Message-ID:	<39D546B8.2310@home.com>
From:	"Salvador I. Ducros" <sducros AT home DOT com>
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Newsgroups:	comp.os.msdos.djgpp
Subject:	question about linker behaviour
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Date:	Sat, 30 Sep 2000 01:49:14 GMT
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

Hello all.
I've been unable to get a straight forward answer from my
usual sources of "expert" information so I thought I'd throw
this question out to UseNet.

My question is basically whether or not the linker (let's say
the DJGPP linker specifically) will include a function from a static
library into an executable if:

	1. the address of the function is assigned to some function pointer
	2. the function pointer is never actually derefernced
	3. the function is never called by name

For example, lets say there is some data structure that contains
one or more function pointers like below:

typedef struct DS {
	void *data;
	unsigned long datalen;

	int (*fn1)(struct DS *);
	int (*fn2)(struct DS *);
	int (*fn3)(struct DS *);
} DS;

Why? The arbitrary data in the structure would be manipulated by the
functions fn1, fn2 and fn3 which would assigned to the structure at some
point where the nature of the data is known. Therefore one set of
functions
from possibly many sets of functions is chosen and the data can be
manipulated in a generic way.

So lets say all the appropriate functions are in a static library and at
some point in a program such a data structure is created and three
distinct
functions (say A, B & C) from the library are assigned to the function
pointers. Lets say further that in the entire program only one of these
pointers is ever actually dereferenced (i.e. the function is called).

e.g.	...
	ds = malloc( sizeof(DS) );
	...
	ds->fn1 = A;
	ds->fn2 = B;
	ds->fn3 = C;
	...
	ds->fn2( ds );
	free( ds );
	...

In my example above, library function 'B' is called by dereferencing the
function pointer 'fn2' of structure 'ds'. Say that none of the library
functions
are ever called by name [i.e. A(ds) or B(ds) or C(ds)] and say also that
for whatever reason the function pointers 'fn1' and 'fn3' are never
dereferenced (i.e. the functions are never called).

Q: Is the linker able to detect that the library functions A & C are
   never executed?

   And, even if the linker could detect this, would the linker still
include
   the functions A & C in the final executable?

My intuition tells me yes (all three functions would be included) and
the
experiments I've run seem to confirm it but my "expert" sources mostly
tell
me no, hence my question. Anybody have any insights?


Salvador I. Ducros
sducros AT cdn-net DOT com

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