diff --git a/include/picrin/string.h b/include/picrin/string.h
index 66ccd374..f2058cde 100644
--- a/include/picrin/string.h
+++ b/include/picrin/string.h
@@ -22,7 +22,10 @@ pic_str *pic_str_new(pic_state *, const char * /* nullable */, size_t);
 pic_str *pic_str_new_cstr(pic_state *, const char *);
 pic_str *pic_str_new_fill(pic_state *, size_t, char);
 
+size_t pic_strlen(pic_state *, pic_str *);
 char pic_str_ref(pic_state *, pic_str *, size_t);
+void pic_str_set(pic_state *, pic_str *, size_t, char);
+
 pic_str *pic_strcat(pic_state *, pic_str *, pic_str *);
 pic_str *pic_substr(pic_state *, pic_str *, size_t, size_t);
 
diff --git a/src/error.c b/src/error.c
index 8faa359f..81cc3403 100644
--- a/src/error.c
+++ b/src/error.c
@@ -167,16 +167,16 @@ pic_error_raise_continuable(pic_state *pic)
 NORETURN static pic_value
 pic_error_error(pic_state *pic)
 {
-  char *str;
-  size_t len, argc;
+  pic_str *str;
+  size_t argc;
   pic_value *argv;
   struct pic_error *e;
 
-  pic_get_args(pic, "s*", &str, &len, &argc, &argv);
+  pic_get_args(pic, "s*", &str, &argc, &argv);
 
   e = (struct pic_error *)pic_obj_alloc(pic, sizeof(struct pic_error), PIC_TT_ERROR);
   e->type = PIC_ERROR_OTHER;
-  e->msg = pic_str_new_cstr(pic, str);
+  e->msg = str;
   e->irrs = pic_list_by_array(pic, argc, argv);
 
   pic_raise(pic, e);
diff --git a/src/file.c b/src/file.c
index 50f7a646..c68a1869 100644
--- a/src/file.c
+++ b/src/file.c
@@ -29,9 +29,8 @@ pic_file_open_input_file(pic_state *pic)
 {
   static const short flags = PIC_PORT_IN | PIC_PORT_TEXT;
   char *fname;
-  size_t size;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   return generic_open_file(pic, fname, "r", flags);
 }
@@ -41,9 +40,8 @@ pic_file_open_input_binary_file(pic_state *pic)
 {
   static const short flags = PIC_PORT_IN | PIC_PORT_BINARY;
   char *fname;
-  size_t size;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   return generic_open_file(pic, fname, "rb", flags);
 }
@@ -53,9 +51,8 @@ pic_file_open_output_file(pic_state *pic)
 {
   static const short flags = PIC_PORT_OUT | PIC_PORT_TEXT;
   char *fname;
-  size_t size;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   return generic_open_file(pic, fname, "w", flags);
 }
@@ -65,9 +62,8 @@ pic_file_open_output_binary_file(pic_state *pic)
 {
   static const short flags = PIC_PORT_OUT | PIC_PORT_BINARY;
   char *fname;
-  size_t size;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   return generic_open_file(pic, fname, "wb", flags);
 }
@@ -76,10 +72,9 @@ pic_value
 pic_file_exists_p(pic_state *pic)
 {
   char *fname;
-  size_t size;
   FILE *fp;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   fp = fopen(fname, "r");
   if (fp) {
@@ -94,9 +89,8 @@ pic_value
 pic_file_delete(pic_state *pic)
 {
   char *fname;
-  size_t size;
 
-  pic_get_args(pic, "s", &fname, &size);
+  pic_get_args(pic, "z", &fname);
 
   if (remove(fname) != 0) {
     pic_error(pic, "file cannot be deleted");
diff --git a/src/load.c b/src/load.c
index 84fc7c29..75d13749 100644
--- a/src/load.c
+++ b/src/load.c
@@ -48,9 +48,8 @@ pic_load_load(pic_state *pic)
 {
   pic_value envid;
   char *fn;
-  size_t len;
 
-  pic_get_args(pic, "s|o", &fn, &len, &envid);
+  pic_get_args(pic, "z|o", &fn, &envid);
 
   return pic_load(pic, fn);
 }
diff --git a/src/port.c b/src/port.c
index 0295ff24..2f460d82 100644
--- a/src/port.c
+++ b/src/port.c
@@ -4,6 +4,7 @@
 
