mruby 4.0.0

We are announcing the first stable release of mruby 4.0 series - mruby 4.0.0.

Describes the new features and changes in mruby 4.0.
The main changes in mruby 4.0 are also described in doc/mruby4.0.md.

User visible changes in `mruby4.0` from `mruby3.4`

“NOTE:” are changes to be aware of.

The language

Pattern Matching

mruby now supports pattern matching (case/in) syntax:

Basic pattern matching with case/in syntax (dadfac6)
Array pattern matching (ec67fd9)
Hash pattern matching (2147263)
Find pattern matching ([*pre, target, *post]) (6c4d98b)
Pin operator (^variable) (1de6340)
Guard clauses (if/unless conditions) (07ac110)
One-line pattern matching (expr in pattern) (e76ce24)
Brace-less hash pattern support (e8096bf)

Other Language Changes

&nil in formal parameters to explicitly opt out of block arguments (b07518e)
Trailing comma in method definition parameters: def foo(a, b,) (f78334b)
Array/Hash/String subclasses can now override [] and []= methods (#6675)
OP_SETIDX optimization for Array and Hash (ddd8fe1)
case/in without else now raises NoMatchingPatternError (d8de35b)
Allow compound statement in parenthesized argument context (919cbd8)

Changes in C API

NOTE: mrb_alloca() renamed to mrb_temp_alloc() (7fe5c2e)
NOTE: mruby/ext/io.h renamed to mruby/io.h (2813f79)
mrb_gc_add_region() for contiguous heap region support (072855a)
mrb_class_outer() to get the outer class/module (3a1b771)
MRB_ENSURE() macro for exception-safe cleanup (3ac682b)
mrb_time_get_tm() for accessing struct tm (daaaafe)
MRB_OPEN_FAILURE() macro for checking mrb_open result (40b0cb9)
mrb_print_error() now handles NULL gracefully (8e50a45)
mrb_open() returns mrb_state with exc set on init failure (05ffe0c)
mrb_utf8_to_buf() for UTF-8 encoding consolidation (7e28e68)
kh_is_end() macro for safe khash iteration (893cc75)
mrb_bigint_p() always defined regardless of bigint gem presence (6c4a8c0)
RInteger and RFloat added to RVALUE union (13dbca0)

ROM Method Tables

All built-in classes and most extension gems now use read-only method tables stored in .rodata instead of heap-allocated hash tables. Method definitions no longer consume heap memory, significantly reducing memory footprint for embedded use.

Core classes converted: BasicObject, Object, Module, Class, Kernel, String, Array, Hash, Numeric, Integer, Float, NilClass, TrueClass, FalseClass, Range, Symbol, Exception, Proc.

Extension gems converted: mruby-string-ext, mruby-array-ext, mruby-set, mruby-struct, mruby-class-ext, mruby-numeric-ext, mruby-random, mruby-kernel-ext, mruby-complex, mruby-rational, mruby-io, mruby-socket, mruby-method, mruby-metaprog, mruby-time, mruby-hash-ext, mruby-proc-ext, mruby-symbol-ext, mruby-range-ext, mruby-object-ext.

GC and Memory

NOTE: MRB_NO_PRESYM removed; presym is now always enabled (81689045)
Replace gcnext gray linked list with fixed-size gray stack, reducing per-object overhead (31fea170)
mrb_gc_add_region() for providing contiguous memory buffers as GC heap pages (072855a)
Chunk-based pool for symbol string allocation (e05bd8f)
Reduce IV_INITIAL_SIZE from 4 to 2 (6bd1f51)
Lossless float encoding using rotation in word boxing (b6148c8)
Lossless rotation encoding for 32-bit float32 word boxing (14a5cfb)
Consolidated irep allocation for .mrb loading (74fb045)
Object shapes (hidden classes) for MRB_TT_OBJECT IV storage, sharing key layouts across objects with the same instance variable assignment order (8d10056)

