Re: [PATCH] split __libc_start_main.c into two files (Wasm)

[Rich Felker <dalias@libc.org>]

On Fri, Dec 15, 2017 at 01:04:14PM +0000, Nicholas Wilson wrote:
> Hi,
> 
> On 15 December 2017 12:33, Szabolcs Nagy wrote:
> > why is exit ugly?
> > halting execution should not be complicated on whatever platform
> 
> In WebAssembly, the runtime environment is (typically) a webpage.
> It's not clear what exit() would be expected to do to the webpage or
> Wasm module. The Wasm interpreter offered by browsers doesn't offer
> a way to cleanly exit/terminate a running WebAssembly function; the
> "trap" instruction to abort is the closest you could get.
> 
> If you have some existing C code that you want to compile/port to
> WebAssembly, whatever the code previously expects to happen on
> exit() won't happen. It will need to be changed when ported to the
> web, the semantics of a Unix process and a web page are unavoidably
> different. For brand-new code specifically targeting a web page, you
> wouldn't deliberately add a dependency on exit(), because it's not
> clearly meaningful for a webpage.
> 
> So, it's "ugly" to me because the lifecycle of the webpage is an
> awkward fit for exit(). We could bodge in some emulation, but it's
> just ugly to pull in the emulation when we don't expect it to be
> used. My preference is "you should only link in the hacks you rely
> on".
> 
> It's not too bad - we will have to implement exit() emulation for
> legacy code, so I could live with it being linked in to every
> application. I just dislike hacks, and don't enjoy seeing them
> bundled into my own applications!

I don't see this as ugly at all -- the obvious behavior for exit is
for it to just go into a "halted" state. This is what you generally
expect on embedded C implementations with no OS, or if pid 1 exits on
a unix-like system. If you don't yet support thread creation, all it
has to do is something like for(;;) wait_for_something(); or similar.
Once you have threads, you may need some nontrivial work to ensure
that all threads enter a halted state, or it might just be as simple
as stopping the interpreter. None of this should involve significant
size/cost.

Rich