I see that some programming languages use "@" in comments. For example, here is a random program in the Linux kernel: https://elixir.bootlin.com/linux/v5.10.11/source/fs/ext4/dir.c#L37
/**
* is_dx_dir() - check if a directory is using htree indexing
* @inode: directory inode
*
* ...
*/
static int is_dx_dir(struct inode *inode)
{
What does the @ mean in @inode? What is this convention of using the "@" symbol called? Is it documented somewhere on the Internet?
This is just a convention of the documentation tool, it's nothing to do with C. You can see the same sorts of comments in Ruby and Java, though I'm sure there's others that take inspiration from the same source.
If this isn't just a habit of the programmer it's to emit documentation automatically and explain the purpose of arguments. For whatever reason, @ was chosen as the "argument identifier" prefix.
It seems that the "@" symbol is indeed for documentation purpose. In the example I gave, the Linux kernel is using Sphinx, and the convention is documented here:
https://www.kernel.org/doc/html/v4.10/doc-guide/kernel-doc.html#writing-kernel-doc-comments