Re: [musl] Linked stream handling

[Rich Felker <dalias@libc.org>]

On Sat, Jan 18, 2025 at 10:58:11AM +0100, Florian Weimer wrote:
> Some of us interpret POSIX that it requires or at least encourages a
> concept of linked streams: reading on from some streams may implicitly
> flush certain other streams.  (The language in the standard around
> that is not particularly clear.)  Linked streams may introduce
> deadlocks (particularly with explicit locking if flockfile), so POSIX
> suggests that implementations provide some form of deadlock detection,
> failing certain stream operations.
> 
> Do I read the sources correctly, and musl does not implement any of
> this?

musl very intentionally does not do any implicit flushing, because (1)
it necessarily incurs deadlocks, and (2) it's useless because portable
programs can't depend on it anyway but need to explicitly flush what
they want, and musl generally favors making programs do the portable
thing when there's no strong reason not to.

I think the recommendation in POSIX to detect deadlocks is misguided
because any deadlock beyond some trivial same-thread stuff that's
unlikely to be the situation at hand is halting-equivalent to detect.
The POSIX recommendation should be to drop the whole misguided legacy
practice of auto-flushing that was invented in a single-threaded world
on C implementations that could only open a small single- or
double-digit number of FILEs at a time.

Rich