When using <c-x><c-o>, vim opens a signature preview window which is different from the one opened when using :ptag.

How to open that one when not completing like when reviewing in normal mode?

I don't want the full file for preview like with :ptag. Just the signature and some type metadata from tags.

I set omnifunc=syntaxcomplete#Complete. Not using any completion plugins.

  • Which filetype? This is usually a documentation popup. It usually only appears when preview is in completeopt.
    – D. Ben Knoble
    Feb 27, 2020 at 16:43
  • filetype C. Yes completeopt has preview by default.
    – bkdm
    Feb 27, 2020 at 16:59
  • Are you sure that’s what omnifunc is set to? I ask only because the default filetype uses a special c completion function. At any rate, the best bet is probably Man for stdlib functions. I think some of the code is hardwired in there, especially when completing tags.
    – D. Ben Knoble
    Feb 27, 2020 at 17:00
  • yes you are right. It is ccomplete#Complete. I am still not able to find the code that opens that preview window.
    – bkdm
    Feb 27, 2020 at 17:05
  • The function ccomplete#Complete doesn't seem to read completeopt or open preview window by itself. May be some autocommand is doing this.
    – bkdm
    Feb 27, 2020 at 17:10

2 Answers 2


Maybe you can use :set cot=. To know more info see cot


The way that "signature window", really "preview window", works is this:

  • The :help 'omnifunc', if there is one, returns a list of dictionaries, one for each completion suggestion.
  • Those dictionaries can have a non-mandatory info field that contains the text to display in the preview window if preview is in :help 'completeopts'.
  • The preview window is triggered automatically when completeopts contains preview AND when the info field is populated.

In this case, that info field is populated on the fly by ccomplete#Complete(), with info gathered from tags, files, etc.. It is the value of that field that is displayed in the preview window and there is currently no built-in way to get that exact text in a preview window or elsewhere without using omni-completion.

Now, all the code used to generate that info field is under $VIMRUNTIME/autoload/ccomplete.vim (and it even has been rewritten in vim9script) so it should be doable to cobble something up out of it.

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