-decn5v sitelibdir\=./.gem.20260124-2254-decn5v clean

current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
make DESTDIR\= sitearchdir\=./.gem.20260124-2254-decn5v sitelibdir\=./.gem.20260124-2254-decn5v
compiling binder.cpp
In file included from binder.cpp:20:
./project.h:25:10: fatal error: 'iostream' file not found
   25 | #include <iostream>
      |          ^~~~~~~~~~
1 error generated.
make: *** [binder.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7 for inspection.
Results logged to /Users/sakuramochi/.gem/ruby/3.4.8/extensions/x86_64-darwin-24/3.4.0-static/eventmachine-1.2.7/gem_make.out

rubyを再インストールしてもう一度jekyllのインストールを試したところ、以下のようなエラーに変わった。

% rm -rf .rubies/ruby-3.4.8
% ruby-install 3.4.8
(omitted)
% ruby --version
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-darwin24]
% gem install jekyll
Building native extensions. This could take a while...
ERROR:  Error installing jekyll:
	ERROR: Failed to build gem native extension.

    current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
/Users/sakuramochi/.rubies/ruby-3.4.8/bin/ruby extconf.rb
checking for pkg-config for openssl... ["-I/usr/local/Cellar/openssl@3/3.6.0/include ", "-L/usr/local/Cellar/openssl@3/3.6.0/lib", "-lssl -lcrypto"]
-----
Using OpenSSL from pkg-config -I/usr/local/Cellar/openssl@3/3.6.0/include  && -L/usr/local/Cellar/openssl@3/3.6.0/lib && -lssl -lcrypto
-----
checking for -lcrypto... yes
checking for -lssl... yes
checking for openssl/ssl.h... yes
checking for openssl/err.h... yes
checking for rb_trap_immediate in ruby.h,rubysig.h... no
checking for rb_thread_blocking_region()... no
checking for rb_thread_call_without_gvl() in ruby/thread.h... yes
checking for rb_thread_fd_select()... yes
checking for rb_fdset_t in ruby/intern.h... yes
checking for rb_wait_for_single_fd()... yes
checking for rb_enable_interrupt()... no
checking for rb_time_new()... yes
checking for inotify_init() in sys/inotify.h... no
checking for __NR_inotify_init in sys/syscall.h... no
checking for writev() in sys/uio.h... yes
checking for pipe2() in unistd.h... no
checking for accept4() in sys/socket.h... no
checking for SOCK_CLOEXEC in sys/socket.h... no
checking for sys/event.h... yes
checking for sys/queue.h... yes
checking for clock_gettime()... yes
checking for CLOCK_MONOTONIC_RAW in time.h... yes
checking for CLOCK_MONOTONIC in time.h... yes
CXXFLAGS=-fdeclspec
creating Makefile

current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
make DESTDIR\= sitearchdir\=./.gem.20260124-35188-iyfb3t sitelibdir\=./.gem.20260124-35188-iyfb3t clean

current directory: /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7/ext
make DESTDIR\= sitearchdir\=./.gem.20260124-35188-iyfb3t sitelibdir\=./.gem.20260124-35188-iyfb3t
compiling binder.cpp
make: *** [binder.o] Error 1

make failed, exit code 2

Gem files will remain installed in /Users/sakuramochi/.gem/ruby/3.4.8/gems/eventmachine-1.2.7 for inspection.
Results logged to /Users/sakuramochi/.gem/ruby/3.4.8/extensions/x86_64-darwin-24/3.4.0-static/eventmachine-1.2.7/gem_make.out

解決

原因は、macOSのシステム付属のclang++が壊れていたことである。

% cat test.cpp
#include <iostream>
int main() {
    return 0;
}
% clang++ test.cpp
test.cpp:1:10: fatal error: 'iostream' file not found
    1 | #include <iostream>
      |          ^~~~~~~~~~
1 error generated.

XCode Command Line Toolsを丸ごと再インストールし、その後rubyを再インストールすることで解決した。

% xcode-select -p
/Library/Developer/CommandLineTools
% sudo rm -rf `xcode-select -p`
Password:
% xcode-select --install
xcode-select: note: install requested for command line developer tools
% clang++ test.cpp
% rm -rf .rubies/ruby-3.4.8
% ruby-install 3.4.8
(omitted)
% ruby --version
ruby 3.4.8 (2025-12-17 revision 995b59f666) +PRISM [x86_64-darwin24]
% gem install jekyll
(omitted)
% jekyll --version
jekyll 4.4.1

再現実験をしてみたところ、ruby-installの実行時点でclang++が壊れていた場合は後者のエラーが、 ruby-installの実行時点で正常に動いていたclang++がgem install jekyll時点で壊れていた場合は前者のエラーが、 それぞれ起こることがわかった。

参考リンク