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From: GIGUERE@WATCSG.BITNET (Eric Giguere)
Newsgroups: comp.sys.amiga.tech
Subject: Prototypes & the default promotions (was about the new RKM stuff..
Message-ID: <8903111954.AA04828@jade.berkeley.edu>
Date: 11 Mar 89 19:55:51 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 55
X-Unparsable-Date: Sat, 11 Mar 1989 14:49 LCL


In article <15751@oberon.USC.EDU>Marco Papa writes:

>Actually things are not as bad as they look, simply because every
>implementation of the 'C' langage has to respect the following rule,
>excerpted from page 186 of K&R:
>
>"Any actual argument of type float are converted to double before the call;
>any of type char or short are converted to int;"
>                    ^^^^^                  ^^^

This is one reason why you shouldn't be using the 1st edition of K&R
anymore as the "final" reference document.  With Lattice 5.0 out now
and Manx 5.0 out in a few months, the ANSI standard will be de rigueur.
Let me quote Section 3.3.2.2 of the draft Standard (Dec 7, 1988):

    "If ... the called function ... does not include a prototype,
     the integral promotions are performed on each argument and
     arguments that have type float are promoted to double."

but then:

    "If ... the called function ... includes a prototype, the
     arguments are implicitly converted, as if by assignment,
     to the types of the corresponding parameters."

So if a prototype is in scope, the default argument promotions are NOT
performed.  In the following example the first call passes an int and
the second a short:

  extern void f();  /* no parameter information */
  short  s = 0;

  f( s );  /* short gets promoted to int */

  extern void f( short );
  short  s = 0;

  f( s );  /* short stays as short */

One of the major changes the Committee made was to make the Standard
Library "invariant" to the default promotions -- i.e., any functions
that previously expected a char or short were redefined to expect an
int so that the presence or absence of a prototype would make no
difference.

Prototypes can be nasty at times.  However, when Manx 5.0 comes out,
proper use of prototypes will solve the 16- vs. 32-bit int problem
quite nicely.

Eric Giguere
Computer Systems Group, University of Waterloo

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