diff --git a/ext/dl/doc/dl.txt b/ext/dl/doc/dl.txt
index 114baafcb7..9964c4e12f 100644
--- a/ext/dl/doc/dl.txt
+++ b/ext/dl/doc/dl.txt
@@ -59,6 +59,18 @@ to wrap the given PtrData object which is, for example, created by DL::malloc().
 DL::malloc() is a function to allocate a memory by using the C library function
 malloc().
 
+We can define a callback using the module function "callback" as follows:
+
+  module Foo
+    extend DL::Importable
+    def my_comp(str1,str2)
+      str1 <=> str2
+    end
+    COMPARE = callback "int my_comp(char*,char*)"
+  end
+
+where Foo::COMPARE is a Symbol object which invokes the method "my_comp".
+
 DL::Importable module is very useful. However, we sometimes encounter a case
 that we must directly use low-level functions such as dlsym(). In such case,
 we would use DL module functions. They are described in next section.
diff --git a/ext/dl/lib/dl/types.rb b/ext/dl/lib/dl/types.rb
index 51489921ff..53680ac536 100644
--- a/ext/dl/lib/dl/types.rb
+++ b/ext/dl/lib/dl/types.rb
@@ -88,6 +88,7 @@ module DL
       ["long",   "L", nil, nil, nil, nil],
       ["float",  "F", nil, nil, nil, nil],
       ["double", "D", nil, nil, nil, nil],
+      [/char\s*\*/,"S",nil, nil, nil, nil],
       [/.+\*/,   "P", nil, nil, nil, nil],
       [/.+\[\]/, "a", nil, nil, nil, nil],
       ["void",   "0", nil, nil, nil, nil],
diff --git a/ext/dl/sample/libc.rb b/ext/dl/sample/libc.rb
index 9975828be3..32816fd331 100644
--- a/ext/dl/sample/libc.rb
+++ b/ext/dl/sample/libc.rb
@@ -35,6 +35,11 @@ module LIBC
     "int tz_minuteswest",
     "int tz_dsttime",
   ]
+
+  def my_compare(ptr1, ptr2)
+    ptr1.ptr.to_s <=> ptr2.ptr.to_s
+  end
+  COMPARE = callback "int my_compare(char**, char**)"
 end
 
 
@@ -54,7 +59,7 @@ p LIBC.strcat("a", "b")
 
 ary = ["a","c","b"]
 ptr = ary.to_ptr
-LIBC.qsort(ptr, ary.length, DL.sizeof('P'), $cb1)
+LIBC.qsort(ptr, ary.length, DL.sizeof('P'), LIBC::COMPARE)
 p ptr.to_a('S', ary.length)
 
 tv = LIBC::Timeval.malloc