diff --git a/doc/format_specifications.rdoc b/doc/format_specifications.rdoc
index bdfdc249536b36..a2755256df3994 100644
--- a/doc/format_specifications.rdoc
+++ b/doc/format_specifications.rdoc
@@ -30,8 +30,9 @@ It consists of:
 
 - A leading percent character.
 - Zero or more _flags_ (each is a character).
-- An optional _width_ _specifier_ (an integer).
-- An optional _precision_ _specifier_ (a period followed by a non-negative integer).
+- An optional _width_ _specifier_ (an integer, or <tt>*</tt>).
+- An optional _precision_ _specifier_ (a period followed by a non-negative
+  integer, or <tt>*</tt>).
 - A _type_ _specifier_ (a character).
 
 Except for the leading percent character,
@@ -125,13 +126,6 @@ Left-pad with zeros instead of spaces:
   sprintf('%6d', 100)  # => "   100"
   sprintf('%06d', 100) # => "000100"
 
-=== <tt>'*'</tt> Flag
-
-Use the next argument as the field width:
-
-  sprintf('%d', 20, 14)  # => "20"
-  sprintf('%*d', 20, 14) # => "                  14"
-
 === <tt>'n$'</tt> Flag
 
 Format the (1-based) <tt>n</tt>th argument into this field:
@@ -152,6 +146,11 @@ of the formatted field:
   # Ignore if too small.
   sprintf('%1d', 100)   # => "100"
 
+If the width specifier is <tt>'*'</tt> instead of an integer, the actual minimum
+width is taken from the argument list:
+
+  sprintf('%*d', 20, 14) # => "                  14"
+
 == Precision Specifier
 
 A precision specifier is a decimal point followed by zero or more
@@ -194,6 +193,11 @@ the number of characters to write:
   sprintf('%s', Time.now)    # => "2022-05-04 11:59:16 -0400"
   sprintf('%.10s', Time.now) # => "2022-05-04"
 
+If the precision specifier is <tt>'*'</tt> instead of a non-negative integer,
+the actual precision is taken from the argument list:
+
+  sprintf('%.*d', 20, 1)    # => "00000000000000000001"
+
 == Type Specifier Details and Examples
 
 === Specifiers +a+ and +A+
diff --git a/io.c b/io.c
index 4ee45c13442f7c..f6daa793990899 100644
--- a/io.c
+++ b/io.c
@@ -4947,7 +4947,7 @@ rb_io_each_codepoint(VALUE io)
             fptr->cbuf.off += n;
             fptr->cbuf.len -= n;
             rb_yield(UINT2NUM(c));
-            rb_io_check_byte_readable(fptr);
+            rb_io_check_char_readable(fptr);
         }
     }
     NEED_NEWLINE_DECORATOR_ON_READ_CHECK(fptr);
diff --git a/test/ruby/test_io.rb b/test/ruby/test_io.rb
index 3ec8726b5e45ce..0025aa5a7dd502 100644
--- a/test/ruby/test_io.rb
+++ b/test/ruby/test_io.rb
@@ -467,6 +467,24 @@ def test_each_codepoint_closed
     }
   end
 
+  def test_each_codepoint_with_ungetc
+    bug21562 = '[ruby-core:123176] [Bug #21562]'
+    with_read_pipe("") {|p|
+      p.binmode
+      p.ungetc("aa")
+      a = ""
+      p.each_codepoint { |c| a << c }
+      assert_equal("aa", a, bug21562)
+    }
+    with_read_pipe("") {|p|
+      p.set_encoding("ascii-8bit", universal_newline: true)
+      p.ungetc("aa")
+      a = ""
+      p.each_codepoint { |c| a << c }
+      assert_equal("aa", a, bug21562)
+    }
+  end
+
   def test_rubydev33072
     t = make_tempfile
     path = t.path