Frustrated trying to be portable

[Rahul Dhesi]

In <1991Feb17.203337.20569 at uokmax.ecn.uoknor.edu>
clgreen at nsslsun.gcn.uoknor.edu (Cliff Green) writes:

     I'm getting somewhat frustrated in making a fairly complex package
     I'm working on to be portable....I'm trying to make portable
     between various hardware and os's (Unix, VMS on VAXen, Sun, SGI,
     etc.).

Good!  Portability is a worthwhile goal.

     Quickly realizing the uses of ANSI C, I ordered the standard
     (thanks, FAQ) and had the sysadmin on our Suns install a copy of
     GCC.

Good!  Conformance to a standard is a worthwhile goal.

     VAX C seems to be pretty close in most of the basic aspects of the
     standard, but there still seems to be a lot missing...Specifically
     on the Sun I'm missing [some include files and functions].

And there's the rub!  You are trying to attain two mutually-exclusive
goals.  Today, given that we are in a transition between traditional C
and ANSI C, one of the best ways to be highly nonportable is to use
pure ANSI C.

So what can you do?  Try this:

1. Use a PARAMS or similar macro to enclose the argument list in
function prototypes in declarations.  Use traditional syntax for
function definitions.

2. Use a symbol STDINCLUDE.  If it's defined, include the ANSI standard
include files.  If not, include traditional C include files only.
Define it wisely.

     #ifdef STDINCLUDE
     # include <stdlib.h>
     # include <string.h>
     #else
       extern char *malloc();
       extern char *strcpy(), *strncpy(), *strcat();
       extern int strlen();
     #endif

(Do not use __STDC__ above -- it's quite possible for a C environment
that isn't strictly ANSI C to still provide ANSI C include files.)

3. If you need functions only provided by ANSI C, either write your own
for other environments.  For example, if you need memmove, use
move_down and move_up instead and include code like the following.
(Check the code for off-by-one errors before using it.)

     #ifdef NEED_MEMMOVE
       char *move_down(dest, src, len) char *dest, *src; unsigned len;
       { char *p = dest; while (len>0) { *dest++ = *src++; --len; } return p; }
       char *move_up(dest, src, len) char *dest, *src; int len;
       { char *p = dest; dest += len; src += len;
	 while (len > 0) { *dest-- = *src--; --len; } return p; }
     #else
     # define move_down   memmove
     # devine move_up     memmove
     #endif

The above hints are the ones that aren't well explained in standard
textbooks.  For many others, see books about portable C programming.
--
Rahul Dhesi <dhesi%cirrusl at oliveb.ATC.olivetti.com>
UUCP:  oliveb!cirrusl!dhesi