Emacs package using Clang for C & C++ completion, on-the-fly syntax checking and more!
Emacs Lisp C++ C
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Build Status

Note: This is a work in progress:

  • documentation might not be up-to-date
  • changes can break compability with earlier versions
  • use at your own risk!
  • open issues, fork-it and create pull-requests!


Optional Parameters


Automatic installation

The easiest way to get irony-mode and its dependencies is to use el-get.

M-x el-get-install RET irony-mode RET

My irony-mode configuration for el-get can help you get started!

You are ready to go, you can jump to: Usage

Manual installation

Download and build

Download with git >= 1.6.5 and later:

git clone --recursive git://github.com/Sarcasm/irony-mode.git

Download with git < 1.6.5:

git clone git://github.com/Sarcasm/irony-mode.git
git submodule update --init


cd irony-mode
mkdir -p build
cd build
cmake .. && make -j 4
make install

Note: It is recommended to proceed to the installation step. This is not a system-wide installation, it shouldn't require any privilege. The installation step will help irony.el to find the binary without additional configuration.

Elisp dependencies

Recommended packages and versions:

Package Version Status Comment
auto-complete 1.4 recommended you can check the version in the auto-complete.el header
auto-complete fork 1.4 as-you-wish conflicts w/ auto-complete AND popup, able to display detailed completions such as overloaded functions
YASnippet All recommended yas--version or yas/version
JSON All needed Built-in since Emacs 23, can be downloaded here: [json.el][json-el-ref]

[json-el-ref]: http://cvs.savannah.gnu.org/viewvc/*checkout*/emacs/lisp/json.el?root=emacs) "json.el"

Emacs configuration

Add irony-mode to the load load-path:

(add-to-list 'load-path (expand-file-name "~/IRONY/MODE/PATH/elisp/"))

And fill-in a basic configuration as shown below in Auto Complete.


If you want the completion to work on a project you will probably need give some information about the flags necessary to compile a file. The best way to achieve this is to use a Compilation Database. Otherwise you can always use the customizable variables: M-x customize-group RET irony RET.

Note: If you want to force the reload of the flags on the server, you can use the command M-x irony-reload-flags. This shouldn't be necessary if you use the compilation database plugin.


To enable a plugin call (irony-enable 'plugin-name). To enable more than one plugin at once call the same function with a list (irony-enable '(plugin-1 plugin-2)).

Auto Complete

Code completion with auto-complete.



(require 'auto-complete)
(require 'yasnippet)
(require 'irony) ;Note: hit `C-c C-b' to open build menu

;; the ac plugin will be activated in each buffer using irony-mode
(irony-enable 'ac)             ; hit C-RET to trigger completion

(defun my-c++-hooks ()
  "Enable the hooks in the preferred order: 'yas -> auto-complete -> irony'."
  ;; be cautious, if yas is not enabled before (auto-complete-mode 1), overlays
  ;; *may* persist after an expansion.
  (auto-complete-mode 1)

  ;; avoid enabling irony-mode in modes that inherits c-mode, e.g: php-mode
  (when (member major-mode irony-known-modes)
    (irony-mode 1)))

(add-hook 'c++-mode-hook 'my-c++-hooks)
(add-hook 'c-mode-hook 'my-c++-hooks)

Note: If my fork of auto-complete is used you should be careful to not have another installation of auto-complete or popup since both of them are present in my fork.

You can eval these lines in the *scratch* buffer:

(let ((requirements (or (require 'auto-complete nil t)
                        (require 'popup nil t))))
  (message "You should remove both auto-complete and popup before using my fork of auto-complete"))

Hit C-x C-e at the end of the expression to evaluate.

Compilation Database

In order to work correctly, irony-mode needs to know the compile flags. This plugin allow aims to provide as automatic as possible compile flags discovery, with minimum user input.

It works great with the following tools:

  • CMake >= 2.8.5

  • Ninja >= 1.2 - Use ninja -t compdb to generate a compilation database for your project.

  • Bear - Bear is a tool that can generate a compile_commands.json file by "monitoring" the build of a project. The typical usage for a make based project will be bear -- make -B.

