Re: libgcov, fork, and mingw (and other targets without the full POSIX set)

[Jakub Jelinek <jakub@redhat.com>]

On Fri, Dec 01, 2023 at 01:03:01PM +0100, Jan Hubicka via Gcc wrote:
> > On Dez 01 2023, Richard Biener via Gcc wrote:
> > 
> > > Hmm, so why's it then referenced and not "GCed"?
> > 
> > This has nothing to do with garbage collection.  It's just the way
> > libgcc avoids having too many source files.  It would be exactly the
> > same if every function were in its own file.
> 
> THe ifdef machinery makes every function to go insto its own .o file
> which are then archived.  So if user code never calls to fork, the .o
> file with fork wrapper should not be picked by linker and we should not
> have link error.
> 
> If user code calls fork, then the .o file with wrapper should be picked
> and we will get linker error on missing fork.  So I think it ought to
> work as it is now.  Does mingw linker behave somehow differently with
> archives?  Or is there problem with a libgcov being DLL or something?

The problem is that the changes to switch to modern C result in calls to
unprototyped function being an error rather than just warning as before.
int foo (void) { return fork (); }
warning: implicit declaration of function ‘fork’ [-Wimplicit-function-declaration]
previously, now
error: implicit declaration of function ‘fork’ [-Wimplicit-function-declaration]
(by default in C99+).

So, as has been discussed earlier, either we should use __builtin_fork ()
rather than fork (), or we need in configure to test for fork prototype and
if missing, prototype it ourselves, or ensure _gcov_fork.o is not compiled
on targets which don't have fork prototyped.

	Jakub