fix(windows): fix MSVC shared library CRT linking in libghostty-vt by eunos-1128 · Pull Request #13151 · ghostty-org/ghostty

eunos-1128 · 2026-07-02T14:40:41Z

Problem

Building libghostty-vt as a shared library on Windows with MSVC fails
due to missing CRT libraries. Explicitly linking these libraries also
exposes a duplicate memset symbol between Ghostty's custom implementation
and libvcruntime.

This blocks downstream projects such as herdr.

Changes

Explicitly link libvcruntime and libucrt for MSVC shared library builds.
Detect the Windows SDK and add its architecture-specific UCRT library path.
Disable the custom memset override when targeting MSVC with libc linked
to avoid the duplicate symbol.

The CRT linking follows the existing approach in GhosttyLib.zig.

Verification

Pending Windows/MSVC build verification after the memset fix.

    
             build-libghostty-vt: 
        
               strategy: 
        
                 matrix: 
        
                   target: [ 
        
                       aarch64-macos, 
        
                       x86_64-macos, 
        
                       aarch64-linux, 
        
                       x86_64-linux, 
        
                       x86_64-linux-musl, 
        
                       x86_64-windows-gnu, 
        
                       # doesn't work yet, we need a way to find msvc libc/c++ headers 
        
                       # x86_64-windows-msvc 
        
                     ] 
        
               runs-on: namespace-profile-ghostty-sm 
        
               needs: test 
        
               env: 
        
                 ZIG_LOCAL_CACHE_DIR: /zig/local-cache 
        
                 ZIG_GLOBAL_CACHE_DIR: /zig/global-cache 
        
               steps: 
        
                 - name: Checkout code 
        
                   uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1 
        
                 - name: Setup Cache 
        
                   uses: namespacelabs/nscloud-cache-action@1124a6f3ce44e5cf84cc22111530961f4d2a15f9 # v1.7.0 
        
                   with: 
        
                     path: | 
        
                       /nix 
        
                       /zig

A helper that's shared between GhosttyLib.zig and GhosttyLibVt.zig will help prevent drift in the future.
It silently ignores a failure to find the SDK.

jcollie · 2026-10-12T06:53:12Z

Oof. Looks like we have some real CI errors.

eunos-1128 · 2026-10-12T08:59:30Z

Oof. Looks like we have some real CI errors.

Would it make sense to disable the custom memset override when targeting MSVC with libc linked?

The Windows CI failures show a duplicate symbol between quirks_memset.zig and libvcruntime.lib.

jcollie · 2026-10-12T14:31:12Z

Would it make sense to disable the custom memset override when targeting MSVC with libc linked?

Yes, I think that makes sense.

eunos-1128 · 2026-10-16T12:31:13Z

@jcollie

Ghostty's custom memset override is now disabled when targeting MSVC with libc linked.

jcollie · 2026-10-29T21:42:41Z

@eunos-1128 can you rebase and fix the conflicts?

Clarified comments regarding Windows DLL requirements and CRT library linking.

) Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com> Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com> Co-authored-by: UENO, M. <eunosium-1128@live.jp>

eunos-1128 · 2026-10-30T17:34:21Z

@eunos-1128 can you rebase and fix the conflicts?

@jcollie I did it.

jcollie

When compiling with -Dsimd=false you can still get duplicate symbols.
Re-enable x86_64-windows-msvc in the CI matrix so that this doesn't bitrot.

jcollie · 2026-10-30T17:56:52Z

+                        .aarch64 => "arm64",
+                        else => "x64",
+                    };
+                    const ucrt_lib_path = std.fmt.allocPrint(


std.fmt.allocPrint is deprecated in favor of b.allocator.print

jcollie · 2026-10-30T17:59:55Z

+                        .x86_64 => "x64",
+                        .x86 => "x86",
+                        .aarch64 => "arm64",
+                        else => "x64",


This should be an error, or at least have a warning logged. Silently falling back to x64 is wrong.

jcollie · 2026-10-30T18:01:52Z

+                        b.allocator,
+                        "{s}\\Lib\\{s}\\ucrt\\{s}",
+                        .{ w10.path, w10.version, arch_str },
+                    ) catch null;


Silent failure to find the SDK is wrong here.

jcollie

A few more changes needed.

