diff --git a/io/sys/darwin/file.c b/io/sys/darwin/file.c
index ddfe6d7..d0b7e39 100644
--- a/io/sys/darwin/file.c
+++ b/io/sys/darwin/file.c
@@ -274,6 +274,18 @@ static enum b_status stream_seek(
 	return b_file_cursor(file, &stream->s_cursor);
 }
 
+static enum b_status stream_tell(const struct b_stream *stream, size_t *pos)
+{
+	const struct b_file *file = stream->s_ptr;
+	off_t v = lseek(file->fd, 0, SEEK_CUR);
+	if (v == (off_t)-1) {
+		return b_status_from_errno(errno, B_ERR_IO_FAILURE);
+	}
+
+	*pos = v;
+	return B_SUCCESS;
+}
+
 const struct b_path *b_file_path(const struct b_file *file)
 {
 	return file->path;
@@ -306,6 +318,7 @@ enum b_status b_file_open_stream(struct b_file *file, struct b_stream **out)
 	stream->s_read = stream_read;
 	stream->s_write = stream_write;
 	stream->s_seek = stream_seek;
+	stream->s_tell = stream_tell;
 
 	*out = stream;
 
@@ -387,11 +400,11 @@ enum b_status b_file_seek(
 	}
 
 	int err = lseek(file->fd, offset, whence);
-	if (err == 0) {
-		return B_SUCCESS;
+	if (err == (off_t)-1) {
+		return b_status_from_errno(errno, B_ERR_IO_FAILURE);
 	}
 
-	return b_status_from_errno(errno, B_ERR_IO_FAILURE);
+	return B_SUCCESS;
 }
 
 enum b_status b_file_swap_shadow(struct b_file *main_file, struct b_file *shadow_file)