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Incremental linking - how to do that?

Ian Lance Taylor ian@airs.com
Tue Feb 19 00:34:00 GMT 2002 
Niv Tweig <rm16029@email.sps.mot.com> writes:

> 1) Suppose you have P1.cc,P2.cc,...,Pn.cc (C++ programs) and you want to
> incrementally link them,
>     how do you do that?
>     I mean, as I understand from the poorly written docs about that
> subject, first you will write probably
>     ld -Ur -o P.exe P1.o P2.o .. Pn.o
>     (the -Ur option is for incremental linking on C++)

-r and -Ur do not do incremental linking.  They link relocateable
objects together to produce a relocateable object.

I guess the documentation for the -i option is misleading.  It should
not use the word ``incremental.''

>     Now suppose that only P1.o changed and you want to update that in
> the exe file,
>     without linking again all the object files, what should you write:
> 
>     ld -Ur -o P.exe P1.o
>         or
>     ld -Ur -o P.exe P1.o P2.o .. Pn.o (Does the linker check the
> timestamps of the files and knows to "update" only P1.o in P.exe?)

Neither.  The first case will essentially copy P1.o while building
global constructor tables.  The second will create a new P.exe from
the input files.

> 2) The documentation for -Ur says :
>     "For anything other than C++ programs, this option is equivalent to
> `-r': it generates relocatable output--i.e., an output file that can in
> turn serve as input to ld. When linking C++ programs, `-Ur' does resolve
> references to constructors, unlike `-r'. It does not work to use `-Ur'
> on files that were themselves linked with `-Ur'; once the constructor
> table has been built, it cannot be added to. Use `-Ur' only for the last
> partial link, and `-r' for the others. "
>     Does that mean that what I suggested in question 1 cannot be done at
> all?
>     i.e. does it mean that P.exe, after being created with the -Ur
> option,
>     cannot be used again with the -Ur option to update itself.
>     If the answer is yes, Then how can incremental links be done in C++.

The answer is yes.

The GNU linker does not support incremental linking.

> 3) What is the exact reason for the separation between C and C++ (the
> options -r and -Ur), what is it with constructors that makes them
> different?

Global objects normally need table of constructors and destructors to
be created.  These tables are used at program startup and termination
to create and destroy the global objects.  The -Ur option directs the
linker to build these tables.  This is useful for certain embedded
operating systems which execute relocateable objects rather than fully
linked executables, such as VxWorks.

Ian
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