Bug #21659: rstring.h error: missing initializer for field ‘len’ of ‘struct RString’ [-Werror=missing-field-initializers] starting in ruby-3.3.10

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: ruby 3.3.10 (2025-10-23 revision 343ea05002) [x86_64-linux]
• Backport: 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN

[wsfulton (William Fulton)]

All extensions now have this error when using -Werror -Wmissing-field-initializers:

In file included from /home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/internal/arithmetic/char.h:29,
                 from /home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/internal/arithmetic.h:24,
                 from /home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/ruby.h:28,
                 from /home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby.h:38,
                 from example_wrap.c:1008:
/home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/internal/core/rstring.h: In function ‘rbimpl_rstring_getmem’:
/home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/internal/core/rstring.h:398:16: error: missing initializer for field ‘len’ of ‘struct RString’ [-Werror=missing-field-initializers]
  398 |         struct RString retval = {RBASIC_INIT};
      |                ^~~~~~~
/home/runner/.rvm/rubies/ruby-3.3.10/include/ruby-3.3.0/ruby/internal/core/rstring.h:206:10: note: ‘len’ declared here
  206 |     long len;
      |          ^~~

Can you please test with warnings turned on as this now breaks all extensions that rely on Ruby.h being warning free.

Looks like it is due to commit https://github.com/ruby/ruby/commit/5a8d7642168f4ea0d9331fded3033c225bbc36c5 and expect that

        struct RString retval = {RBASIC_INIT};

should have been changed to:

        struct RString retval = {RBASIC_INIT, 0};

in rstring.h as well as all in the actual Ruby code base.

[nobu (Nobuyoshi Nakada)]

• Status changed from Open to Feedback

In configure.ac, we add -Wno-missing-field-initializers explicitly if available.
Are you adding that -Werror -Wmissing-field-initializer by yourself?

[wsfulton (William Fulton)]

This happens when compiling an EXTENSION. We have always used -Wmissing-field-initializers in our extensions and now they are broken. New Ruby releases really MUST NOT introduce new warnings for extensions which have to include Ruby.h.

[jeremyevans0 (Jeremy Evans)]

wsfulton (William Fulton) wrote in #note-2:

This happens when compiling an EXTENSION. We have always used -Wmissing-field-initializers in our extensions and now they are broken. New Ruby releases really MUST NOT introduce new warnings for extensions which have to include Ruby.h.

New Ruby releases should not introduce errors in extensions. I think it is unreasonable to expect that they will not introduce warnings if you are explicitly configuring non-default warning flags. If you are combining non-default warning flags with -Werror, I think you are responsible for handling the issues that result from that, since it is your actions that are causing the errors. I realize this may have worked in the past for you, but that was by chance, it is not something that Ruby has officially or deliberately supported.

[wsfulton (William Fulton)]

Keep code warning free improves the code quality. Fixing this warning, in particular, removes undefined behaviour. Are the core Ruby developers supportive of these goals and consider a patch to fix this warning and possibly even making it a supported warning flag? It will lead to an improved Ruby ecosystem by using the C++ tools available to automatically find time wasting bugs, not just in Ruby itself, but in all the Ruby extensions.

[jeremyevans0 (Jeremy Evans)]

wsfulton (William Fulton) wrote in #note-4:

Fixing this warning, in particular, removes undefined behaviour.

Can you explain what the undefined behavior is? My understanding is that in both C and C++, the behavior is defined to zero-initialize all uninitialized fields.

[ufuk (Ufuk Kayserilioglu)]

Isn't this related to this problem with GCC? https://bugs.ruby-lang.org/issues/21655#change-114967

[alanwu (Alan Wu)]

• Status changed from Feedback to Rejected

-Wmissing-field-initializers is in -Wextra, not even in -Wall. From the manual (emphasis mine)

Warn if a structure’s initializer has some fields missing. For example, the following code causes such a warning, because x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

All fields are initialized, just some are done implicitly. In fact, before the patch you point to, all fields were actually uninitialized; there is strictly more initialization happening now.

The warning acts more like a lint if you want to conform to a certain coding style, and isn't a flag for some material problem that ends up in the compiled artifact. It's fine to use this warning in your code, but it's unreasonable to expect ruby to use the same style as you. You can toss in a #pragma around #include "ruby.h" to fix your build.

In general, -Werror makes your build forward incompatible with future compilers because they release with new warnings all the time.