🚀 go-pugleaf

In <1993Jan6.084522.25921@emr1.emr.ca> jagrant@emr1.emr.ca (John Grant) writes:

>			static STUFF stufftable[MAXSTUFF];
>   and in another module that referenced it:
>			extern STUFF stufftable[];
>Ok, that was fine and everything worked ok.

I'm not sure that it should. Since stufftable is explicitly
static to a module (file) you shouldn't have gotten at it
from anywhere else. If you did, that's a bug.

>			static STUFF *stufftable;
>			stufftable=malloc(MAXSTUFF*sizeof(STUFF));
>and in the other module, I left it as:
>			extern STUFF stufftable[];

This of course is totally bogus. It tells the compiler that
you have a data area consisting of items, and that saying
"stufftable" will generate the address of that data area.

>			extern STUFF *stufftable;
>and re-built, it ran perfectly.

Well, this is the correct thing, saying there's this variable
stufftable pointing to data items, and saying "stufftable"
gives you the contents of that variable.

However, since stufftable is static in the first module, you
should not have been able to get at it; the linker should
have given you an unresolved external error.

>and explain why it is so dangerous here, but it's perfectly ok
>to use a pointer or a [] in a function declaration.

I did, above. Better yet, buy Kernighan & Ritchie,
"The C Programming Language" which explain this and a lot
of other things about C.

One common and recommended way of avoiding this kind of
problems is to have the extern declaration in a header,
that both the defining and using file uses.

foo.h:

extern fooItem * fooPointer ;


foo.c:

#include "foo.h"

fooItem * fooPointer = NULL ;

void
InitFoo ( void )
{
	fooPointer = ( fooItem * ) calloc ( sizeof ( fooItem ) , 47 ) ;
}


bar.c:

#include "foo.h"

...
	fooPointer [ 4 ] = ...

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 -- Jon W{tte, h+@nada.kth.se, Mac Hacker Deluxe --

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