From 8b2d0ce218db8d874cec1c11b3f186955af620c1 Mon Sep 17 00:00:00 2001
From: gDator <infos.g.infos@gmail.com>
Date: Thu, 23 Jul 2026 23:06:12 +0200
Subject: [PATCH] Io.Threaded: Implement fileLength without fileStat on Windows
 (#36293)

Closes #36179

Reviewed-on: https://codeberg.org/ziglang/zig/pulls/36293
Reviewed-by: Ryan Liptak <squeek502@noreply.codeberg.org>
---
 lib/std/Io/Threaded.zig | 21 ++++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index 04384b3d68e4e8c26f610731479f4ec435c7cc44..457e4a255f857a80bd7f487b52c9db83027a4370 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -3858,7 +3858,26 @@ fn fileLength(userdata: ?*anyopaque, file: File) File.LengthError!u64 {
             }
         }
     } else if (is_windows) {
-        // TODO call NtQueryInformationFile and ask for only the size instead of "all"
+        var io_status_block: windows.IO_STATUS_BLOCK = undefined;
+        var info: windows.FILE.STANDARD_INFORMATION = undefined;
+        const syscall: Syscall = try .start();
+        while (true) switch (windows.ntdll.NtQueryInformationFile(
+            file.handle,
+            &io_status_block,
+            &info,
+            @sizeOf(windows.FILE.STANDARD_INFORMATION),
+            .Standard,
+        )) {
+            .SUCCESS => break syscall.finish(),
+            .INVALID_PARAMETER => |err| return syscall.ntstatusBug(err),
+            .ACCESS_DENIED => return syscall.fail(error.AccessDenied),
+            .CANCELLED => {
+                try syscall.checkCancel();
+                continue;
+            },
+            else => |s| return syscall.unexpectedNtstatus(s),
+        };
+        return @as(u64, @bitCast(info.EndOfFile));
     }
 
     const stat = try fileStat(t, file);
-- 
2.54.0