jcollie · 2026-10-30T19:10:47Z

+            const ucrt_lib_path = try b.allocator.print(
+                "{s}\\Lib\\{s}\\ucrt\\{s}",
+                .{ w10.path, w10.version, arch_str },
+            );


The code in GhosttyLib.zig that handles MSVC builds has the same problems, but isn't getting updated. We need a shared helper function so that this all gets handled in one place.

jcollie · 2026-10-30T19:13:43Z

+
+      # Duplicate symbols (e.g. memset) can show up without SIMD.
+      - name: Build libghostty-vt (x86_64-windows-msvc, no SIMD)
+        run: zig build -Demit-lib-vt -Dtarget=x86_64-windows-msvc -Dsimd=false


This will only detect the duplicate symbols, it's not fixed. Either only link the CRT when libc is linked, or stop making the memset condition depend on link_libc.

jcollie · 2026-10-30T19:15:50Z


+      - name: Build libghostty-vt (x86_64-windows-msvc)
+        run: zig build -Demit-lib-vt -Dtarget=x86_64-windows-msvc
+


We should be running zig build test-lib-vt -Dtarget=x86_64-windows-msvc so that loading the DLL is actually tested.

The MSVC CRT linking logic (libvcruntime, the Windows SDK ucrt library path, and libucrt) was duplicated in GhosttyLibVt.zig and GhosttyLib.zig, and the two copies had drifted: libghostty silently ignored a missing SDK and fell back to x64 for unknown architectures, while libghostty-vt reported errors. Move the logic into a shared WindowsCrt.link helper that both call. The memset override was skipped for MSVC only when libc was linked, so builds without libc (e.g. -Dsimd=false) could still produce a duplicate memset symbol against libvcruntime. Skip the override for all MSVC targets instead of depending on link_libc. CI now runs test-lib-vt for x86_64-windows-msvc so that the DLL is actually loaded rather than only built, which replaces the separate build steps, including the no-SIMD one.

ghostty-vouch Bot closed this Jul 2, 2026

eunos-1128 mentioned this pull request Jul 3, 2026

Fix MSVC shared library CRT linking in libghostty-vt #13164

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pluiedev reopened this Jul 3, 2026

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eunos-1128 changed the title ~~Fix MSVC shared library CRT linking in libghostty-vt~~ fix(windows): Fix MSVC shared library CRT linking in libghostty-vt Jul 3, 2026

eunos-1128 changed the title ~~fix(windows): Fix MSVC shared library CRT linking in libghostty-vt~~ fix(build): Fix MSVC shared library CRT linking in libghostty-vt Jul 3, 2026

eunos-1128 changed the title ~~fix(build): Fix MSVC shared library CRT linking in libghostty-vt~~ fix(windows): fix MSVC shared library CRT linking in libghostty-vt Jul 5, 2026

mitchellh approved these changes Jul 5, 2026

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eunos-1128 requested a review from mitchellh July 27, 2026 16:35

Copilot AI mentioned this pull request Jul 27, 2026

fix(build): update WindowsSdk.find to 4-arg form in GhosttyLibVt.zig eunos-1128/ghostty#1

Merged

eunos-1128 marked this pull request as draft July 28, 2026 04:22

eunos-1128 marked this pull request as ready for review July 28, 2026 04:55

jcollie closed this Oct 12, 2026

jcollie reopened this Oct 12, 2026

eunos-1128 and others added 5 commits October 30, 2026 17:27

fix(windows): add MSVC CRT linking for ghostty-vt shared builds

6daabcf

Update comments on Windows DLL and CRT linking

1739d58

Clarified comments regarding Windows DLL requirements and CRT library linking.

Update library linking for MSVC in GhosttyLibVt.zig

6767d9e

fix: disable custom memset for MSVC CRT

4733f8d

eunos-1128 force-pushed the bugfix/libghostty-vt-msvc-ucrt branch from 9e54e06 to 4733f8d Compare October 30, 2026 17:29

jcollie requested changes Oct 30, 2026

View reviewed changes

fix(build): enhance Windows SDK detection and add MSVC target builds

d3d1667

jcollie requested changes Oct 30, 2026

View reviewed changes

eunos-1128 requested a review from jcollie October 31, 2026 07:58


		- name: Build libghostty-vt (x86_64-windows-msvc)
		run: zig build -Demit-lib-vt -Dtarget=x86_64-windows-msvc

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eunos-1128 commented Jul 2, 2026 •

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