Ruby: include/ruby/internal/attr/noalias.h File Reference

Ruby 3.4.7p58 (2025-10-08 revision 7a5688e2a27668e48f8d6ff4af5b2208b98a2f5e)

Macros
noalias.h File Reference

Defines RBIMPL_ATTR_NOALIAS. More...

`#include "ruby/internal/compiler_since.h"`
`#include "ruby/internal/has/declspec_attribute.h"`

Go to the source code of this file.

## Macros
#define | RBIMPL_ATTR_NOALIAS()
| Wraps (or simulates) __declspec((noalias))

## Detailed Description

Defines RBIMPL_ATTR_NOALIAS.

Author
  Ruby developers ruby-.nosp@m.core.nosp@m.@ruby.nosp@m.-lan.nosp@m.g.org

Copyright
  This file is a part of the programming language Ruby. Permission is hereby granted, to either redistribute and/or modify this file, provided that the conditions mentioned in the file COPYING are met. Consult the file for details.

Warning
  Symbols prefixed with either RBIMPL or rbimpl are implementation details. Don't take them as canon. They could rapidly appear then vanish. The name (path) of this header file is also an implementation detail. Do not expect it to persist at the place it is now. Developers are free to move it anywhere anytime at will.

Note
  To ruby-core: remember that this header can be possibly recursively included from extension libraries written in C++. Do not expect for instance __VA_ARGS__ is always available. We assume C99 for ruby itself but we don't assume languages of extension libraries. They could be written in C++98.

### Q&A

- Q: There are seemingly similar attributes named RBIMPL_ATTR_CONST, RBIMPL_ATTR_PURE, and RBIMPL_ATTR_NOALIAS. What are the difference?
- A: Allowed operations are different.
  - RBIMPL_ATTR_CONST ... Functions attributed by this are not allowed to read/write any pointers at all (there are exceptional situations when reading a pointer is possible but forget that; they are too exceptional to be useful). Just remember that everything pointer- related are NG.
  - RBIMPL_ATTR_PURE ... Functions attributed by this can read any nonvolatile pointers, but no writes are allowed at all. The ability to read any nonvolatile pointers makes it possible to mark VALUE- taking functions as being pure, as long as they are read-only.
  - RBIMPL_ATTR_NOALIAS ... Can both read/write, but only through pointers passed to the function as parameters. This is a typical situation when you create a C++ non-static member function which only concerns this. No global variables are allowed to read/write. So this is not a super-set of being pure. If you want to read something, that has to be passed to the function as a pointer. VALUE -taking functions thus cannot be attributed as such.

Definition in file noalias.h.

## Macro Definition Documentation

## ◆ RBIMPL_ATTR_NOALIAS

#define RBIMPL_ATTR_NOALIAS | ( |  | ) |

Value:
/* void */

Wraps (or simulates) __declspec((noalias))

Definition at line 66 of file noalias.h.

Referenced by rb_fd_set().