struct sockaddr_storage is insufficient as well. A Unix domain socket path can be longer than `sizeof ((struct sockaddr_un){ 0}).sun_path`. That's a major reason why all the socket APIs take a separate socklen_t argument. Most people just assume that a domain socket path is limited to a relatively short string, but it's not (except possibly Minix, IIRC).
> A Unix domain socket path can be longer than `sizeof ((struct sockaddr_un){ 0}).sun_path`
Hm, I didn't realize this, or if I knew this I had forgotten. It makes sense because sun_path is usually pretty small, I believe 108 chars is the most common choice, and typically file paths are allowed to be much longer.
Do you have a citation for this behavior? I can't seem to find it, though I'm not looking very hard.
I guess you are right that any syscall taking a struct sockaddr * also has a length passed to it... Some systems have sa_len inside struct sockaddr to indicate length, but IIRC linux does not. I've often thought that length parameter was sort of redundant, because (1) some platforms have sa_len, and (2) even without that, you should be able to derive length from family. But your Unix domain socket example breaks (2). Without being able to do that, I start to imagine that the kernel would need to probe for NUL chars terminating the C string anytime it inspects a struct sockaddr_un, rather than block-copying the expected size of the structure -- that would be needlessly complicated.
So I just reran some tests on my existing VMs and it turns out I remembered wrong. Here's the actual break down:
* Solaris 11.4: .sun_path: 108; bind/connect path maximum: 1023. Length seems to be same as open. Interestingly, open path maximum seems to be 1023 (judged by trying ls -l /path/to/sock), although I always thought it was unbounded on Solaris.
* MacOS 10.14: .sun_path: 104, bind/connect path maximum: 253. Length can be bigger than .sun_path but less than open path limit.
* NetBSD 8.0: .sun_path: 104, bind/connect path maximum: 253. Same as MacOS.
* Linux 5.4: .sun_path: 108, bind/connect path maximum: 108.
* AIX 7.1: .sun_path: 1023, bind/connect path maximum: 1023. Yes, .sun_path is statically sized to 1023! And like Solaris, open path maximum seems to be 1023 (as judged by trying ls -l /path/to/socket). Thanks to Polar Home, polarhome.com, for the free AIX shell account.
Note that all the above lengths are exclusive of NUL, and the passed socklen_t argument did not include a NUL terminator.
For posterity: on all these systems you can still create sockets with long paths, you just have to chdir or use bindat/connectat if available. My test code confirmed as much. And AFAICT getsockname/getpeername will only return the .sun_path path (if anything) used to bind or connect, but that's a more complex topic (see https://github.com/wahern/cqueues/blob/e3af1f63/PORTING.md#g...)
Linux also has the unusual extension of: if sun_path[0] is NUL, the path is not a filesystem path and the rest of the name buffer is an ID. I don't remember if that can have embedded NULs in that ID. I believe so.
