fix(BunFile) .slice offset on macOS (#4991)

* fix offset

* simplify error

2 files changed, 15 insertions, 0 deletions

diff --git a/src/io/io_darwin.zig b/src/io/io_darwin.zig
index 226a4d284..cb2d15afb 100644
--- a/src/io/io_darwin.zig
+++ b/src/io/io_darwin.zig
@@ -1350,6 +1350,12 @@ pub fn read(
         struct {
             fn doOperation(op: anytype) ReadError!usize {
                 while (true) {
+                    if (op.positional) {
+                        const rc = os.system.lseek(op.fd, @intCast(op.offset), 0);
+                        if (rc == -1) {
+                            return error.Unseekable;
+                        }
+                    }
                     const rc = os.system.read(
                         op.fd,
                         op.buf,
diff --git a/test/js/bun/io/bun-write.test.js b/test/js/bun/io/bun-write.test.js
index b67df9405..f435d2ceb 100644
--- a/test/js/bun/io/bun-write.test.js
+++ b/test/js/bun/io/bun-write.test.js
@@ -291,6 +291,15 @@ it.skip("Bun.write('output.html', HTMLRewriter.transform(Bun.file)))", async don
   done();
 });
 
+it("offset should work #4963", async () => {
+  const filename = tmpdir() + "/bun.test.offset.txt";
+  await Bun.write(filename, "contents");
+  const file = Bun.file(filename);
+  const slice = file.slice(2, file.size);
+  const contents = await slice.text();
+  expect(contents).toBe("ntents");
+});
+
 it("#2674", async () => {
   const file = path.join(import.meta.dir, "big-stdout.js");