Re: How to look up where a structure is defined?

[Konstantin Kharlamov <hi-angel@yandex.ru>]

On Wed, 2021-03-03 at 16:24 -0600, Peng Yu wrote:
> On Wed, Mar 3, 2021 at 2:23 PM Konstantin Kharlamov <hi-angel@yandex.ru>
> wrote:
> > 
> > On Wed, 2021-03-03 at 23:09 +0300, Konstantin Kharlamov wrote:
> > > On Wed, 2021-03-03 at 13:51 -0600, Peng Yu wrote:
> > > > This seems to be a complicated solution. I just want to get a database
> > > > (a TSV file should be fine) of types and the header they appear. I
> > > > don't want to build the project just to get this info.
> > > 
> > > I see, well, the Universal Ctags I mentioned should work for you. It
> > > doesn't
> > > require building the project: you just run `ctags -R` or `ctags -Re`
> > > (first
> > > for
> > > vim-style tags file, second one for emacs-style) over the repository, and
> > > you
> > > get a `tags` or `TAGS` file with a list of definitions.
> > > 
> > > Possible drawbacks on ctags I mentioned in the other email. Basically it's
> > > that
> > > it doesn't take context into consideration.
> > > 
> > > Regarding usage: the tags file it generates, although can be read for
> > > human,
> > > supposed to be read by text editors/IDEs. Since you mention a CSV file, I
> > > assume
> > > you might want something human-readable. Please see option --output-
> > > format= in
> > > `man ctags` for details: I think you might want the `xref` format. (I
> > > never
> > > tried it myself, just reading the man it seems like it what you're after).
> > 
> > Although, I wouldn't hold my breath that reading a resulting xref file would
> > be easy The reason being is that I expect a tags file created from glibc
> > repo to be some hundreds of megabytes. For reference, a TAGS file I
> > generated long ago for libreoffice project is sized at 183M.
> > 
> > So yeah, you will probably want to use the file from an IDE or text editor,
> > rather than reading it manually.
> 
> ctags can partially solve the problem. The declaration and definition
> of a struct are all in the header. So finding the definition is OK.
> But it can not return function declaration. Is there a way to show
> function declaration as well? (For example, ./socket/sys/socket.h for
> setsockopt().)
> 
> $ grep '^icmphdr\>' tags
> icmphdr sysdeps/gnu/netinet/ip_icmp.h /^struct icmphdr$/;" s
> $ grep ^setsockopt tags
> setsockopt sysdeps/unix/sysv/linux/setsockopt.c /^setsockopt (int fd,
> int level, int optname, const void *optval, socklen_t len)$/;" f
> 
> ./socket/sys/socket.h
> 215:extern int setsockopt (int __fd, int __level, int __optname,

But, in the grep output you show you have both the declaration and the definition of setsockopt, no? In particular, you have the declaration from `socket.h` file.

I think you're confused because your editor/IDE navigates by default to the first match in the tags file, which probably happened to be the definition. I know vim does that in particular. What to do here is depends on the editor/IDE you're using. For vim specifically there is some command to go to the next tag (I think it's `:tn`).

In Emacs it is implemented in a better way: it shows you a list of all matches when there's more than one, and you get to chose which one you want. Idk, perhaps there's a plugin that implements similar behavior for vim…? Anyway, point being is that the answer depends on what you're using with the tags file.

On a side note, you do need to figure it out for your editor/IDE, because, for example, with struct fields you might get tons of matches, depending on circumstances. Remember, tags does not include context. So if project has multiple structs with a field `int foo;`, all of them will match when you'll try navigate to `foo` by using tags.