From b2776d097d4407a8d76bea88e4289297eed65e3e Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Thu, 15 Jan 2026 10:30:25 -0800
Subject: [PATCH] std.Io.Threaded: implement MemoryMap.setLength for Windows

---
 lib/std/Io/Threaded.zig | 43 +++++++++++++++++++++++++++--------------
 lib/std/Io/test.zig     |  5 +++--
 lib/std/os/windows.zig  |  2 ++
 3 files changed, 34 insertions(+), 16 deletions(-)

diff --git a/lib/std/Io/Threaded.zig b/lib/std/Io/Threaded.zig
index bfa012c8c2f177773cf4bc2cb6bd8d077c924bac..c8b2f26d273405000abbe3cfa708f5a313c82f8e 100644
--- a/lib/std/Io/Threaded.zig
+++ b/lib/std/Io/Threaded.zig
@@ -16232,7 +16232,7 @@ fn createFileMap(
     protection: std.process.MemoryProtection,
     offset: u64,
     populate: bool,
-    aligned_len: usize,
+    len: usize,
 ) CreateFileMapError!File.MemoryMap {
     if (is_windows) {
         try Thread.checkCancel();
@@ -16251,7 +16251,7 @@ fn createFileMap(
                 .STANDARD = .{ .RIGHTS = .REQUIRED },
             },
             null,
-            @constCast(&@as(i64, @intCast(aligned_len))),
+            @constCast(&@as(i64, @intCast(len))),
             .{ .READWRITE = true },
             .{ .COMMIT = populate },
             file.handle,
@@ -16261,12 +16261,11 @@ fn createFileMap(
             .INVALID_FILE_FOR_SECTION => return error.OperationUnsupported,
             else => |status| return windows.unexpectedStatus(status),
         }
-        const current_process: windows.HANDLE = @ptrFromInt(@as(usize, @bitCast(@as(isize, -1))));
-        var contents_ptr: ?[*]u8 = null;
-        var contents_len = aligned_len;
+        var contents_ptr: ?[*]align(std.heap.page_size_min) u8 = null;
+        var contents_len = len;
         switch (windows.ntdll.NtMapViewOfSection(
             section,
-            current_process,
+            windows.current_process,
             @ptrCast(&contents_ptr),
             null,
             0,
@@ -16284,7 +16283,7 @@ fn createFileMap(
         return .{
             .file = file,
             .offset = offset,
-            .memory = @alignCast(contents_ptr.?[0..contents_len]),
+            .memory = contents_ptr.?[0..contents_len],
             .section = section,
         };
     } else if (have_mmap) {
@@ -16308,17 +16307,17 @@ fn createFileMap(
         const contents = while (true) {
             const syscall: Syscall = try .start();
             const casted_offset = std.math.cast(i64, offset) orelse return error.Unseekable;
-            const rc = mmap_sym(null, aligned_len, prot, flags, file.handle, casted_offset);
+            const rc = mmap_sym(null, len, prot, flags, file.handle, casted_offset);
             syscall.finish();
             const err: posix.E = if (builtin.link_libc) e: {
                 if (rc != std.c.MAP_FAILED) {
-                    break @as([*]align(page_align) u8, @ptrCast(@alignCast(rc)))[0..aligned_len];
+                    break @as([*]align(page_align) u8, @ptrCast(@alignCast(rc)))[0..len];
                 }
                 break :e @enumFromInt(posix.system._errno().*);
             } else e: {
                 const err = posix.errno(rc);
                 if (err == .SUCCESS) {
-                    break @as([*]align(page_align) u8, @ptrFromInt(rc))[0..aligned_len];
+                    break @as([*]align(page_align) u8, @ptrFromInt(rc))[0..len];
                 }
                 break :e err;
             };
@@ -16356,8 +16355,7 @@ fn fileMemoryMapDestroy(userdata: ?*anyopaque, mm: *File.MemoryMap) void {
     const memory = mm.memory;
     if (mm.section) |section| switch (native_os) {
         .windows => {
-            const current_process: windows.HANDLE = @ptrFromInt(@as(usize, @bitCast(@as(isize, -1))));
-            _ = windows.ntdll.NtUnmapViewOfSection(current_process, memory.ptr);
+            _ = windows.ntdll.NtUnmapViewOfSection(windows.current_process, memory.ptr);
             windows.CloseHandle(section);
         },
         .wasi => unreachable,
@@ -16394,8 +16392,25 @@ fn fileMemoryMapSetLength(
         }
         switch (native_os) {
             .windows => {
-                _ = section;
-                @panic("TODO");
+                var contents_ptr: ?[*]align(page_align) u8 = null;
+                var contents_len = new_len;
+                switch (windows.ntdll.NtMapViewOfSection(
+                    section,
+                    windows.current_process,
+                    @ptrCast(&contents_ptr),
+                    null,
+                    0,
+                    null,
+                    &contents_len,
+                    .Unmap,
+                    .{},
+                    .{ .READWRITE = true },
+                )) {
+                    .SUCCESS => {},
+                    .SECTION_PROTECTION => return error.PermissionDenied,
+                    else => |status| return windows.unexpectedStatus(status),
+                }
+                mm.memory = contents_ptr.?[0..contents_len];
             },
             .wasi => unreachable,
             .linux => {
diff --git a/lib/std/Io/test.zig b/lib/std/Io/test.zig
index 00a2d872418167d53c5d09d3ce2d8072d21026fa..0f4285b9bd92183cb9efa265e22ac2aec4042baa 100644
--- a/lib/std/Io/test.zig
+++ b/lib/std/Io/test.zig
@@ -640,9 +640,10 @@ test "memory mapping" {
 
         try expectEqualStrings("this9is9my", mm.memory);
 
-        try mm.setLength(io, .{ .len = "this9is9my data123".len });
+        // Cross a page boundary to require an actual remap.
+        try mm.setLength(io, .{ .len = std.heap.pageSize() * 2 });
         try mm.read(io);
 
-        try expectEqualStrings("this9is9my data123", mm.memory);
+        try expectEqualStrings("this9is9my data123\x00\x00", mm.memory[0.."this9is9my data123\x00\x00".len]);
     }
 }
diff --git a/lib/std/os/windows.zig b/lib/std/os/windows.zig
index 1e864d14c277f4d09954f388559bf4255a4bd821..91f3cb6513fd6969b98dd0cc67b2e922268bde42 100644
--- a/lib/std/os/windows.zig
+++ b/lib/std/os/windows.zig
@@ -28,6 +28,8 @@ pub const ws2_32 = @import("windows/ws2_32.zig");
 pub const crypt32 = @import("windows/crypt32.zig");
 pub const nls = @import("windows/nls.zig");
 
+pub const current_process: HANDLE = @ptrFromInt(@as(usize, @bitCast(@as(isize, -1))));
+
 pub const FILE = struct {
     // ref: km/ntddk.h
 
-- 
2.54.0