diff --git a/ext/iconv/iconv.c b/ext/iconv/iconv.c
index 2801049..77fae7e 100644
--- a/ext/iconv/iconv.c
+++ b/ext/iconv/iconv.c
@@ -188,7 +188,7 @@ static VALUE iconv_convert _((iconv_t cd, VALUE str, long start, long length, in
 static VALUE iconv_s_allocate _((VALUE klass));
 static VALUE iconv_initialize _((int argc, VALUE *argv, VALUE self));
 static VALUE iconv_s_open _((int argc, VALUE *argv, VALUE self));
-static VALUE iconv_s_convert _((struct iconv_env_t* env));
+static VALUE iconv_s_convert _((VALUE env));
 static VALUE iconv_s_iconv _((int argc, VALUE *argv, VALUE self));
 static VALUE iconv_init_state _((VALUE cd));
 static VALUE iconv_finish _((VALUE self));
@@ -204,7 +204,7 @@ static VALUE charset_map;
  * Returns the map from canonical name to system dependent name.
  */
 static VALUE
-charset_map_get(void)
+charset_map_get(VALUE klass)
 {
     return charset_map;
 }
@@ -642,7 +642,7 @@ iconv_s_allocate(VALUE klass)
 }
 
 static VALUE
-get_iconv_opt_i(VALUE i, VALUE arg)
+get_iconv_opt_i(RB_BLOCK_CALL_FUNC_ARGLIST(i, arg))
 {
     VALUE name;
 #if defined ICONV_SET_TRANSLITERATE || defined ICONV_SET_DISCARD_ILSEQ
@@ -784,8 +784,9 @@ iconv_s_open(int argc, VALUE *argv, VALUE self)
 }
 
 static VALUE
-iconv_s_convert(struct iconv_env_t* env)
+iconv_s_convert(VALUE env_value)
 {
+    struct iconv_env_t* env = (struct iconv_env_t*)env_value;
     VALUE last = 0;
 
     for (; env->argc > 0; --env->argc, ++env->argv) {
@@ -906,7 +907,7 @@ list_iconv(unsigned int namescount, const char *const *names, void *data)
 
 #if defined(HAVE_ICONVLIST) || defined(HAVE___ICONV_FREE_LIST)
 static VALUE
-iconv_s_list(void)
+iconv_s_list(VALUE klass)
 {
 #ifdef HAVE_ICONVLIST
     int state;