Fix ARGF.read(length) short read [Bug #18074]

2 files changed, 10 insertions, 2 deletions

diff --git a/io.c b/io.c
index bbecc427db..4d5c3acd3a 100644
--- a/io.c
+++ b/io.c
@@ -12512,8 +12512,7 @@ argf_read(int argc, VALUE *argv, VALUE argf)
     else if (argc >= 1) {
 	long slen = RSTRING_LEN(str);
 	if (slen < len) {
-	    len -= slen;
-            argv[0] = LONG2NUM(len);
+            argv[0] = LONG2NUM(len - slen);
 	    goto retry;
 	}
     }
diff --git a/test/ruby/test_argf.rb b/test/ruby/test_argf.rb
index 7922e5ad07..76a2720d47 100644
--- a/test/ruby/test_argf.rb
+++ b/test/ruby/test_argf.rb
@@ -1110,4 +1110,13 @@ class TestArgf < Test::Unit::TestCase
       assert_raise(TypeError, bug11610) {gets}
     };
   end
+
+  def test_sized_read
+    [@t1, @t2, @t3].each { |t|
+      open(t.path, "wb") { |f| f.write "t" }
+    }
+    ruby('-e', "print ARGF.read(3).size", @t1.path, @t2.path, @t3.path) do |f|
+      assert_equal("3", f.read)
+    end
+  end
 end