Fails to build on macOS with Ruby 3.4.7

[coder2000]

Curses gem fails to build on Ruby 3.4.7 with macOS (arm64): "convertible type of chtype... failed"

Environment

• Ruby version: 3.4.7
• Curses gem version: 1.5.3
• Operating System: macOS 26.1 (Build 25B78)
• Architecture: arm64 (Apple Silicon)
• Compiler: Apple clang version 21.0.0
• ncurses: 6.5 (installed via Homebrew at /opt/homebrew/opt/ncurses)

Description

The curses gem fails to compile on Ruby 3.4.7 during the native extension build. The build fails at the chtype type conversion test in extconf.rb.

Steps to Reproduce

gem install curses -- --use-system-libraries --with-ncurses-dir=/opt/homebrew/opt/ncurses

Error Output

From gem_make.out:

checking for convertible type of chtype... failed
*** extconf.rb failed ***
Could not create Makefile due to some reason, probably lack of necessary
libraries and/or headers.

Root Cause

From analyzing mkmf.log, the issue appears to be twofold:

1. Ruby 3.4.7 header warning: Ruby's internal header (ruby/internal/core/rstring.h:398) triggers a compiler warning:

   warning: default initialization of an object of type 'struct RString' with const member 
   leaves the object uninitialized [-Wdefault-const-init-field-unsafe]
   

2. chtype conversion test failure: The test to determine the C type equivalent of chtype fails with:

   conftest.c:8:20: error: 'conftest_const' declared as an array with a negative size
   int conftest_const[((rbcv_typedef_)-1 < 0) ? 1 : -1];
   

The test program attempts to determine if chtype is signed, but the compilation fails before it can complete the type detection logic.

Attempted Workarounds

• Setting --with-cflags="-Wno-error=default-const-init-field-unsafe" does not resolve the issue
• The warning flag is applied correctly, but the chtype conversion test still fails

Relevant Log Excerpt

convertible_int: checking for convertible type of chtype... -------------------- failed

DYLD_LIBRARY_PATH=.:/Users/coder2000/.local/share/mise/installs/ruby/3.4.7/lib ASAN_OPTIONS=detect_leaks=0 "clang -I/Users/coder2000/.local/share/mise/installs/ruby/3.4.7/include/ruby-3.4.0/arm64-darwin25 -I/Users/coder2000/.local/share/mise/installs/ruby/3.4.0/ruby/backward -I/Users/coder2000/.local/share/mise/installs/ruby/3.4.7/include/ruby-3.4.0 -I. -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE -D_DARWIN_UNLIMITED_SELECT -D_REENTRANT   -Wno-error=default-const-init-field-unsafe -arch arm64  -c conftest.c"
...
conftest.c:8:20: error: 'conftest_const' declared as an array with a negative size
    8 | int conftest_const[((rbcv_typedef_)-1 < 0) ? 1 : -1];

Expected Behavior

The gem should compile successfully and the chtype type conversion should be detected properly.

Additional Notes

• This appears to be specific to Ruby 3.4.x on macOS with Apple Silicon
• The same issue may affect other recent Ruby 3.4.x versions
• ncurses libraries are found correctly - all function checks pass up until the chtype conversion test

Question

Is there a known incompatibility with Ruby 3.4.x's internal headers and the type detection logic in extconf.rb? Would it be possible to update the type detection mechanism to be more robust against compiler warnings in Ruby headers?

[gillisd]

I was experiencing the same issue, and was able to work around it by creating a temporary proxy script for clang to use in compilation. The problem is that ruby sets its own Werror flag, so you need a way to remove it. You can intercept the call to clang and remove the flag like so:

WRAPPER_DIR="$(mktemp -d)"

cat > "$WRAPPER_DIR/clang" << 'EOF'
#!/bin/bash
args=()
for arg in "$@"; do
     # here we check for ruby's flag and ensure clang doesn't use it
    [[ "$arg" != "-Werror" ]] && args+=("$arg")
done
exec /usr/bin/clang "${args[@]}"
EOF

chmod +x "$WRAPPER_DIR/clang"

PATH="$WRAPPER_DIR:$PATH" gem install curses -- \
  --with-ncursesw-dir="$(brew --prefix ncurses)"

rm -rf "$WRAPPER_DIR"

[coder2000]

That works, thanks. Hopefully the issue can be resolved where it's not needed.