Build & Configuration

NOTE: MRB_WORDBOX_NO_FLOAT_TRUNCATE renamed to MRB_WORDBOX_NO_INLINE_FLOAT (old name still works) (59e1fe2)
NOTE: MRB_INT64 on 32-bit now requires MRB_NO_BOXING (other boxing modes cannot guarantee alignment for heap-allocated 64-bit integers) (eaaa66b)
Amalgamation support via rake amalgam task (d995ca2)
New Platform: Cosmopolitan Libc (#6681)
Emscripten: use native WASM exception handling (ca364e3)
HAL (Hardware Abstraction Layer) for platform abstraction in mruby-io, mruby-socket, mruby-dir, mruby-task (74ca22f)
MRUBY_MIRB_READLINE environment variable to control readline library selection (0aafb83)
MSYS2 drive letter support in build script (77f6ffe)
Inter-gem headers separated from external API headers (#6671)

Changes in mrbgems

New Gems

mruby-task: Cooperative multitasking with preemptive scheduling (ae0d7a0)
mruby-benchmark: Benchmarking gem (2f40f3d)
mruby-strftime: Time#strftime implementation (b31e22f)

mruby-bin-mirb Improvements

Custom multi-line editor replacing readline (527018c)
Syntax highlighting for keywords, strings, result values, hash key symbols (624272b, 1713d4a)
Automatic light/dark theme detection via OSC 11 (db4c8d9)
Tab completion support (2f15282)
Colored output for prompts and errors (b36e0b4)
Auto-indentation and auto-dedent (d52f318, e901b6d)
Command history with Up/Down navigation (5f85c1b)
Line numbers in multi-line prompts (5a3f0e2)
UTF-8 multibyte character support (4a97da3)

mruby-bigint Improvements

Toom-3 multiplication for large numbers (99620804)
Karatsuba multiplication for medium-sized numbers (85e81072)
Balance multiplication for asymmetric operands (0220ec2b)
Divide-and-conquer optimization for to_s (990ff90f)
Consolidated mpn layer for low-level limb operations (9ef3362f)
Always use 32-bit limbs by default (c747c77f)

Other Gem Changes

NOTE: Hash#deconstruct_keys removed for CRuby compatibility (34b9412)
mruby-enum-lazy: Fix Lazy#flat_map to handle non-enumerable block return values (#6765)
mruby-array-ext: Add Array#find and Array#rfind methods
mruby-io: Add IO#putc and Kernel#putc (baff6e6)
mruby-random: Replace xoshiro with PCG for better memory efficiency (f1bab01)
mruby-compiler: Variable-sized AST nodes for reduced memory usage
mruby-compiler: no_return_value context flag for script optimization (613b03a)
initialize_copy and respond_to_missing? defined as private (#6708)
Struct keyword argument initialization (#6574)

Compiler Improvements

Variable-sized AST nodes for reduced memory consumption (821b989)
Pattern matching bytecode optimizations (21d4135)
Optimized masgn to generate literals directly into target registers (fb5d966)
Optimized splat of literal arrays in args/literals (1cb8d73)
Early termination after too many parse errors (510ebd7)
Chunk array literals at 64 elements to reduce register pressure (f98d641)
Chunk %w() and %i() literals to reduce register pressure (62cf0dc)

VM Optimizations

New super-instructions that fuse common opcode sequences to reduce bytecode size and improve performance:

OP_SEND0/OP_SSEND0: Zero-argument method call, avoiding argument count setup (9123ef4)
OP_TDEF/OP_SDEF: Fused method definition combining TCLASS/SCLASS+METHOD+DEF into single instruction, saving 4 bytes per method (8d4f47e)
OP_GETIDX0: Fast path for array[0] and Array#first access (680f7ec)
OP_ADDILV/OP_SUBILV: Local variable increment/decrement fusion for i += n patterns (43f64b9)
OP_RETSELF: Single-byte instruction for return self pattern (a71db8c)
OP_RETNIL: Single-byte instruction for return nil pattern (64e30bf)
OP_RETTRUE/OP_RETFALSE: Single-byte instructions for return true/return false patterns (0b15727)
OP_MATCHERR: Pattern matching error with conditional execution (944168a)
OP_BLKCALL: Direct block call for yield, bypassing method dispatch (13-17% faster) (3aa2872)

