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From: mcdaniel@amara.uucp (Tim McDaniel)
Newsgroups: comp.lang.c,gnu.gcc
Subject: Re: How do I cast to "pointer to function returning int" ?
Message-ID: <MCDANIEL.90Apr12201648@orenda.amara.uucp>
Date: 13 Apr 90 00:16:48 GMT
References: <6090.2621f6c2@csv.viccol.edu.au>
	<90100.172535CMH117@psuvm.psu.edu>
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In-reply-to: CMH117@psuvm.psu.edu's message of 10 Apr 90 21:25:35 GMT

CMH117@psuvm.psu.edu (Charles Hannum) writes:
   Casting a void function to an int function is extremely non-portable.

Not quite.  ANSI C, and many existing compilers, requires that
function pointers all be the same size.  Any pointer to a function can
be cast into any other pointer-to-function type and back again,
producing the same pointer value.  However, the value can't be called
while it is cast to a different type.  For example:

   typedef anytype   (*PF_ANY  )();
   typedef othertype (*PF_OTHER)();
   /*
    * PF_ANY is the type "pointer to function () returning anytype".
    * PF_OTHER is the type "pointer to function () returning othertype".
    * Assume that anytype and othertype are different types.
    */
   PF_ANY   f1, f2;
   PF_OTHER g;
   ...
   g  = (PF_OTHER) f1;	  /* legal under ANSI C */
   f2 = (PF_ANY) g;	  /* legal under ANSI C */
   if (f1 != f2) {
      /* then this compiler is not ANSI C compliant. */
      }
   (*f2)();		  /* exact same result as "(*f1)();" */
   (*g)();		  /* illegal under ANSI C */

And, by the way,
   f2 = (PF_ANY) (void *) f1;
is illegal in ANSI C.  "void *" is the universal pointer only for DATA
objects, not functions.  You can convert any FUNCTION pointer to any
other FUNCTION pointer type.

To rephrase, then:
   Casting a pointer to void function to a pointer to int function
   type, and calling that result, is extremely non-portable.

--
Tim McDaniel
Applied Dynamics International, Ann Arbor, MI
Internet: mcdaniel%amara.uucp@mailgw.cc.umich.edu
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