Check buffers¶

Flycheck provides two Emacs minor modes for automatic syntax checking: Flycheck Mode to enable syntax checking in the current buffer, and Global Flycheck Mode to enable syntax checking in all buffers whenever possible.

Minor Mode Flycheck Mode¶: Enable automatic syntax checking in the current buffer.

Minor Mode Global Flycheck Mode¶

Enable Flycheck Mode in all buffers where syntax checking is possible.

Note

This mode does not enable Flycheck Mode in remote files (via TRAMP) and encrypted files. Checking remote files may be very slow depending on the network connections, and checking encrypted files would leak confidential data to temporary files and subprocesses.

You can manually enable Flycheck Mode in these buffers nonetheless, but we do not recommend this for said reasons.

Add the following to your init file to enable syntax checking permanently:

(add-hook 'after-init-hook #'global-flycheck-mode)

You can exclude specific major modes from syntax checking with flycheck-global-modes:

defcustom flycheck-global-modes¶

Major modes for which Global Flycheck Mode turns on Flycheck Mode:

t (the default): Turn Flycheck Mode on for all major modes.
(foo-mode …): Turn Flycheck Mode on for all major modes in this list, i.e. whenever the value of major-mode is contained in this list.
(not foo-mode …): Turn Flycheck Mode on for all major modes not in this list, i.e. whenever the value of major-mode is not contained in this list.

Note

Global Flycheck Mode never turns on Flycheck Mode in major modes whose mode-class property is special, regardless of the value of this option. Syntax checking simply makes no sense in special buffers which are typically intended for non-interactive display rather than editing.

Check automatically¶

By default Flycheck Mode automatically checks a buffer whenever

it is enabled,
the buffer is saved,
the buffer is reverted (e.g. via global-auto-revert-mode),
a new line is inserted,
or a short time after the last change was made in a buffer.

You can customise this behaviour with flycheck-check-syntax-automatically:

defcustom flycheck-check-syntax-automatically¶

A list of events which trigger a syntax check in the current buffer:

save

Check the buffer immediately after it was saved.

new-line

Check the buffer immediately after a new line was inserted.

idle-change

Check the buffer a short time after the last change. The delay is customisable with flycheck-idle-change-delay:

defcustom flycheck-idle-change-delay¶: Seconds to wait after the last change to the buffer before starting a syntax check.

idle-buffer-switch

Check the buffer a short time after switching to it from another buffer. The delay is customisable with flycheck-idle-buffer-switch-delay:

defcustom flycheck-idle-buffer-switch-delay¶

Seconds to wait after switching to a buffer before starting a syntax check.

If you switch to several buffers in rapid succession, the behavior depends on flycheck-buffer-switch-check-intermediate-buffers:

defcustom flycheck-buffer-switch-check-intermediate-buffers¶: If non-nil, then a buffer you switch to will have a syntax check run even if you switch to another buffer before it starts. If nil, then only the current buffer can have a syntax check run. Note that syntax checks can still be run in other buffers due to changes to their contents.

mode-enabled

Check the buffer immediately after Flycheck Mode was enabled.

For instance with the following setting Flycheck Mode will only check the buffer when it was saved:

(setq flycheck-check-syntax-automatically '(mode-enabled save))

When a change-driven syntax check (idle-change or save) is triggered while a recently started check is still running, the running check is interrupted and the new one starts immediately, since its results would no longer match the buffer. Checks triggered on every keystroke (the new-line condition) or by buffer switches coalesce behind the running check instead, and syntax checkers that cannot be interrupted or that have already made substantial progress are left to finish:

defcustom flycheck-interrupt-running-checks¶: Whether a new syntax check interrupts a running one. With the default value of 10, only running checks younger than ten seconds are interrupted; checks that have made more progress are left to complete, and the new check is deferred until they finish, so slow syntax checkers still publish their results. Set to t to always interrupt, or nil to never interrupt and always defer, like older Flycheck versions did. Setting nil file- or directory-locally is handy for projects whose syntax checkers you never want interrupted.

Check manually¶

You can also start a syntax check explicitly with C-c ! c:

C-c ! c¶
M-x flycheck-buffer¶: Check syntax in the current buffer.