TIL: Show current C function in vim status line sans plugins

13 Mar 2024

I primarily work on the Linux kernel (which is written almost entirely in C) and I frequently find myself needing to know which function I'm currently in while browsing the codebase. There is, of course, the dictum that functions should be as small as possible and not more than a screen's length long; this is mostly just a guiding principle.

There are plugins to do this sort of thing, but I do not want to become overly dependent on them. I am more partial towards things I can use that are just plain vanilla vimscript because I can just stick them in my .vimrc when setting up a new workstation or logging into a remote server somewhere.

The function

Here's the function in all its inscrutable beauty:

" Show function name on demand, mapped to key ','
fun! ShowFuncName()
    echohl ModeMsg
    echo getline(search("^[^ \t#/]\\{2}.*[^:]\s*$", 'bWn'))
    echohl None
endfun

nmap , :call ShowFuncName()<CR>

I couldn't locate the StackOverflow answer I had sourced this from (even though this post says TIL, I did not learn this literally today) so I used Gemini AI to break this down for me.

The breakdown

echohl ModeMsg: Sets the highlighting for the echo output to the mode message style. This ensures the function name visually stands out.

getline(search("^[^ \t#/]\\{2}.*[^:]\s*$", 'bWn')): This line is the core logic for finding the function name. Let's break down the search pattern:

• ^[^ \t#/] - Matches the beginning of the line (^) and excludes whitespace ( and \t), tabs (/), and comments (#).
• \\{2} - Matches exactly two characters (the function name can have at least two characters).
• .* - Matches any characters (captures the function name).
• [^:] - Excludes the colon (:) which might follow the function name.
• \s*$ - Matches whitespace at the end of the line.
• bWn - Flags:
  • b - Starts searching backward from the current line.
  • W - Considers word boundaries.
  • n - Doesn't include the matching pattern in the returned line.

This search essentially finds the line before the current line that starts with two non-whitespace, non-tab, non-comment characters, followed by any characters (the function name), excluding the colon and ending with whitespace.

echo: Prints the captured line containing the function name.

echohl None: Resets the highlighting back to the default.

The demo

If you were at this line in drivers/mfd/qcom-pm8008.c (which is a driver I authored) and pressed , to trigger the above command, you would see this in the vim statusline:

static int pm8008_probe(struct i2c_client *client)

which is, in my view, a very clean way of doing things.