Re: [PATCH 3/3] Misc: Add <sys/cstack.h> and the cstack_* family of functions

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 24/05/2021 20:41, Paul Eggert wrote:
> On 5/24/21 12:11 PM, Adhemerval Zanella via Libc-alpha wrote:
> 
>> If this is really a requirement, I think the best options would be to add
>> an extra flag to opt-in.
> 
> Opt-in works for me. Although the GNU apps I help maintain typically don't have nested functions, nested functions are a longstanding GNU C feature and are a good thing to have when you need them.
> 
> Part of the issue here (in case this isn't clear) is that we might want to have a Gnulib version (or even just a copy) of cstack_* to help port GNU apps to other platforms. On these other platforms, mmap might not be the right way to make a stack executable (or might not be needed). I imagine that it'd be better to have this implementation detail done by the cstack_* implementation, than to have each nested-function-using app port this detail by hand.
> 
>> The cstack_t is basically another heap object and I think it should be
>> handled as such.
> 
> I hope that "handled as such" doesn't mean "the user must always manually free it". That is, even if the cstack_t object is on the heap, surely it'd be more convenient for cstack_* users if the object's storage is automatically reclaimed when a thread exits and the object is no longer needed.

The problem is how we go to tie the object lifetime with the thread.
Because if the idea is to provide a different stack, I think extending
the pthread_attr_t with a size and a flag is way more straightforward
than this cstack_t functions. Something like:

  pthread_attr_t attr;
  pthread_attr_init (&attr);
  pthread_attr_setalternatestack_np (262144, PTHREAD_ATTR_NOBOTTOMGUARD);

In this manner the pthread_create will be responsible to allocate and 
deallocate it (on execution ending, pthread_exit, or pthread_cancel).

The issue is how will it would mingle with current pthread_attr stack 
functions, but I think the functionality your are proposing also has
the same issue.