Other optimizations:

1.5x stack growth instead of linear growth for reduced reallocations (f7988c93)
Skip keyword argument hash duplication (5970e350)

Fixed GitHub Issues

#5531 Hash recursion detection
#6506 Constant lookup in singleton class
#6507 tally multi-values
#6508 Enumerable#sum index
#6509 scope_new nregs initialization
#6515 y.tab.c in repository
#6516 Private backquote
#6554 Socket private #initialize
#6570 instance_eval crash
#6613 const_added hook during bootstrapping
#6635, #6636 Colon3 constant lookup
#6637 arm64 mingw64 builtin setjmp/longjmp
#6642 Task segfault when sleep called from C
#6645 Set memory leak from double initialization
#6646 IO#gets negative length
#6647 IO#ungetc buffer overflow
#6648 sprintf buffer overread
#6649 Array#sort! use-after-realloc
#6650 Array#fill validation
#6652 Array comparison use-after-realloc
#6657 Exception handling for = on class variables
#6659 Super with keyword arguments
#6660 Regression on struct/array/hash == override with super
#6662 Array set operations use-after-free
#6664 Set#flatten memory leak
#6666 Regexp literal with encoding
#6668 Method#== for aliased methods and comparison bug
#6671 Separate inter-gem headers from external API headers
#6674 Document pattern matching limitations
#6675 Allow Hash#[] to be aliased again
#6687 Expand MRB_SYM/MRB_GVSYM support for symbols with special characters
#6698 Bigint tests fail on architectures other than x86_64 and i386
#6701 Heap-use-after-free in mrb_vm_exec involving mruby-rational / mruby-bigint
#6702 mruby-bigint doesn’t compile in C++ project
#6704 Heap-buffer-overflow in mrb_vm_exec via malformed source code
#6705 Can’t get outer class of an object in C
#6713 mruby-polarssl not work
#6720 Random float range: different behavior from CRuby
#6722 RBreak size overflow on 32-bit platforms with MRB_NO_BOXING
#6740 %w()/%i() register pressure with large literals
#6741 case/in without else should raise NoMatchingPatternError
#6760 mrb_gc_unregister() not removing all matching entries

