From 7ef9110b1eaefbcbf72f25ad941c2c3b7ed3db01 Mon Sep 17 00:00:00 2001
From: Nobuyoshi Nakada <nobu@ruby-lang.org>
Date: Fri, 21 Feb 2025 19:40:58 +0900
Subject: [PATCH] [ruby/stringio] Fix SEGV at unget to a null device StringIO

https://github.com/ruby/stringio/commit/eb4ee49218
---
 ext/stringio/stringio.c        | 4 +++-
 test/stringio/test_stringio.rb | 4 ++++
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/ext/stringio/stringio.c b/ext/stringio/stringio.c
index aa6fbf0cac..0c0f63aac7 100644
--- a/ext/stringio/stringio.c
+++ b/ext/stringio/stringio.c
@@ -180,7 +180,9 @@ check_modifiable(struct StringIO *ptr)
     else if (OBJ_FROZEN_RAW(ptr->string)) {
 	rb_raise(rb_eIOError, "not modifiable string");
     }
-    rb_str_modify(ptr->string);
+    else {
+	rb_str_modify(ptr->string);
+    }
 }
 
 static VALUE
diff --git a/test/stringio/test_stringio.rb b/test/stringio/test_stringio.rb
index ffaa2efa0a..570b3d7ad6 100644
--- a/test/stringio/test_stringio.rb
+++ b/test/stringio/test_stringio.rb
@@ -64,6 +64,10 @@ class TestStringIO < Test::Unit::TestCase
     assert_nil io.gets
     io.puts "abc"
     assert_nil io.string
+
+    # Null device StringIO just drop ungot string
+    io.ungetc '#'
+    assert_nil io.getc
   end
 
   def test_truncate