Syntax checkers

This document explains how Flycheck selects external tools to check a buffer.

Flycheck does not check buffers on its own. Instead it delegates this task to external syntax checkers which are external programs or services that receive the contents of the current buffer and return a list of errors in the buffer, together with metadata that tells Flycheck how to run the program, how to pass buffer contents to it, and how to extract errors.

See also

Supported Languages
A complete list of all syntax checkers included in Flycheck

Like everything else in Emacs syntax checkers have online documentation which you can access with C-c ! ?:

C-c ! ?
M-x flycheck-describe-checker

Prompt for the name of a syntax checker and pop up a Help buffer with its documentation.

The documentation includes the name of the program or service used, a list of major modes the checker supports and a list of all options for this syntax checker.

Automatic syntax checker selection

Normally Flycheck automatically selects the best syntax checkers for the current buffer from flycheck-checkers whenever it needs to check the buffer:

User option flycheck-checkers

A list of all syntax checkers available for syntax checking.

A syntax checker in this list is a registered syntax checker.

Flycheck picks the first syntax checker from this list which exists and supports the current major mode, and runs it over the current buffer. When the checker has finished Flycheck whether it asks for a next syntax checker to run, and if so, runs the next syntax checker, and so on, until there is no more syntax checker for the current buffer. This process repeats whenever Flycheck needs to check the buffer according to flycheck-check-syntax-automatically.

For instance, the first syntax checker for Emacs Lisp is emacs-lisp which checks Emacs Lisp with Emacs’ own byte compiler. This syntax checker asks for emacs-lisp-checkdoc to run next, which checks for stylistic issues in Emacs Lisp docstrings. Thus Flycheck will first run the byte compiler and then checkdoc in an Emacs Lisp buffer.

Manual syntax checker selection

Alternatively you can tell Flycheck explicitly which syntax checker to start with in the current buffer:

C-c ! s
M-x flycheck-select-checker

Prompt for a syntax checker and use this syntax checker as the first syntax checker for the current buffer.

Flycheck may still run further syntax checkers from flycheck-checkers if the selected syntax checker asks for it.

Flycheck will use the selected syntax checker as “entry point” for syntax checks in the current buffer, just as if it had selected this syntax checker automatically. It will automatically run further syntax checkers from flycheck-checkers if the selected syntax checker asks for it.

Under the hood C-c ! s sets flycheck-checker:

Variable flycheck-checker

The name of a syntax checker to use for the current buffer.

If nil (the default) let Flycheck automatically select the best syntax checker from flycheck-checkers.

If set to a syntax checker Flycheck will use this syntax checker as the first one in the current buffer, and run subsequent syntax checkers just as if it had selected this one automatically.

If the syntax checker in this variable does not work in the current buffer signal an error.

This variable is buffer-local.

We recommend to set flycheck-checker via directory local variables to enforce a specific syntax checker for a project. For instance, Flycheck usually prefers javascript-eslint for Javascript buffers, but if your project uses javascript-jshint instead you can tell Flycheck to use javascript-jshint for all Javascript buffers of your project with the following command in the top-level directory of your project: M-x add-dir-local-variable RET js-mode RET flycheck-checker RET javascript-jshint. A new buffer pops up that shows the newly created entry in the directory variables. Save this buffer and kill it. From now on Flycheck will check all Javascript files of this project with JSHint.

See also

General information about local variables.
Directory Variables(emacs)
Information about directory variables.

To go back to automatic selection either set flycheck-checker to nil or type C-u C-c ! s:

C-u C-c ! s
C-u M-x flycheck-select-checker

Remove any selected syntax checker and let Flycheck again select a syntax checker automatically.

Disabled syntax checkers

Even if you select a checker manually Flycheck may still use a syntax checker that you’d not like to use. To completely opt out from a specific syntax checker disable it:

C-c ! x
M-x flycheck-disable-checker

Prompt for a syntax checker to disable in the current buffer.

For instance if you do not care for documentation conventions of Emacs Lisp you can opt out from emacs-lisp-checkdoc which checks your code against these conventions with C-c ! x emacs-lisp-checkdoc. After the next check all checkdoc warnings will be gone from the buffer.

Internally this command changes the buffer-local flycheck-disabled-checkers:

User option flycheck-disabled-checkers

A list of disabled syntax checkers. Flycheck will never use disabled syntax checkers to check a buffer.

This option is buffer-local. You can customise this variable with M-x customize-variable RET flycheck-disabled-checkers or set the default value in your init file to permanently disable specific syntax checkers. For instance:

(setq-default flycheck-disabled-checkers '(c/c++-clang))

will permanently disable c/c++-clang in all buffers.

You can also disable syntax checkers per project with directory local variables. For instance type M-x add-dir-local-variable RET emacs-lisp-mode RET flycheck-disabled-checkers RET emacs-lisp-checkdoc in your user emacs directory to disable emacs-lisp-checkdoc for all Emacs Lisp files in your personal configuration.

See also

General information about local variables.
Directory Variables(emacs)
Information about directory variables.

To enable a disabled checker again, remove it from flycheck-disabled-checkers or use C-u C-c ! x:

C-u C-c ! x
C-u M-x flycheck-disable-checker

Prompt for a disabled syntax checker to enable again in the current buffer.