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mruby Architecture

This document provides a map of mruby's internals for developers who want to understand, debug, or contribute to the codebase.

Overview

mruby's execution pipeline:

Ruby source → Parser → AST → Code Generator → Bytecode (irep)
                                                    ↓
                                                   VM → Result

The design priority is memory > performance > readability.

Object Model

All heap-allocated Ruby objects share a common header (MRB_OBJECT_HEADER):

struct RBasic (8 bytes on 64-bit)
┌──────────────┬─────┬──────────┬────────┬───────┐
│ RClass *c    │ tt  │ gc_color │ frozen │ flags │
│ (class ptr)  │ 8b  │ 3b       │ 1b     │ 20b   │
└──────────────┴─────┴──────────┴────────┴───────┘

All object structs embed this header via MRB_OBJECT_HEADER:

Struct Ruby Type Extra Fields
RObject Object instances iv (instance variables)
RClass Class/Module iv, mt (method table), super
RString String embedded or heap buffer, length
RArray Array embedded or heap buffer, length
RHash Hash hash table or k-v array
RProc Proc/Lambda irep or C function, environment
RData C data wrapper void *data, mrb_data_type
RFiber Fiber mrb_context
RException Exception iv

Immediate values (Integer, Symbol, true, false, nil) are encoded directly in mrb_value without heap allocation. The encoding depends on the boxing mode (see boxing.md).

Objects must fit within 5 words (mrb_static_assert_object_size).

Virtual Machine

The VM is register-based, using two stacks: a value stack for registers (locals, temporaries, arguments) and a call info stack for tracking method/block call frames. Each method call pushes a mrb_callinfo frame with the method symbol, proc, PC, and argument counts.

The dispatch loop in mrb_vm_exec() decodes opcodes and operates on registers. Method dispatch looks up the receiver's class method table (with a per-state method cache), then either calls a C function directly or pushes a new call frame for Ruby methods.

Exception handling uses setjmp/longjmp (or C++ exceptions if configured). Rescue/ensure handler tables are stored in each irep and searched during stack unwinding.

See vm.md for detailed VM internals, opcode.md for the full instruction set.

Garbage Collector

The GC uses tri-color incremental mark-and-sweep with an optional generational mode. Objects are colored white (unmarked), gray (marked, children pending), black (fully marked), or red (static/ROM).

The three-phase cycle (root scan, incremental marking, sweep) runs in small steps between VM instructions to avoid long pauses. Write barriers (mrb_field_write_barrier, mrb_write_barrier) maintain correctness during incremental marking.

The GC arena protects newly created objects in C code. Heap regions (mrb_gc_add_region) support embedded systems with fixed memory banks.

See gc.md for detailed GC internals, ../guides/gc-arena-howto.md for arena usage patterns, ../guides/memory.md for memory management.

Compiler Pipeline

The compiler transforms Ruby source code through three stages:

  1. Parser (Prism): the lib/prism submodule parses the source into a pm_node_t tree, with local variables already resolved per scope.
  2. Code Generator (codegen.c): walks the tree and emits bytecode into mrc_irep structures (instruction sequence, literal pool, symbol table, child ireps).
  3. Execution: the VM runs the irep once it is converted to mrb_irep and wrapped in an RProc. mrbc takes the other path and writes the mrc_irep out in .mrb binary format as it is.

Alternative loading paths include mrb_load_string() (compile and run), mrb_load_irep() (load precompiled bytecode), and mrbc (ahead-of-time compilation).

See compiler.md for detailed compiler internals, opcode.md for the instruction set.

Source File Map

Core (src/)

File Responsibility
vm.c Bytecode dispatch loop, method invocation
state.c mrb_state init/close, irep management
gc.c Garbage collector (mark-sweep, incremental)
class.c Class/module definition, method tables
object.c Core object operations
variable.c Instance/class/global variables, object shapes
proc.c Proc/Lambda/closure handling
array.c Array implementation
string.c String implementation (embedded, shared, heap)
hash.c Hash implementation (open addressing)
numeric.c Integer/Float arithmetic
symbol.c Symbol table and interning
range.c Range implementation
error.c Exception creation, raise, backtrace
kernel.c Kernel module methods
load.c .mrb bytecode loading
dump.c Bytecode serialization (write .mrb)
print.c Print/puts/p output
backtrace.c Stack trace generation
codedump.c Bytecode disassembler (mrb_codedump_all)
debug.c Debug info (pc → file/line) lookup
refinement.c Refinements (MRB_USE_REFINEMENTS)

Compiler (mrbgems/mruby-compiler/src/)

File Responsibility
compile.c Parse and compile entry points
codegen.c Prism tree → bytecode (irep)
dump.c .mrb writer for the compiler's irep
mruby_compat.c mrb_parser_state API over the above

The parser itself is Prism, vendored as the lib/prism submodule.

Key Headers (include/mruby/)

Header Contents
mruby.h mrb_state, core API declarations
value.h mrb_value, type enums, value macros
object.h RBasic, RObject, object header
class.h RClass, method table types
string.h RString, string macros
array.h RArray, array macros
hash.h RHash, hash API
data.h RData, C data wrapping
irep.h mrb_irep, bytecode structures
compile.h Compiler context, mrb_load_string
boxing_*.h Value boxing implementations

mrbgems System

Gems are the module system for mruby. Each gem lives in mrbgems/mruby-*/ and contains:

mruby-example/
├── mrbgem.rake       gem specification (name, deps, bins)
├── src/              C source files
├── mrblib/           Ruby source files (compiled to bytecode)
├── include/          C headers
├── test/             mrbtest test files
└── bintest/          binary test files (CRuby)

At build time, gem Ruby files are compiled with mrbc and linked into libmruby.a. Gem initialization runs in dependency order via gem_init.c (auto-generated).

GemBoxes (mrbgems/*.gembox) define named collections of gems (e.g., default.gembox includes stdlib, stdlib-ext, stdlib-io, math, metaprog, and binary tools).