Merged Pull Requests

#6418 Add ls-lint with GitHub Actions
#6492 fix a typo, update specs
#6493 Fix TYPO in memory.md
#6495 Remove MRB_ENDIAN_LOHI() that is no longer in use
#6497 gha: update build.yml try windows-2025 image
#6498 Clean up and standardize the pre-commit config
#6501 Update pre-commit Node.js version to v22.14.0 LTS
#6502 pre-commit: update prettier to the latest version
#6503 misc: fix typos
#6505 mrbgems: fix spelling
#6510 Fixed class method visibility via module_function
#6511 Exclude the external project “lrama” from pre-commit
#6513 mruby 3.4.0 released
#6517 core/codegen.c: remove unneeded duplicate semicolon
#6518 Change mrbc_args.flags bit width from 2 to 3
#6519 Add tools/lrama to .prettierignore
#6520 pre-commit: autoupdate and update node LTS version
#6521 Add codespell config file .codespellrc
#6522 gha: label more files
#6523 add rand(Range) and unify implementations of Random#rand and Kernel#rand
#6524 Fix Kernel#p when no argument
#6525 Skip adding empty input to mirb history
#6526 Add build config for Luckfox Pico embedded SBC
#6528 misc: fix spelling
#6530 Revert “class.c (find_visibility_scope): when callinfo returns, *ep == NULL; #6512”
#6531 Improve method table performance by rehashing at 75% load factor
#6532 Reverted method table optimizations to prioritize memory savings
#6533 Fix calling extended callback
#6534 Add descriptive comment to mrb_read_float function
#6535 Added descriptive comments for functions/macros in src/mempool.c
#6536 Add descriptive comments to public functions in src/debug.c
#6537 Updated comments in cdump.c to remove the @brief tag
#6539 Add descriptive comments for functions in src/load.c
#6540 Add descriptive comments to MRB_API functions in object.c
#6541 Add descriptive comments for MRB_API functions in src/array.c
#6542 Add descriptive comments to MRB_API functions in src/symbol.c
#6543 Add descriptive comments to several functions in src/dump.c
#6544 Fix build strings that must be mutable
#6545 Add descriptive comments for MRB_API functions in src/class.c
#6548 Add descriptive comments to MRB_API functions in src/etc.c
#6549 Add descriptive comments to kernel functions
#6550 Add descriptive comments for MRB_API functions in src/proc.c
#6551 Add descriptive comments for MRB_API functions in src/state.c
#6552 Fix: Correct placement of comments in src/variable.c
#6553 Add descriptive comments for MRB_API functions in src/vm.c
#6555 mrb_mt_foreach() needs to update the pointer at each loop
#6556 iv_foreach() needs to update the pointer at each loop
#6560 Refactor: Improve Set GC marking and freeing
#6561 pre-commit updates and fix prettier entrypoint
#6562 misc: fix spelling word case
#6563 pre-commit add rubocop with one rule spaces for indentation
#6564 Remove jumanjihouse pre-commit hooks no longer maintained
#6565 Rubocop: fix target Ruby version; add two more cops; fix lint error
#6566 Removed unreferenced variables in CrossBuild#run_bintest
#6567 Avoid array object creation in cmd_bin method in bintest
#6568 mruby-bin-debugger depends on mruby-bin-mrbc in bintest
#6569 sed s/Mruby/MRuby/g
#6571 Update limitations.md to add behavior on small hash
#6572 Add Claude Code GitHub Workflow
#6573 pre-commit fixes and updates
#6574 Support initializing structs via keyword arguments
#6575 Fix typo in file time methods
#6581 Merge mrb_obj_iv_inspect() into mrb_obj_inspect()
#6582 Stricter type tag in mrb_obj_alloc()
#6583 Add fallback to local build_config.rb before using default configuration
#6585 Fix typo in mruby3.2 docs
#6586 Makefile: refactor add docs and add command line help target
#6587 Add Set#hash tests
#6588 Add CodeQL Analysis for GitHub Actions
#6589 Add pre-commit hook check-zip-file-is-not-committed
#6591 mruby-eval fix license link in README
#6593 README: Add Contributors Avatars, Star History, Table of Contents
#6598 Fix heap buffer overflow in #method_missing
#6599 pre-commit: run markdown-link-check, oxipng, prettier manually
#6600 dreamcast_shelf build config: update to use KallistiOS wrappers
#6601 fix: skip local build_config.rb when working in MRUBY_ROOT
#6602 Improved iseq annotations for new and !=
#6604 pre-commit config updates
#6607 fix bigint on raspberry pi
#6610 Extract golden ratio prime into constant
#6614 Fix uninitialized variable in io_gets causing segmentation fault
#6617 Fix various minor problems and speed up build
#6618 Stop generating unnecessary C++ files in mruby-bin-mruby
#6621 Set up all GEMS before mruby core tasks definition
#6624 Fixed wrong MRuby::Build.current at the top level of mrbgem.rake
#6628 Revert File.absolute_path logic
#6629 pre-commit update
#6631 Revert “Rakefile: make the whole thing parallel unless SERIAL=1”
#6633 Fix a heap-buffer-overflow in str strip! methods
#6643 Fix crash caused by an incorrect node type check in codegen_masgn
#6651 Address stack-use-after-return in the mruby bigint implementation
#6653 Improve HAL-related components for MinGW
#6655 Preventing Memory Leaks in Array#__combination_init
#6656 Fix integer overflow in allocation size calculation
#6663 Added the kh_is_end() macro function
#6665 Fixed use-after-free with Set#join
#6670 Arranging VM dispatch macros
#6673 Adjust broken license links; clean up Markdown
#6677 gha: run pre-commit with --color=always
#6678 Put ls-lint and pre-commit in separate workflow files
#6679 pre-commit autoupdate; update node and prettier
#6681 Add Cosmopolitan Libc build configuration
#6689 docs: fix pre-commit manual hooks; fix link
#6694 Fix mirb build under Cosmopolitan
#6695 Dependabot: add a cooldown period for new releases
#6696 Fix parse error with required kwargs and omitted parens
#6699 Fix mruby-task for PicoRuby Integration
#6700 Fix float/double pack/unpack on s390x
#6706 Refactor task class to use symbol IDs
#6708 initialize_copy and respond_to_missing? defined as private
#6709 Add the MRB_ENSURE() macro
#6711 Fix out of bounds read and write in IO.select
#6714 Fix OP_DEBUG operand type and add NULL check for debug_op_hook
#6716 Fixes identity for proc object
#6717 Fix mruby-task: wrapping by critical section and setting initial task receiver
#6718 Add installation instructions for conda and Homebrew
#6723 Add RInteger and RFloat to RVALUE
#6727 Language documentation: update wording of “overloading” section
#6729 Simplifying dependency addition for gensym task
#6730 Simplifying presym file generation actions
#6733 Include mruby/presym.h for all source files
#6734 Chunk array literals at 64 elements to reduce register pressure
#6735 Prevent full recompilation without changes to presym file
#6739 Fix MSYS2 build error with drive letters
#6743 Chunk %w() and %i() literals to reduce register pressure
#6744 Raise NoMatchingPatternError in case/in without else
#6747 Correctly handle empty hash as default named argument
#6749 Fix microcontroller profile
#6750 Fix out-of-bounds read and divide-by-zero in Array#product
#6752 Fix attr_reader-generated methods accepting extra arguments
#6753 Further optimize Array#product
#6754 Mark attr_reader procs as noarg
#6755 Reload ci after mrb_hash_delete_key() in keyword argument handling
#6756 Avoid impact of object modifications caused by mrb_vm_exec() calls
#6758 Don’t assign result of mrb_funcall() directly to regs
#6759 Define mrb_bigint_p() always
#6761 Fix mrb_gc_unregister() to remove all matching entries
#6762 Write generated test C files atomically to avoid build race condition
#6765 Fix Lazy#flat_map to handle non-enumerable block return values
#6767 Allow compound statement in parenthesized argument context
#6780 Fix String#prepend with self-referencing arguments
#6781 Protect sprintf format string from mutation during callbacks
#6783 Pin GitHub Actions workflows to commit hashes