 #include <stdlib.h>
 #include <string.h>
+#include <limits.h>
 
 #include "picrin.h"
 #include "picrin/proc.h"
@@ -264,9 +265,8 @@ pic_port_open_input_string(pic_state *pic)
 {
   struct pic_port *port;
   char *str;
-  size_t len;
 
-  pic_get_args(pic, "s", &str, &len);
+  pic_get_args(pic, "z", &str);
 
   port = (struct pic_port *)pic_obj_alloc(pic, sizeof(struct pic_port *), PIC_TT_PORT);
   port->file = xmopen();
@@ -465,23 +465,22 @@ static pic_value
 pic_port_write_string(pic_state *pic)
 {
   char *str;
-  size_t len;
   struct pic_port *port;
   int start, end, n, i;
 
-  n = pic_get_args(pic, "s|pii", &str, &len, &port, &start, &end);
+  n = pic_get_args(pic, "z|pii", &str, &port, &start, &end);
   switch (n) {
   case 1:
     port = pic_stdout(pic);
   case 2:
     start = 0;
   case 3:
-    end = len;
+    end = INT_MAX;
   }
 
   assert_port_profile(port, PIC_PORT_OUT | PIC_PORT_TEXT, PIC_PORT_OPEN, "write-string");
 
-  for (i = start; i < end; ++i) {
+  for (i = start; i < end && str[i] != '\0'; ++i) {
     xfputc(str[i], port->file);
   }
   return pic_none_value();
diff --git a/src/string.c b/src/string.c
index d35bedfb..56e6c78f 100644
--- a/src/string.c
+++ b/src/string.c
@@ -48,6 +48,14 @@ pic_str_new_fill(pic_state *pic, size_t len, char fill)
   return pic_str_new(pic, cstr, len);
 }
 
+size_t
+pic_strlen(pic_state *pic, pic_str *str)
+{
+  UNUSED(pic);
+
+  return str->len;
+}
+
 char
 pic_str_ref(pic_state *pic, pic_str *str, size_t n)
 {
@@ -56,6 +64,14 @@ pic_str_ref(pic_state *pic, pic_str *str, size_t n)
   return str->str[n];
 }
 
+void
+pic_str_set(pic_state *pic, pic_str *str, size_t i, char c)
+{
+  UNUSED(pic);
+
+  str->str[i] = c;
+}
+
 pic_str *
 pic_strcat(pic_state *pic, pic_str *a, pic_str *b)
 {
@@ -204,36 +220,34 @@ pic_str_make_string(pic_state *pic)
 static pic_value
 pic_str_string_length(pic_state *pic)
 {
-  char *str;
-  size_t len;
+  pic_str *str;
 
-  pic_get_args(pic, "s", &str, &len);
+  pic_get_args(pic, "s", &str);
 
-  return pic_int_value(len);
+  return pic_int_value(pic_strlen(pic, str));
 }
 
 static pic_value
 pic_str_string_ref(pic_state *pic)
 {
-  char *str;
-  size_t len;
+  pic_str *str;
   int k;
 
-  pic_get_args(pic, "si", &str, &len, &k);
+  pic_get_args(pic, "si", &str, &k);
 
-  return pic_char_value(str[k]);
+  return pic_char_value(pic_str_ref(pic, str, k));
 }
 
 static pic_value
 pic_str_string_set(pic_state *pic)
 {
-  char *str, c;
-  size_t len;
+  pic_str *str;
+  char c;
   int k;
 
-  pic_get_args(pic, "sic", &str, &len, &k, &c);
+  pic_get_args(pic, "sic", &str, &k, &c);
 
-  str[k] = c;
+  pic_str_set(pic, str, k, c);
   return pic_none_value();
 }
 
@@ -273,45 +287,38 @@ DEFINE_STRING_CMP(ge, >=)
 static pic_value
 pic_str_string_copy(pic_state *pic)
 {
-  size_t len;
-  char *str;
-  int n, start, end, i;
-  pic_str *copy;
+  pic_str *str;
+  int n, start, end;
 
-  n = pic_get_args(pic, "s|ii", &str, &len, &start, &end);
+  n = pic_get_args(pic, "s|ii", &str, &start, &end);
 
   switch (n) {
   case 1:
     start = 0;
   case 2:
-    end = len;
+    end = pic_strlen(pic, str);
   }
 
