diff --git a/internal/proc/files.go b/internal/proc/files.go
index 5a5e1c2..712713a 100644
--- a/internal/proc/files.go
+++ b/internal/proc/files.go
@@ -6,8 +6,12 @@ import (
 )
 
 func ExtraFile(cmd *exec.Cmd, f *os.File) (fd uintptr) {
+	return ExtraFileSlice(&cmd.ExtraFiles, f)
+}
+
+func ExtraFileSlice(extraFiles *[]*os.File, f *os.File) (fd uintptr) {
 	// ExtraFiles: If non-nil, entry i becomes file descriptor 3+i.
-	fd = uintptr(3 + len(cmd.ExtraFiles))
-	cmd.ExtraFiles = append(cmd.ExtraFiles, f)
+	fd = uintptr(3 + len(*extraFiles))
+	*extraFiles = append(*extraFiles, f)
 	return
 }