Security Fixes

Buffer overflow in bigint uadd (3f2611e)
Stack buffer overflow in Montgomery reduction (edce0a3)
Buffer overflow in pack_uu encoding (2993302)
Buffer overflow in IO#ungetc (01ab2ff)
Heap-buffer-overflow in pattern alternation codegen (eea9e30)
Out of bounds read and write in IO.select (44831711)
Off-by-one in bounds check for symbol names and pool strings in load.c (b3b8c01)
Use-after-free in Set operations (a6b55e7)
Use-after-free in Array set operations (729b84c)
Use-after-free in Set#join (0e653eb)
Use-after-realloc in Array#sort! (eb39897)
Heap-use-after-free in insertion_sort (099d2c47)
Integer overflow in str_check_length (6afff1c3)
Integer overflow in Integer#lcm (070bef24)
Heap buffer overflow in #method_missing (550d10a)
Out-of-bounds read and divide-by-zero in Array#product (8441eaf)
Heap buffer overflow in String#prepend with self-referencing arguments (18ba026)
Use-after-free in sprintf via to_s callback mutating format string (48fc422)
Multiple memory leak fixes in bigint, Set, Array, and Task gems

We have done 1,844 commits to 473 files with 82,458 additions and 14,976 deletions since mruby 3.4.0. For more detail of the updates, see Commit Log.

Thanks to all the contributors who have worked on bug fixes and improvements in the release of mruby 4.0.0.