Re: Can we go GTK-only?

[Daniel Colascione <dancol@dancol.org>]

On 11/01/2016 09:49 AM, Eli Zaretskii wrote:
>> Cc: raeburn@raeburn.org, monnier@iro.umontreal.ca, emacs-devel@gnu.org
>> From: Paul Eggert <eggert@cs.ucla.edu>
>> Date: Tue, 1 Nov 2016 09:28:34 -0700
>>
>> On 11/01/2016 08:11 AM, Eli Zaretskii wrote:
>>  > Only C11 mandates that malloc/realloc/free shall be thread-safe, and
>> we don't yet require C11.
>>
>> This is too pessimistic. C11 was the first C standard to talk about
>> threads, which is why it's the first C standard to specify whether
>> malloc is thread-safe. In practice it should be safe to assume that
>> malloc is thread-safe on multithreaded platforms, as C programmers would
>> have revolted en masse otherwise.
>
> "It should be safe" and "it's safe" are 2 different things.
>
>>  > gmalloc is only thread-safe if Emacs is built with pthreads.
>>
>> Yes, and that's what one would expect. If you build Emacs in
>> single-threaded mode, malloc won't be thread-safe. But in the normal
>> case nowadays, malloc should be thread-safe.
>
> pthreads is not the only way to have threads.

On any modern system POSIX system it is. Counterexample, please.

>>  > ralloc is not thread-safe at all.
>>
>> Yes, and ralloc as it stands should not be used on modern platforms. We
>> clearly need to move in that direction anyway.
>
> We do move in that direction, but we aren't there yet.
>
>>  > xmalloc calls memory_full, which manipulates global state and calls
>> xsignal, so that is not thread-safe, either.
>>
>> That's fine, so long as xmalloc is called only in the Emacs Lisp thread.
>
> I'd imagine any code that wants to allocate from the heap will call
> xmalloc, as we never call malloc directly in Emacs AFAIK.

That's because we want xmalloc to handle memory exhaustion in a sane 
way. A thread calling system malloc on its own can handle memory 
exhaustion a different way. Memory exhaustion being reported in some way 
is what really matters.

> Like I said: we are barely out of the woods, so allocations from the
> heap in non-main threads should be avoided.

This restriction makes a big class of programs very difficult to write. 
Besides, in my Emacs right now, I have in addition to the main thread a 
generic glib worker thread, a dconf worker thread, and a gdbus thread. 
I'd be amazed if none of these threads allocated memory.