-  copy = pic_str_new(pic, NULL, end - start);
-  for (i = 0; i < end - start; ++i) {
-    copy->str[i] = str[start + i];
-  }
-  return pic_obj_value(copy);
+  return pic_obj_value(pic_substr(pic, str, start, end));
 }
 
 static pic_value
 pic_str_string_copy_ip(pic_state *pic)
 {
-  size_t to_len, from_len;
-  char *to_str, *from_str;
+  pic_str *to, *from;
   int n, at, start, end;
 
-  n = pic_get_args(pic, "sis|ii", &to_str, &to_len, &at, &from_str, &from_len, &start, &end);
+  n = pic_get_args(pic, "sis|ii", &to, &at, &from, &start, &end);
 
   switch (n) {
   case 3:
     start = 0;
   case 4:
-    end = from_len;
+    end = pic_strlen(pic, from);
   }
 
   while (start < end) {
-    to_str[at++] = from_str[start++];
+    pic_str_set(pic, to, at++, pic_str_ref(pic, from, start++));
   }
   return pic_none_value();
 }
@@ -319,44 +326,40 @@ pic_str_string_copy_ip(pic_state *pic)
 static pic_value
 pic_str_string_append(pic_state *pic)
 {
-  size_t argc, len, i;
+  size_t argc, i;
   pic_value *argv;
-  char *buf;
+  pic_str *str;
 
   pic_get_args(pic, "*", &argc, &argv);
 
-  len = 0;
-  buf = NULL;
+  str = pic_str_new(pic, NULL, 0);
   for (i = 0; i < argc; ++i) {
     if (! pic_str_p(argv[i])) {
       pic_error(pic, "type error");
     }
-    buf = pic_realloc(pic, buf, len + pic_str_ptr(argv[i])->len);
-    /* copy! */
-    memcpy(buf + len, pic_str_ptr(argv[i])->str, pic_str_ptr(argv[i])->len);
-    len += pic_str_ptr(argv[i])->len;
+    str = pic_strcat(pic, str, pic_str_ptr(argv[i]));
   }
-  return pic_obj_value(pic_str_new(pic, buf, len));
+  return pic_obj_value(str);
 }
 
 static pic_value
 pic_str_string_fill_ip(pic_state *pic)
 {
-  size_t len;
-  char *str, c;
+  pic_str *str;
+  char c;
   int n, start, end;
 
-  n = pic_get_args(pic, "sc|ii", &str, &len, &c, &start, &end);
+  n = pic_get_args(pic, "sc|ii", &str, &c, &start, &end);
 
   switch (n) {
   case 1:
     start = 0;
   case 2:
-    end = len;
+    end = pic_strlen(pic, str);
   }
 
   while (start < end) {
-    str[start++] = c;
+    pic_str_set(pic, str, start++, c);
   }
   return pic_none_value();
 }
diff --git a/src/system.c b/src/system.c
index be814089..afe4301a 100644
--- a/src/system.c
+++ b/src/system.c
@@ -83,9 +83,8 @@ static pic_value
 pic_system_getenv(pic_state *pic)
 {
   char *str, *val;
-  size_t len;
 
-  pic_get_args(pic, "s", &str, &len);
+  pic_get_args(pic, "z", &str);
 
   val = getenv(str);
 
diff --git a/src/vm.c b/src/vm.c
index 2bd8f27e..02675ba3 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -170,19 +170,33 @@ pic_get_args(pic_state *pic, const char *format, ...)
       break;
     }
     case 's': {
+      pic_str **str;
+      pic_value v;
+
+      str = va_arg(ap, pic_str **);
+      if (i < argc) {
+        v = GET_OPERAND(pic,i);
+        if (pic_str_p(v)) {
+          *str = pic_str_ptr(v);
+        }
+        else {
+          pic_error(pic, "pic_get_args: expected string");
+        }
+        i++;
+      }
+      break;
+    }
+    case 'z': {
       pic_value str;
       char **cstr;
-      size_t *len;
 
       cstr = va_arg(ap, char **);
-      len = va_arg(ap, size_t *);
       if (i < argc) {
         str = GET_OPERAND(pic,i);
         if (! pic_str_p(str)) {
           pic_error(pic, "pic_get_args: expected string");
         }
         *cstr = pic_str_ptr(str)->str;
-        *len = pic_str_ptr(str)->len;
         i++;
       }
       break;