diff --git a/include/pocketpy/common/str.h b/include/pocketpy/common/str.h
index 7452a359..0ed6dbd8 100644
--- a/include/pocketpy/common/str.h
+++ b/include/pocketpy/common/str.h
@@ -15,22 +15,29 @@ typedef struct c11_sv{
     int size;
 } c11_sv;
 
+/* string */
+typedef struct c11_bytes{
+    int size;
+    const char data[];
+} c11_bytes;
+
+// int size | char[] | '\0'
+typedef c11_bytes c11_string;
+
 int c11_sv__cmp(c11_sv self, c11_sv other);
 int c11_sv__cmp2(c11_sv self, const char* other, int size);
 int c11_sv__cmp3(c11_sv self, const char* other);
 
-// int size | char[] | '\0'
-typedef const char c11_string;
-
 c11_string* c11_string__new(const char* data);
 c11_string* c11_string__new2(const char* data, int size);
+void c11_string__ctor(c11_string* self, const char* data);
+void c11_string__ctor2(c11_string* self, const char* data, int size);
 c11_string* c11_string__copy(c11_string* self);
 void c11_string__delete(c11_string* self);
-int c11_string__len(c11_string* self);
 c11_sv c11_string__sv(c11_string* self);
 c11_string* c11_string__replace(c11_string* self, char old, char new_);
 
-int c11_string__u8_length(const c11_string* self);
+int c11_string__u8_length(c11_string* self);
 c11_sv c11_string__u8_getitem(c11_string* self, int i);
 c11_string* c11_string__u8_slice(c11_string* self, int start, int stop, int step);
 
diff --git a/include/pocketpy/pocketpy.h b/include/pocketpy/pocketpy.h
index 872f1a30..8a658605 100644
--- a/include/pocketpy/pocketpy.h
+++ b/include/pocketpy/pocketpy.h
@@ -321,7 +321,6 @@ const char* py_tpname(py_Type type);
 /// %p: void*
 /// %t: py_Type
 /// %n: py_Name
-const char* py_fmt(const char* fmt, ...);
 
 #define MAGIC_METHOD(x) extern uint16_t x;
 #include "pocketpy/xmacros/magics.h"
diff --git a/src/common/sourcedata.c b/src/common/sourcedata.c
index 6862b7f2..67bb235a 100644
--- a/src/common/sourcedata.c
+++ b/src/common/sourcedata.c
@@ -27,7 +27,7 @@ static void pk_SourceData__ctor(struct pk_SourceData* self,
     }
     self->source = pk_SStream__submit(&ss);
     self->is_precompiled = (strncmp(source, "pkpy:", 5) == 0);
-    c11_vector__push(const char*, &self->line_starts, self->source);
+    c11_vector__push(const char*, &self->line_starts, self->source->data);
 }
 
 static void pk_SourceData__dtor(struct pk_SourceData* self) {
@@ -77,7 +77,7 @@ c11_string* pk_SourceData__snapshot(const struct pk_SourceData* self,
     pk_SStream ss;
     pk_SStream__ctor(&ss);
 
-    pk_sprintf(&ss, "  File \"%S\", line %d", &self->filename, lineno);
+    pk_sprintf(&ss, "  File \"%s\", line %d", self->filename->data, lineno);
 
     if(name && *name) {
         pk_SStream__write_cstr(&ss, ", in ");
diff --git a/src/common/sstream.c b/src/common/sstream.c
index e9f7cf0b..8bda68c6 100644
--- a/src/common/sstream.c
+++ b/src/common/sstream.c
@@ -1,5 +1,6 @@
 #include "pocketpy/common/sstream.h"
 #include "pocketpy/common/config.h"
+#include "pocketpy/common/str.h"
 #include "pocketpy/common/utils.h"
 #include "pocketpy/pocketpy.h"
 
