Weak Linking on Linux
lazarus / 32-Linux

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Kevan S. Hashemi · 2020-11-11 18:48

Dear All,

I'm compiling a dynamic library (DLL) with FPC. At compile time, I
have available all routines the DLL references external to itself. At
run-time I sometimes have all these routines available, in which case
the DLL runs just fine on Linux, Windows, and MacOS (different DLL for
each OS). I sometimes do not have all these routines available, but
that should be okay because in those cases, the DLL won't call any
unresolved external routines. On MacOS and Windows, the DLL runs just
fine, because of what I believe is called "weak linking". On Linux,
the DLL returns an "undefined symbol" error and fails to load. I need
weak linking on Linux. Looking at this page.

https://wiki.freepascal.org/FPC_HowToDo#Weak_linking

it appears that weak linking for Linux is possible with an "external
assembler". I am hoping for an FPC equivalent to the GCC
"__attribute__((weak))" directive, so that the DLL with ignore the
unresolved references at run-time. But if I must mess around in the
assembler, I'd like to give that a try, if instructions exist.

If example code would help, I'll construct three example objects that
illustrate the problem.

Best Wishes, Kevan

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MarkMLl · 2020-11-11 19:23

I've never played with that, and I'd be a bit wary looking at how old
those additions to the page are (i.e. without more recent comment or
updates).

I've done a lot of explicit dynamic loading, and a fair amount of
explicit negotiation relating to plugin versions etc., https://forum.l
azarus.freepascal.org/index.php/topic,49502.msg383415.html#msg383415
is relevant but there's a great deal of "boilerplate" which has to be
written to get things to work.

MarkMLl

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marcov · 2020-11-11 20:28

There is a weakexternal directive,

Quotefunction objc_getProtocol(name:pchar): pobjc_protocol; cdecl;
weakexternal libname;

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MarkMLl · 2020-11-11 21:31

Quote from: marcov on November 11, 2020, 09:28:12 pmThere is a
weakexternal directive,
Quote
Thanks for that. What happens if it's not satisfied?

MarkMLl

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marcov · 2020-11-11 21:59

I assume it is nil then.

It could be that only referencing can be done by directive, and
_defining_ a weak symbol only works in external assembler.

The usage seems to be mostly OS X though (grep for it in rtl/inc) and
some startup code like

riscv64/si_c.inc:  libc_init_proc: TProcedure; weakexternal name
'_init';
riscv64/si_c.inc:  libc_fini_proc: TProcedure; weakexternal name
'_fini';
xtensa/si_c.inc:  libc_init_proc: TProcedure; weakexternal name
'_init';
xtensa/si_c.inc:  libc_fini_proc: TProcedure; weakexternal name
'_fini';

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Kevan S. Hashemi · 2020-11-11 22:19

Hi Marcov,

> weakexternal libname;

Sweet! That works perfectly. For some reason, weak external is the
default on MacOS and Windows. Now it is explicit in all platforms.

I search on-line for "Free Pascal Compiler weak external directive"
and come up with nothing. Where should I have looked for documentation
for this directive?

Thank you, Kevan

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marcov · 2020-11-11 22:29

Afaik it is undocumented