FrozenError when executing sample/figmemo_sample.rb

[mtasaka]

Using:
ruby 4.0.1 (2026-01-13 revision e04267a14b) +PRISM [x86_64-linux]

Well, just trying to execute some random example program, I've found that sample/figmemo_sample.rb causes error with backtrace:

$ ruby ./sample/figmemo_sample.rb
/builddir/GIT/tk/lib/tk.rb:2813:in 'TclTkIp#_toUTF8': can't modify frozen String: "10c" (FrozenError)
	from /builddir/GIT/tk/lib/tk.rb:2813:in 'TclTkIp#_toUTF8'
	from /builddir/GIT/tk/lib/tk.rb:676:in 'TkComm#_toUTF8'
	from /builddir/GIT/tk/lib/tk.rb:1962:in 'TkUtil#_conv_args'
	from /builddir/GIT/tk/lib/tk.rb:1962:in 'TkCore#_tk_call_core'
	from /builddir/GIT/tk/lib/tk.rb:1990:in 'TkCore#tk_call'
	from /builddir/GIT/tk/lib/tk.rb:3925:in 'TkConfigMethod#__configure_core'
	from /builddir/GIT/tk/lib/tk.rb:3948:in 'TkConfigMethod#configure'
	from /builddir/GIT/tk/lib/tk/composite.rb:329:in 'TkComposite#configure'
	from /builddir/GIT/tk/lib/tk.rb:4846:in 'TkObject#method_missing'
	from ./sample/figmemo_sample.rb:96:in 'PhotoCanvas#initialize'
	from ./sample/figmemo_sample.rb:415:in 'TkKernel.new'
	from ./sample/figmemo_sample.rb:415:in '<main>'

The following workarounds this:

diff --git a/lib/tk.rb b/lib/tk.rb
index 1d84d93..c2fe525 100644
--- a/lib/tk.rb
+++ b/lib/tk.rb
@@ -2810,7 +2810,7 @@ if (/^(8\.[1-9]|9\.|[1-9][0-9])/ =~ Tk::TCL_VERSION && !Tk::JAPANIZED_TK)
 
         if enc_name
           # str has its encoding information
-          encstr = __toUTF8(str, enc_name)
+          encstr = __toUTF8(+str, enc_name)
           encstr.force_encoding(Tk::Encoding::UTF8_NAME)
           return encstr
         else

[jeremyevans]

Thank you for your report. Do you think it's best to duplicate the frozen string there, or inside __toUTF8?

[mtasaka]

Honestly saying, I think perhaps people would not expect that the usage __toUTF8(str, enc_name) cannot work is str is frozen, so it is best that __toUTF8 can work normally even if str is frozen (i.e. I think it is best that str is duplicated inside __toUTF8 if needed)

[mtasaka]

Instead this seems to work, but I am not familiar with Ruby in C (so I maybe wrong.....)

diff --git a/ext/tk/tcltklib.c b/ext/tk/tcltklib.c
index 1d2b036..f2b93bc 100644
--- a/ext/tk/tcltklib.c
+++ b/ext/tk/tcltklib.c
@@ -7555,7 +7555,7 @@ ip_restart(VALUE self)
 static VALUE
 lib_toUTF8_core(VALUE ip_obj, VALUE src, VALUE encodename)
 {
-    volatile VALUE str = src;
+    VALUE str = rb_str_dup(src);
 
 #ifdef TCL_UTF_MAX
 # if 0

[jeremyevans]

We should probably only duplicate the string if it is frozen. With chilled string literals, we really need to use String#+@, since checking if the string is frozen will return false, but modifying the string will cause a warning. I'm guessing we need to use VALUE str = rb_funcall(src, rb_intern("+@"), 0);. I'll have to do some testing to see whether that works.

[jeremyevans]

It's interesting, this affects sample/figmemo_sample.rb only on Tcl/Tk 9.0. The line of codes causing the bug:

    @scr_region = [-width, -height, width, height]
    self.scrollregion(@scr_region)

On Tcl/Tk 8.6, @src_region is [-381, -267, 381, 267]. On Tcl/Tk 9.0, @src_region is ["10c", "7c", "10c", "7c"]. The code is expecting width and height to be numbers, but in Tcl/Tk 9.0, they are strings, so the -width and -height are calling String#-@, not Integer#-@, and result in the string being frozen.

I've submitted #84 to fix the issue. @mtasaka, can you please test and let me know whether it works for you?