@@ -9,7 +10,7 @@
 #include <ctype.h>
 #include <math.h>
 
-const static int C11_STRING_HEADER_SIZE = sizeof(int);
+const static int C11_STRING_HEADER_SIZE = sizeof(c11_string);
 
 void pk_SStream__ctor(pk_SStream* self) {
     c11_vector__ctor(&self->data, sizeof(char));
@@ -24,14 +25,14 @@ void pk_SStream__write_char(pk_SStream* self, char c) { c11_vector__push(char, &
 void pk_SStream__write_int(pk_SStream* self, int i) {
     // len('-2147483648') == 11
     c11_vector__reserve(&self->data, self->data.count + 11 + 1);
-    int n = sprintf(self->data.data, "%d", i);
+    int n = snprintf(self->data.data, 11 + 1, "%d", i);
     self->data.count += n;
 }
 
 void pk_SStream__write_i64(pk_SStream* self, int64_t val) {
     // len('-9223372036854775808') == 20
     c11_vector__reserve(&self->data, self->data.count + 20 + 1);
-    int n = sprintf(self->data.data, "%lld", (long long)val);
+    int n = snprintf(self->data.data, 20 + 1, "%lld", (long long)val);
     self->data.count += n;
 }
 
@@ -106,9 +107,9 @@ void pk_SStream__write_ptr(pk_SStream* self, void* p) {
 c11_string* pk_SStream__submit(pk_SStream* self) {
     c11_vector__push(char, &self->data, '\0');
     c11_array arr = c11_vector__submit(&self->data);
-    int* p = arr.data;
-    *p = arr.count - C11_STRING_HEADER_SIZE - 1;
-    return (c11_string*)(p + 1);
+    c11_string* retval = (c11_string*)arr.data;
+    retval->size = arr.count - C11_STRING_HEADER_SIZE - 1;
+    return retval;
 }
 
 void pk_vsprintf(pk_SStream* ss, const char* fmt, va_list args) {
@@ -186,18 +187,3 @@ void pk_sprintf(pk_SStream* ss, const char* fmt, ...) {
     pk_vsprintf(ss, fmt, args);
     va_end(args);
 }
-
-const char* py_fmt(const char* fmt, ...) {
-    PK_THREAD_LOCAL pk_SStream ss;
-    if(ss.data.elem_size == 0) {
-        pk_SStream__ctor(&ss);
-    } else {
-        c11_vector__clear(&ss.data);
-    }
-    va_list args;
-    va_start(args, fmt);
-    pk_vsprintf(&ss, fmt, args);
-    va_end(args);
-    pk_SStream__write_char(&ss, '\0');
-    return (const char*)ss.data.data;
-}
diff --git a/src/common/str.c b/src/common/str.c
index 58f90b72..cdb36bdf 100644
--- a/src/common/str.c
+++ b/src/common/str.c
@@ -11,54 +11,53 @@
 c11_string* c11_string__new(const char* data) { return c11_string__new2(data, strlen(data)); }
 
 c11_string* c11_string__new2(const char* data, int size) {
-    int* p = malloc(sizeof(int) + size + 1);
-    *p++ = size;
+    c11_string* retval = malloc(sizeof(c11_string) + size + 1);
+    c11_string__ctor2(retval, data, size);
+    return retval;
+}
+
+void c11_string__ctor(c11_string* self, const char* data) {
+    c11_string__ctor2(self, data, strlen(data));
+}
+
+void c11_string__ctor2(c11_string* self, const char* data, int size) {
+    self->size = size;
+    char* p = (char*)self->data;
     memcpy(p, data, size);
-    ((char*)p)[size] = '\0';
-    return (c11_string*)p;
+    p[size] = '\0';
 }
 
 c11_string* c11_string__copy(c11_string* self) {
-    int* p = (int*)self - 1;
-    int total_size = sizeof(int) + *p + 1;
-    int* q = malloc(total_size);
-    memcpy(q, p, total_size);
-    return (c11_string*)(q + 1);
+    int total_size = sizeof(c11_string) + self->size + 1;
+    c11_string* retval = malloc(total_size);
+    memcpy(retval, self, total_size);
+    return retval;
 }
 
 void c11_string__delete(c11_string* self) {
-    int* p = (int*)self - 1;
-    free(p);
-}
-
-int c11_string__len(c11_string* self) {
-    int* p = (int*)self - 1;
-    return *p;
+    free(self);
 }
 
 c11_sv c11_string__sv(c11_string* self) {
-    int* p = (int*)self - 1;
-    return (c11_sv){self, *p};
+    return (c11_sv){self->data, self->size};
 }
 