  • .clang_complete - A file at the root of your project containing the compilation flags, one per line. This is compatible with the with plugin Rip-Rip/clang_complete. If you want to generate the .clang_complete automatically, take a look there: cc_args.py documentation.

The JSON Compilation Database Format Specification page might reference some new tools in the future supporting the compile_commands.json format (such as cmake and Bear described above). irony-mode support that file format and hopefully it should work out-of-the-box for such tool.

Compilation DB demo


This is not really a plugin but something built-in irony-mode core, as such it doesn't require any activation code. Just hit C-c C-b to display the build configuration menu.

The menu should be self explanatory, if it's not the case open an issue please.


It's slow, why?

A bug in old version of Clang (at least '3.1-8') caused the completion to fail on the standard library types. To eliminate this bug an optimisation has been disabled in the parsing of a translation unit. This result in a slower parsing.

This only affect old versions of Clang (< 3.2), it is suggested to update your libclang installation if you want to take advantage of the optimizations.

How do I use a custom version of libclang?

On a system with multiple versions of libclang installed, it is possible to hint CMake in order to pick-up the preferred one. Two variables can be specified when invoking CMake:

  1. LIBCLANG_INCLUDE_PATH=/path/to/libclang/include-dir: The path to the include directory that contains the header file clang-c/Index.h.

  2. LIBCLANG_LIBRARY_PATH=/path/to/libclang/libraries: The path to the directory that contains the libclang library, e.g: libclang.so on Linux.

Example of a CMake invocation using a custom libclang installation:

mkdir build
cd build
cmake -DLIBCLANG_INCLUDE_PATH=~/llvm-3.4/include/ \
      -DLIBCLANG_LIBRARY_PATH=~/llvm-3.4/lib/ ..

CMake doesn't find libclang...

When libclang is installed in a non-standard path CMake may produce the following error:

CMake Error at /usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:108 (message):
Could NOT find LibClang (missing: LibClang_LIBRARY LibClang_INCLUDE_DIR)
Call Stack (most recent call first):
/usr/share/cmake-2.8/Modules/FindPackageHandleStandardArgs.cmake:315 (FPHSAFAILURE_MESSAGE)
cmake/FindLibClang.cmake:35 (find_package_handle_standard_args)
server/CMakeLists.txt:4 (find_package)

This can be fixed by using the same method as described above, using a custom version of libclang.

Please report an issue if the include and library paths are the standard libclang installation paths on your system. Other users may benefit from these paths being handled natively by irony-mode.

I got an error due to 'stdarg.h'...

If by looking into the log file (/tmp/irony.$$PID.log on Linux) you can have the following error:

'stdarg.h' file not found

Assuming (otherwise it might be necessary to adjust the code):

  • Clang version is 3.2
  • Clang is installed in /usr/

You can use the following configuration:

;; The Clang installation missed the system include directory
;; "/usr/lib/clang/3.2/include/"
(when (file-exists-p "/usr/lib/clang/3.2/include/")
  (setq irony-libclang-additional-flags
        '("-isystem" "/usr/lib/clang/3.2/include/")))

This is issue is a known problem:

Make sure that Clang is using its own <stddef.h>. It will be in a directory ending in clang/3.2/include/ where 3.2 is the version of clang that you are using. You may need to explicitly add it to your header search. Usually clang finds this directory relative to the executable with CompilerInvocation::GetResourcesPath(Argv0, MainAddr), but using just the libraries, it can't automatically find it.

libclang.so: cannot open shared object file...

Compiling irony-server succeed but you have the following message when you try to run the irony-server executable:

'irony-server: error while loading shared libraries: libclang.so: cannot open shared object file: No such file or directory

Maybe it's due to a non-standard location for your installation of libclang. A path such as /usr/local/lib might not be in the path list of the dynamic loader (see ld.so.conf).

To solve this issue it is possible to build irony-server with the following command:

cmake -DUSE_RPATH=ON ..

auto-complete acts strangely, it tries to complete inside string literals...

In order to enable header completion, such as:

#include "heade[COMP]

The ac plugin allows auto-complete to complete inside string literals in irony-ac-enable.

(setq ac-disable-faces (delq 'font-lock-string-face ac-disable-faces))

Please create an issue if you find this unacceptable.