Re: [PATCH v2 1/2] system_data_types.7: Add 'void *'

[Vincent Lefevre]

On 2020-10-01 18:55:04 +0200, Alejandro Colomar via Gcc wrote:
> On 2020-10-01 18:38, Michael Kerrisk (man-pages) wrote:
> > > +According to the C language standard,
> > > +a pointer to any object type may be converted to a pointer to
> > > +.I void
> > > +and back.
> > > +POSIX further requires that any pointer,
> > > +including pointers to functions,
> > > +may be converted to a pointer to
> > > +.I void
> > > +and back.
> > I know you are correct about POSIX, but which part of the
> > standard did you find this information in? The only
> > reference that I find in POSIX is the dlsym() spec. Is it
> > covered also somewhere else in the standrd?
[...]
> I've bean searching, and dlsym is the only one:
[...]
> The most explicit paragraph in dlsym is the following:
> 
> [[
> Note that conversion from a void * pointer to a function pointer as in:
> 
> fptr = (int (*)(int))dlsym(handle, "my_function");
> 
> is not defined by the ISO C standard.
> This standard requires this conversion to work correctly
> on conforming implementations.
> ]]

I think that "this conversion" applies only to the dlsym context,
and the conversion isn't defined in general. Imagine that the
void * pointer to function pointer conversion requires the compiler
to generate additional code. The compiler may be able to detect
that dlsym will not be used in some contexts (e.g. because of
always false condition) and do not generate such additional code,
making the conversion to have undefined behavior.

-- 
Vincent Lefèvre  - Web: 
100% accessible validated (X)HTML - Blog: 
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)

gcc-8-20201008 is now available

[GCC Administrator via Gcc]

Snapshot gcc-8-20201008 is now available on
  https://gcc.gnu.org/pub/gcc/snapshots/8-20201008/
and on various mirrors, see http://gcc.gnu.org/mirrors.html for details.

This snapshot has been generated from the GCC 8 git branch
with the following options: git://gcc.gnu.org/git/gcc.git branch releases/gcc-8 
revision 311183d74e4f3fd5a37749cfbb0960e655e715fb

You'll find:

 gcc-8-20201008.tar.xzComplete GCC

  SHA256=6e9ab2050f7e7c6a42abfbf3734ac14e89651c5b6380b0ea277bf863f6bfc257
  SHA1=cca8305cc284e07ae950294cbe3458afd147a4c1

Diffs from 8-20201001 are available in the diffs/ subdirectory.

When a particular snapshot is ready for public consumption the LATEST-8
link is updated and a message is sent to the gcc list.  Please do not use
a snapshot before it has been announced that way.