Fix Windows build and provider loading issues

[youngwise]

Fix Windows build issues: OSSL_provider_init export and C4702 unreachable code

This PR fixes two issues encountered when building and running gost-engine on Windows using MSVC.

1. Fix missing export for OSSL_provider_init

Problem
When attempting to load the provider on Windows, OpenSSL fails with the following error:

1C240000:error:1280006A:DSO support routines:win32_bind_func:could not bind to the requested symbol name:crypto\dso\dso_win32.c:182:symname(OSSL_provider_init)
1C240000:error:1280006A:DSO support routines:DSO_bind_func:could not bind to the requested symbol name:crypto\dso\dso_lib.c:171:
1C240000:error:0788010C:common libcrypto routines:provider_init:unsupported:crypto\provider_core.c:962:name=gostprov, provider has no provider init function

Cause
On Windows, the entry point function OSSL_provider_init must be explicitly exported from the DLL. While OPENSSL_EXPORT is used, in some build configurations with MSVC and macro aliasing, the symbol might not be correctly added to the export table.

Solution
Added #pragma comment(linker, "/EXPORT:OSSL_provider_init") for Windows builds to ensure the symbol is visible to OpenSSL's DSO loader regardless of the build environment specifics.

2. Fix MSVC Warning/Error C4702 (Unreachable Code)

Problem
Build fails in Release configuration with the following error:

[ 95%] Building C object CMakeFiles/gost12sum.dir/gost12sum.c.obj gost12sum.c
engine\gost12sum.c(91) : error C2220: following warning treated as error
engine\gost12sum.c(91) : warning C4702: unreachable code

Cause
In gost12sum.c, the help() function internally calls exit(). Therefore, the exit(0) following the help() call in the switch statement is unreachable. This triggers a compiler error when "Warnings as Errors" is enabled.

Solution
Removed the redundant exit(0) call after help().

Environment

• OS: Windows 11 Pro 25H2 x64
• Compiler: Visual Studio 2026 Community (v18.2.1) / MSVC Toolset v14.50.18.0
• OpenSSL Version: OpenSSL 3.6.0 1 Oct 2025 (Library: OpenSSL 3.6.0 1 Oct 2025)

[chipitsine]

we do have windows builds in CI

https://github.com/gost-engine/engine/blob/201c1f9a24918626fb9d10e0f5b47c4e19686806/.github/workflows/windows.yml

no issues encountered. can you provide repro steps ?
or compare steps taken by you with listed in workflow file

[chipitsine]

btw, your pull request seems to failed on CI, please have a look

[youngwise]

Thanks for the feedback! I investigated the differences between the CI environment and my setup, and here is what I found:

1. Regarding the export issue (OSSL_provider_init):
The CI uses windows-latest (currently Visual Studio 2022 / MSVC 19.4x). I am building with Visual Studio 2026 (MSVC 19.5x).
It appears that the newer MSVC toolset handles symbol aliasing differently, causing __declspec(dllexport) to be ignored for the aliased OSSL_provider_init symbol. I verified this with dumpbin /exports: the export table was empty on my build without the fix.

MSVC 19.5 (Visual Studio 26):

Microsoft (R) COFF/PE Dumper Version 14.50.35723.0
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file bin\Debug\gostprov.dll

File Type: DLL

  Summary

        1000 .00cfg
        F000 .data
        5000 .idata
        4000 .pdata
      19B000 .rdata
        A000 .reloc
        1000 .rsrc
       FD000 .text

MSVC 19.4 (Visual Studio 22):

Microsoft (R) COFF/PE Dumper Version 14.44.35222.0
Copyright (C) Microsoft Corporation.  All rights reserved.


Dump of file bin\Debug\gostprov.dll

File Type: DLL

  Section contains the following exports for gostprov.dll

    00000000 characteristics
    FFFFFFFF time date stamp
        0.00 version
           1 ordinal base
           1 number of functions
           1 number of names

    ordinal hint RVA      name

          1    0 0000122B OSSL_provider_init = @ILT+550(OSSL_provider_init)

  Summary

        1000 .00cfg
        F000 .data
        5000 .idata
        4000 .pdata
      19B000 .rdata
        A000 .reloc
        1000 .rsrc
       FE000 .text

Update:
Instead of adding #pragma to the source code, I modified CMakeLists.txt to use target_link_options for MSVC. This explicitly forces the export at the linker level:

if(MSVC)
  target_link_options(gost_prov PRIVATE "/EXPORT:OSSL_provider_init")
endif()

2. Regarding the unreachable code (C4702):
The CI workflow runs tests in Debug mode. The C4702 error occurs specifically in Release builds where MSVC performs aggressive static analysis and detects that exit(0) after help() is unreachable.