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From: chips@usfvax2.UUCP (Chip Salzenberg)
Newsgroups: comp.lang.c
Subject: Re: How big is a (??? *)
Message-ID: <767@usfvax2.UUCP>
Date: Fri, 22-May-87 13:10:11 EDT
Article-I.D.: usfvax2.767
Posted: Fri May 22 13:10:11 1987
Date-Received: Sat, 23-May-87 17:28:24 EDT
References: <737@edge.UUCP>
Organization: AT Engineering, Tampa, FL
Lines: 35
Summary: Use void * instead of char *
Endurium: 120 cubic meters

Missile-Launcher: Class 5


In article <737@edge.UUCP>, doug@edge.UUCP (Doug Pardee) writes:
> To the "outside world", each object is known only by a handle, which is
> typically (here it comes, folks) a pointer to that malloc'ed struct.
> [...] 
> Possibility #1:  We cavalierly declare that no pointers are longer than
> char pointers:

Does the word "assume" ring a bell?

> Possibility #2:  We cavalierly declare that all pointers to pointers are
> the same length:

Again, an unsafe assumption.  If, for example, "char *" is bigger than "int *",
then "char *" may have more stringent alignment requirements than "int *".
Therefore, "char **" could be smaller than "int **".  (Any resemblance to
real architectures, living or dead, is purely coincidental.)

> Possibility #3:  We observe that malloc is declared as "char *", and
> so we assume that all pointers returned by malloc are truly (char *),
> even if we subsequently cast them to (struct foo *) and put a struct foo
> into that memory.

Use "void *" instead.  Void pointers are guaranteed to hold any pointer value
without losing precision.  Cast the return value of malloc() to "void *" and 
use the resulting void pointer as a handle.

If you want your code to be portable to compilers lacking a "void" type, make
a typedef (typedef void *HANDLE) so that you can conveniently change your mind.



-- 
Chip Salzenberg		    Address: "{gatech,cbatt,akgua}!usfvax2!ateng!chip"
AT Engineering, Tampa, FL   Redress: "chips@usfvax2.UUCP"
"Use the Source, Luke!"	    My opinions do not necessarily agree with anything.