 c11_string* c11_string__replace(c11_string* self, char old, char new_) {
     c11_string* retval = c11_string__copy(self);
-    char* p = (char*)retval;
-    int size = c11_string__len(retval);
-    for(int i = 0; i < size; i++) {
+    char* p = (char*)retval->data;
+    for(int i = 0; i < retval->size; i++) {
         if(p[i] == old) p[i] = new_;
     }
     return retval;
 }
 
 int c11_string__u8_length(c11_string* self) {
-    int size = c11_string__len(self);
-    return c11__byte_index_to_unicode(self, size);
+    return c11__byte_index_to_unicode(self->data, self->size);
 }
 
 c11_sv c11_string__u8_getitem(c11_string* self, int i) {
-    i = c11__unicode_index_to_byte(self, i);
-    int size = c11__u8_header(self[i], false);
+    i = c11__unicode_index_to_byte(self->data, i);
+    int size = c11__u8_header(self->data[i], false);
     return c11_sv__slice2(c11_string__sv(self), i, i + size);
 }
 
diff --git a/src/compiler/compiler.c b/src/compiler/compiler.c
index eec27067..c73a056c 100644
--- a/src/compiler/compiler.c
+++ b/src/compiler/compiler.c
@@ -2054,11 +2054,11 @@ Error* pk_compile(pk_SourceData_ src, CodeObject* out) {
     if(err) return err;
 
     Token* data = (Token*)tokens.data;
-    printf("%s\n", src->filename);
+    printf("%s\n", src->filename->data);
     for(int i = 0; i < tokens.count; i++) {
         Token* t = data + i;
         c11_string* tmp = c11_string__new2(t->start, t->length);
-        printf("[%d] %s: %s\n", t->line, pk_TokenSymbols[t->type], tmp);
+        printf("[%d] %s: %s\n", t->line, pk_TokenSymbols[t->type], tmp->data);
         c11_string__delete(tmp);
     }
 
diff --git a/src/compiler/lexer.c b/src/compiler/lexer.c
index d6a2188d..2cb1a704 100644
--- a/src/compiler/lexer.c
+++ b/src/compiler/lexer.c
@@ -40,7 +40,7 @@ const static TokenValue EmptyTokenValue;
 static void pk_Lexer__ctor(pk_Lexer* self, pk_SourceData_ src){
     PK_INCREF(src);
     self->src = src;
-    self->curr_char = self->token_start = src->source;
+    self->curr_char = self->token_start = src->source->data;
     self->current_line = 1;
     self->brackets_level = 0;
     c11_vector__ctor(&self->nexts, sizeof(Token));
@@ -549,7 +549,7 @@ static Error* lex_one_token(pk_Lexer* self, bool* eof){
 
 static Error* from_precompiled(pk_Lexer* self) {
     TokenDeserializer deserializer;
-    TokenDeserializer__ctor(&deserializer, self->src->source);
+    TokenDeserializer__ctor(&deserializer, self->src->source->data);
 
     deserializer.curr += 5;  // skip "pkpy:"
     c11_sv version = TokenDeserializer__read_string(&deserializer, '\n');
@@ -576,8 +576,8 @@ static Error* from_precompiled(pk_Lexer* self) {
         if(is_raw_string_used(t.type)) {
             int64_t index = TokenDeserializer__read_uint(&deserializer, ',');
             c11_string* p = c11__getitem(c11_string*, precompiled_tokens, index);
-            t.start = p;
-            t.length = c11_string__len(p);
+            t.start = p->data;
+            t.length = p->size;
         } else {
             t.start = NULL;
             t.length = 0;
@@ -937,7 +937,7 @@ double TokenDeserializer__read_float(TokenDeserializer* self, char c){
     // TODO: optimize this
     c11_string* nullterm = c11_string__new2(sv.data, sv.size);
     char* end;
-    double retval = strtod(nullterm, &end);
+    double retval = strtod(nullterm->data, &end);
     c11_string__delete(nullterm);
     assert(*end == 0);
     return retval;
diff --git a/src/interpreter/ceval.c b/src/interpreter/ceval.c
index 2e02e33c..d0be6c63 100644
--- a/src/interpreter/ceval.c
+++ b/src/interpreter/ceval.c
@@ -495,7 +495,7 @@ pk_FrameResult pk_VM__run_top_frame(pk_VM* self) {
                 }
                 SP() = begin;
                 c11_string* res = pk_SStream__submit(&ss);
-                py_newstrn(SP()++, res, c11_string__len(res));
+                py_newstrn(SP()++, res->data, res->size);
                 c11_string__delete(res);
                 DISPATCH();
             }
diff --git a/src/public/cast.c b/src/public/cast.c
index 36ba3bb4..3c38cc3b 100644
--- a/src/public/cast.c
+++ b/src/public/cast.c
@@ -4,65 +4,58 @@
 #include "pocketpy/objects/object.h"
 #include "pocketpy/interpreter/vm.h"
 