[shugo]

I've heard from @nobu that the Ruby 3.4.7 header warning has already been fixed in the ruby_3_4 branch.
We are investigating the chtype conversion test failure now.

[nobu]

I can't reproduce the second issue "chtype conversion test failure" with Apple clang version 17.0.0.
And Apple clang version 21 doesn't look released yet.
Are you using developers beta or something?

[nobu]

@coder2000 Could you show your mkmf.log file?

[coder2000]

Here is the mkmf.log file as requested.

mkmf.log

[coder2000]

Any update? Still experiencing this issue.

[hsbt]

I couldn't reproduce this.

❯ ruby -v
ruby 3.4.8 (2026-01-06 revision c04363c0a0) +YJIT +MN +PRISM [arm64-darwin25]

❯ pkgutil --pkg-info=com.apple.pkg.CLTools_Executables
package-id: com.apple.pkg.CLTools_Executables
version: 26.4.0.0.1770878581
volume: /
location: /
install-time: 1771319824

❯ brew info ncurses
==> ncurses ✔: stable 6.6 (bottled) [keg-only]
Text-based UI library
https://invisible-island.net/ncurses/announce.html
Installed
/opt/homebrew/Cellar/ncurses/6.6 (4,086 files, 10.5MB)
  Poured from bottle using the formulae.brew.sh API on 2026-01-04 at 19:55:53
From: https://github.com/Homebrew/homebrew-core/blob/HEAD/Formula/n/ncurses.rb
License: X11-distribute-modifications-variant
==> Caveats
ncurses is keg-only, which means it was not symlinked into /opt/homebrew,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have ncurses first in your PATH, run:
  echo 'export PATH="/opt/homebrew/opt/ncurses/bin:$PATH"' >> /Users/hsbt/.config/zsh/.zshrc

For compilers to find ncurses you may need to set:
  export LDFLAGS="-L/opt/homebrew/opt/ncurses/lib"
  export CPPFLAGS="-I/opt/homebrew/opt/ncurses/include"

For pkgconf to find ncurses you may need to set:
  export PKG_CONFIG_PATH="/opt/homebrew/opt/ncurses/lib/pkgconfig"
==> Analytics
install: 154,048 (30 days), 451,719 (90 days), 1,409,501 (365 days)
install-on-request: 1,791 (30 days), 7,911 (90 days), 17,020 (365 days)
build-error: 251 (30 days)

❯ uname -a
Darwin euler.local 25.4.0 Darwin Kernel Version 25.4.0: Sun Feb 15 21:26:30 PST 2026; root:xnu-12377.100.625.0.3~12/RELEASE_ARM64_T6000 arm64 arm Darwin

❯ gem i curses
Fetching curses-1.5.4.gem
Building native extensions. This could take a while...
Successfully installed curses-1.5.4
1 gem installed

[shugo]

@coder2000
It seems that the issue was fixed by ruby/ruby@2bb6fe3 and has been backported to Ruby 3.4.8.

Could you try Ruby 3.4.8?

[shugo]

@nobu

I can't reproduce the second issue "chtype conversion test failure" with Apple clang version 17.0.0. And Apple clang version 21 doesn't look released yet. Are you using developers beta or something?

The "chtype conversion test failure" is a symptom of the "Ruby 3.4.7 header warning". The sequence is:

1. try_signedness succeeds fine (no -Werror there); chtype is correctly identified as unsigned
2. convertible_int then searches for the compatible type using try_compile(..., werror: true), which appends -Werror
3. Every candidate type (unsigned int, unsigned long, etc.) fails; not because of a type mismatch, but because
   -Werror promotes the rstring.h warning to a fatal error