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Message-ID: <24d8214c-476a-4d6b-8fc0-be2c4320d52c@SystematicSW.ab.ca>
Date: Thu, 22 May 2025 13:19:46 -0600
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User-Agent: Mozilla Thunderbird
Subject: Re: gcc links to libcmain when generating a dll
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To: cygwin AT cygwin DOT com
References: <5b9f8d4b-0a5a-4aea-99cb-f2c24a6a127a AT slipbits DOT com>
Organization: Systematic Software
In-Reply-To: <5b9f8d4b-0a5a-4aea-99cb-f2c24a6a127a@slipbits.com>
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From: Brian Inglis via Cygwin <cygwin AT cygwin DOT com>
Reply-To: cygwin AT cygwin DOT com
Cc: Brian Inglis <Brian DOT Inglis AT SystematicSW DOT ab DOT ca>
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On 2025-05-22 12:37, Arthur Schwarz via Cygwin wrote:
> I can't figure this one out.
> 
>  > clear;g++ -flinker-output=dyn -o slip.dll *.o
> 
> generates:
> 
> /usr/lib/gcc/x86_64-pc-cygwin/12/../../../../x86_64-pc-cygwin/bin/ld: /usr/lib/ 
> gcc/x86_64-pc-cygwin/12/../../../../lib/libcygwin.a(libcmain.o): in function 
> `main':
> /usr/src/debug/cygwin-3.6.1-1/winsup/cygwin/lib/libcmain.c:37: 
> (.text.startup+0x79): undefined reference to `WinMain'
> collect2: error: ld returned 1 exit status
> 
> But I am developing a dll, not a windows application. The gcc.pdf manual says:
> 
>     -flinker-output=type
>     This option controls code generation of the link-time optimizer. By
>     default the
>     linker output is automatically determined by the linker plugin. For
>     debugging
>     the compiler and if incremental linking with a non-LTO object file
>     is desired,
>     it may be useful to control the type manually.
> 
>     If type is ‘dyn’, code generation produces a shared library. In this
>     case ‘-fpic’
>     or ‘-fPIC’ is preserved, but not enabled automatically. This allows
>     to build
>     shared libraries without position-independent code on architectures
>     where this
>     is possible, i.e. on x86.
> 
> My code is not executable and nowhere links to or uses any Windows code. It does 
> not have a WinMain method and there is no link to libcmain.o in the object 
> folder (*.o does not have libcmain.o).
> 
> How do I get rid of this link error?

Tell the linker (ld/gcc/g++) not the LTO code generator!
If your "code" is not executable (data?) then LTO is irrelevant.

Please note:

$ info gcc flinker-output | tail -8
---------- Footnotes ----------

(1) On some systems, ‘gcc -shared’ needs to build supplementary stub
code for constructors to work.
On multi-libbed systems, ‘gcc -shared’ must select the correct support libraries 
to link against.
*Failing to supply the correct flags may lead to subtle defects.*
Supplying them in cases where they are not necessary is innocuous.

A couple of examples from cygport scripts:

$ grep -iA3 'CC.*-shared' **/*.cygport
expect.cygport:	cygmake SHLIB_LD='$(CC) -shared -Wl,--out-implib,$@.a'
--
libvpx.cygport:    $CC -shared -fpic -o cygvpx-$ABI.dll			\
libvpx.cygport-		-Wl,--out-implib,libvpx.dll.a			\
libvpx.cygport-		-Wl,--whole-archive,libvpx.a,--no-whole-archive	\
libvpx.cygport-		-Wl,--version-script,$EXPORT_FILE

Your program will need to link with the implib.

-- 
Take care. Thanks, Brian Inglis              Calgary, Alberta, Canada

La perfection est atteinte                   Perfection is achieved
non pas lorsqu'il n'y a plus rien à ajouter  not when there is no more to add
mais lorsqu'il n'y a plus rien à retrancher  but when there is no more to cut
                                 -- Antoine de Saint-Exupéry

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