-int64_t py_toint(const py_Ref self){
+int64_t py_toint(const py_Ref self) {
     assert(self->type == tp_int);
     return self->_i64;
 }
 
-double py_tofloat(const py_Ref self){
+double py_tofloat(const py_Ref self) {
     assert(self->type == tp_float);
     return self->_f64;
 }
 
-bool py_castfloat(const py_Ref self, double* out){
-    switch(self->type){
-        case tp_int:
-            *out = (double)self->_i64;
-            return true;
-        case tp_float:
-            *out = self->_f64;
-            return true;
-        default:
-            return false;
+bool py_castfloat(const py_Ref self, double* out) {
+    switch(self->type) {
+        case tp_int: *out = (double)self->_i64; return true;
+        case tp_float: *out = self->_f64; return true;
+        default: return false;
     }
 }
 
-bool py_tobool(const py_Ref self){
+bool py_tobool(const py_Ref self) {
     assert(self->type == tp_bool);
     return self->_bool;
 }
 
-py_Type py_totype(const py_Ref self){
+py_Type py_totype(const py_Ref self) {
     assert(self->type == tp_type);
     py_Type* ud = py_touserdata(self);
     return *ud;
 }
 
-const char* py_tostr(const py_Ref self){
+const char* py_tostr(const py_Ref self) {
     assert(self->type == tp_str);
-    int* p = PyObject__value(self->_obj);
-    return (const char*)(p+1);
+    c11_string* ud = PyObject__value(self->_obj);
+    return ud->data;
 }
 
-const char* py_tostrn(const py_Ref self, int* size){
+const char* py_tostrn(const py_Ref self, int* size) {
     assert(self->type == tp_str);
-    int* p = PyObject__value(self->_obj);
-    *size = *p;
-    return (const char*)(p+1);
+    c11_string* ud = PyObject__value(self->_obj);
+    *size = ud->size;
+    return ud->data;
 }
 
-const unsigned char* py_tobytes(const py_Ref self, int* size){
+const unsigned char* py_tobytes(const py_Ref self, int* size) {
     assert(self->type == tp_bytes);
     int* ud = PyObject__value(self->_obj);
     *size = *ud;
     return (unsigned char*)(ud + 1);
 }
 
-void* py_touserdata(const py_Ref self){
+void* py_touserdata(const py_Ref self) {
     assert(self && self->is_ptr);
     return PyObject__value(self->_obj);
 }
 
-bool py_istype(const py_Ref self, py_Type type){
-    return self->type == type;
-}
+bool py_istype(const py_Ref self, py_Type type) { return self->type == type; }
diff --git a/src/public/values.c b/src/public/values.c
index 02af220b..621ae9ea 100644
--- a/src/public/values.c
+++ b/src/public/values.c
@@ -46,10 +46,10 @@ void py_newstr(py_Ref out, const char* data) {
 
 void py_newstrn(py_Ref out, const char* data, int size) {
     pk_ManagedHeap* heap = &pk_current_vm->heap;
-    int total_size = sizeof(int) + size + 1;
+    int total_size = sizeof(c11_string) + size + 1;
     PyObject* obj = pk_ManagedHeap__gcnew(heap, tp_str, 0, total_size);
-    int* p = PyObject__value(obj);
-    *p = size;
+    c11_string* ud = PyObject__value(obj);
+    c11_string__ctor2(ud, data, size);
     out->type = tp_str;
     out->is_ptr = true;
     out->_obj = obj;