Re: [Bug] __wait_status_to_int() is expected to be a macro

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Jun  8 12:43, Eric Blake wrote:
> On 06/08/2018 12:26 PM, Hans-Bernhard Bröker wrote:
> > Am 08.06.2018 um 17:16 schrieb Denis Nikiforov:
> > > /usr/include/boost/process/detail/posix/is_running.hpp:18:1: error:
> > > non-constant condition for static assertion
> > >   static_assert(!WIFEXITED(still_active), "Internal Error");
> > >   ^~~~~~~~~~~~~
> > > 
> > > __wait_status_to_int must be a macros but it's redefined as a function.

I don't know the surrounding code, but in how for does this make
sense at all?  This expression is never static, afaics:

  int status;
  wait (&status);
  WIFEXITED(status);

The sense of the above expression with a constant "still_active" is
somehow beyond me.

And this has nothing to do with macro vs. inline function, nor ...

> > It must?  Says who, based on what standard document?
> > 
> > For starters, nobody gets to make any such claims about
> > __wait_status_to_int(), because that's an internal implementation detail
> > of Cygwin.  If at all, such a claim might be made about WIFEXITED(),
> > which is a POSIX standard element.  But POSIX makes no such requirement,
> > as far as I could see on short examination.
> 
> POSIX requires:
> 
> http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/sys_wait.h.html#tag_13_71
> 
> "The <sys/wait.h> header shall define the following macros for analysis of
> process status values:
> 
> WEXITSTATUS
>     Return exit status.
> WIFCONTINUED
>     [XSI] [Option Start] True if child has been continued. [Option End]
> WIFEXITED
>     True if child exited normally.
> ..."
> 
> and:
> 
> http://pubs.opengroup.org/onlinepubs/9699919799/functions/waitpid.html
> 
> "Regardless of its value, this information may be interpreted using the
> following macros, which are defined in <sys/wait.h> and evaluate to integral
> expressions; the stat_val argument is the integer value pointed to by
> stat_loc.
> 
> WIFEXITED(stat_val)
>     Evaluates to a non-zero value if status was returned for a child process
> that terminated normally.
> WEXITSTATUS(stat_val)"
> 
> 
> Nothing in that wording requires WIFEXITED(foo) to be a compile-time
> constant if foo is also a compile-time constant, so on that grounds, Boost
> is buggy for trying to assume that WIFEXITED(constant) can be used in a
> static_assert().  On the other hand, the implementation of WIFEXITED() is
> more like Linux if it DOES result in a compile-time constant when used on a
> compile-time constant argument; so in that regards, it would ALSO be worth
> patching Cygwin, whether or not the Boost portability bug is fixed.

...with Cygwin vs. Linux.

But, assuming I'm completely off-track above and a macro is really
desired:

Defining __wait_status_to_int as function is C++-specific.  I don't think
this is really required.  A quick test implies the C macro definition in
sys/wait.h is sufficent for C++ as well.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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