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Symbols

Symbols in mruby C source code is represented by mrb_sym which is alias of uint32_t. Lower 30 bits are used for symbols so that higher 2 bits can be used as flags, e.g. struct mt_elem in class.c.

struct mt_elem {
  union mt_ptr ptr;
  size_t func_p:1;
  size_t noarg_p:1;
  mrb_sym key:sizeof(mrb_sym)*8-2;
};

C API

We provide following C API for symbols.

Generate Symbols

mrb_sym mrb_intern(mrb_state*,const char*,size_t)

Get a symbol from a string.

mrb_sym mrb_intern_check_cstr(mrb_state*,const char*)

Get a symbol from a NULL terminated (C) string.

mrb_sym mrb_intern_str(mrb_state*,mrb_value)

Get a symbol from a Ruby string object.

mrb_intern_lit(mrb_state*,const char*)

Get a symbol from a C string literal. The second argument should be a C string literal, otherwise you will get a compilation error. It does not copy C string given the fact it's a literal.

mrb_sym mrb_intern_check(mrb_state*,const char*,size_t)

Get a symbol from a string if the string has been already registered as a symbol, otherwise return 0. We also provide variants mrb_intern_check_str() (from Ruby string) and mrb_intern_check_cstr() (from C string).

const char *mrb_sym_name(mrb_state*,mrb_sym)

Get a string representation of a symbol as a C string.

const char *mrb_sym_name_len(mrb_state*,mrb_sym,mrb_int*)

Get a string representation of a symbol, and its length.

Preallocate Symbols

To save RAM, mruby can use compile-time allocation of some symbols. You can use following macros to get preallocated symbols by including mruby/presym.h header.

  • MRB_SYM(xor) //=> xor (Word characters)
  • MRB_SYM_B(xor) //=> xor! (Method with Bang)
  • MRB_SYM_Q(xor) //=> xor? (Method with Question mark)
  • MRB_SYM_E(xor) //=> xor= (Method with Equal)
  • MRB_GVSYM(xor) //=> $xor (Global Variable)
  • MRB_CVSYM(xor) //=> @@xor (Class Variable)
  • MRB_IVSYM(xor) //=> @xor (Instance Variable)
  • MRB_OPSYM(xor) //=> ^ (Operator)

For MRB_OPSYM(), specify the names corresponding to operators (see MRuby::Presym::OPERATORS in lib/mruby/presym.rb for the names that can be specified for it). Other than that, describe only word characters excluding leading and ending punctuation.

These macros are converted to static symbol IDs at compile time. The _2 suffix variants (e.g., MRB_SYM_2) are kept for backward compatibility only; they accept an explicit mrb_state* parameter but ignore it. New code should use the standard macros above.

The IDs are given by the build from a scan of the sources, the core's symbols first and then the ones each gem adds, so an ID depends on the parts of the build before the one that brings it and not on the ones after (see doc/guides/compile.md, "File prefix map", for what that gives a compiler cache). They are not stable across versions of mruby, so keep them out of anything that outlives a build.

The names of the IDs are in the debug information of libmruby, as the enumerators of enum mruby_presym; in a debugger, a symbol ID reads as its name through a cast:

(gdb) p (enum mruby_presym)sym
$1 = MRB_SYM__initialize

A dynamic symbol, one interned at run time, has no name there; p mrb_sym_name(mrb, sym) reads either.