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Subject: Re: free() and implicit conversion to a function pointer
To: cygwin@cygwin.com
References: <c5af0608-17c4-2270-dbba-c3b704c9226e@t-online.de> <f76885f4-f99f-dc50-2a19-cb892a62db7e@t-online.de>
From: Eric Blake <eblake@redhat.com>
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Message-ID: <d9b1d7d9-a5bc-13fa-35e7-3b289edb349b@redhat.com>
Date: Mon, 20 Mar 2017 13:43:24 -0500
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From: Eric Blake <eblake@redhat.com>
To: cygwin@cygwin.com
Message-ID: <d9b1d7d9-a5bc-13fa-35e7-3b289edb349b@redhat.com>
Subject: Re: free() and implicit conversion to a function pointer
References: <c5af0608-17c4-2270-dbba-c3b704c9226e@t-online.de>
 <f76885f4-f99f-dc50-2a19-cb892a62db7e@t-online.de>
In-Reply-To: <f76885f4-f99f-dc50-2a19-cb892a62db7e@t-online.de>


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On 03/16/2017 02:24 PM, Hans-Bernhard Br=C3=B6ker wrote:
>=20
> The reason this is wrong is that C by design treats data and functions
> as living in separate realms, i.e. its virtual machine has a Harvard
> architecture.  One of the consequences of this is that pointers to
> functions and pointers to data are incommensurable, i.e. any and all
> conversions or comparisons across this divide are wrong.  (void *) are
> compatible to all data pointers, but not to function pointers.

That's true of strict C99, but not true of POSIX (which adds the
additional requirements above-and-beyond C99 that NULL be equivalent to
((void*)0) and that any function pointer can be converted to void* and
back without loss of information, in part because of dlsym() and friends).

--=20
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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