realloc

[Walter Bright]

In article <9118 at alice.UUCP< ark at alice.UUCP (Andrew Koenig) writes:
<In article <10032 at ihlpb.ATT.COM<, gregg at ihlpb.ATT.COM (Wonderly) writes:
<< I read the man page for realloc and it said nothing about this.  Is it
<< not possible for you to type
<< 	if (ptr == NULL)
<< 		ptr = malloc (nbytes);
<< 	else
<< 		ptr = realloc (ptr, nbytes);
<Hm.  Here's what my draft ANSI C spec has to say about realloc:
<	[stuff deleted]
<Of course, not all C implementations behave this way.  This
<leaves C programmers in a bind: rely on this behavior or not?  If
<not, how does one determine which behavior can be trusted?  If
<so, what does one do when one's code breaks on various machines?

I am involved daily with porting code between PCs, Suns, Apples, Vaxes,
etc. The quality of C compilers varies widely, as does the conformance
to ANSI C. What I have done is create a file called 'missing.c' in
which I write functional equivalents of ANSI C library functions. They
are #ifdef'd in for those primitive compilers that don't have them.
For instance, BSD Unix doesn't have memcpy, so I have:

#if BSDUNIX || VAX11C
char *memcpy(t,f,n)
#if BSDUNIX
char *t,*f;
int n;
{
	bcopy(f,t,n);
	return t;
}
#else
register char *t,*f;
register int n;
{
	char *retval = t;

	while (n--) *t++ = *f++;
	return retval;
}
#endif
#endif

(VAX11C is my euphamism for DEC's C compiler.)

I have dealt with the primitive realloc's by always using a function
mem_realloc, and defining it as:

#if __STDC__
#define mem_realloc realloc
#else
void *mem_realloc(oldmem_ptr,newnumbytes)
void *oldmem_ptr;
unsigned newnumbytes;
{   extern void *realloc(),*malloc();
    void *p;

    if (oldptr == NULL)
	p = newnumbytes ? malloc(newnumbytes) : NULL;
    else if (newnumbytes == 0)
    {	free(oldptr);
	p = NULL;
    }
    else
	p = realloc(oldptr,newnumbytes);
    /*printf("realloc(x%lx,%d) = x%lx\n",oldptr,newnumbytes,p);*/
    return p;
}
#endif

If I'm using a compiler which has a broken realloc(), (which is common!)
I can reimplement my mem_realloc as using a malloc/memcpy/free combination.

Using this approach I can use ANSI library function semantics and still
work with archaic compilers. The application code doesn't have to be
littered with crutches for this stuff, it's isolated away.