diff --git a/qse/include/qse/cmn/str.h b/qse/include/qse/cmn/str.h
index 436c4125..e81b6520 100644
--- a/qse/include/qse/cmn/str.h
+++ b/qse/include/qse/cmn/str.h
@@ -2736,6 +2736,12 @@ QSE_EXPORT qse_size_t qse_mbs_fcat (
 	...
 );
 
+QSE_EXPORT qse_size_t qse_mbs_fmt (
+	qse_mbs_t*         str,
+	const qse_mchar_t* fmt,
+	...
+);
+
 /**
  * The qse_wcs_open() function creates a dynamically resizable wide-character
  * string.
@@ -2927,6 +2933,12 @@ QSE_EXPORT qse_size_t qse_wcs_fcat (
 	...
 );
 
+QSE_EXPORT qse_size_t qse_wcs_fmt (
+	qse_wcs_t*         str,
+	const qse_wchar_t* fmt,
+	...
+);
+
 #if defined(QSE_CHAR_IS_MCHAR)
 #	define qse_str_setmmgr(str,mmgr)    qse_mbs_wetmmgr(str,mmgr)
 #	define qse_str_getmmgr(str)         qse_mbs_getmmgr(str)
@@ -2955,6 +2967,7 @@ QSE_EXPORT qse_size_t qse_wcs_fcat (
 #	define qse_str_trm(str)             qse_mbs_trm(str)
 #	define qse_str_pac(str)             qse_mbs_pac(str)
 #	define qse_str_fcat                 qse_mbs_fcat
+#	define qse_str_fmt                  qse_mbs_fmt
 #else
 #	define qse_str_setmmgr(str,mmgr)    qse_wcs_wetmmgr(str,mmgr)
 #	define qse_str_getmmgr(str)         qse_wcs_getmmgr(str)
@@ -2983,6 +2996,7 @@ QSE_EXPORT qse_size_t qse_wcs_fcat (
 #	define qse_str_trm(str)             qse_wcs_trm(str)
 #	define qse_str_pac(str)             qse_wcs_pac(str)
 #	define qse_str_fcat                 qse_wcs_fcat
+#	define qse_str_fmt                  qse_wcs_fmt
 #endif
 
 
diff --git a/qse/lib/cmn/str-dyn.c b/qse/lib/cmn/str-dyn.c
index 13c4b304..09ffe6e7 100644
--- a/qse/lib/cmn/str-dyn.c
+++ b/qse/lib/cmn/str-dyn.c
@@ -32,20 +32,34 @@ static int put_wchar_null (qse_wchar_t c, void* ctx)
 	return 1;
 }
 
-static int put_mchar (qse_mchar_t c, void* ctx)
+static int put_mchar_check (qse_mchar_t c, void* ctx)
 {
 	qse_mbs_t* str = (qse_mbs_t*)ctx;
 	if (str->val.len < str->capa) str->val.ptr[str->val.len++] = c;
 	return 1;
 }
 
-static int put_wchar (qse_wchar_t c, void* ctx)
+static int put_wchar_check (qse_wchar_t c, void* ctx)
 {
 	qse_wcs_t* str = (qse_wcs_t*)ctx;
 	if (str->val.len < str->capa) str->val.ptr[str->val.len++] = c;
 	return 1; 
 }
 
+static int put_mchar_nocheck (qse_mchar_t c, void* ctx)
+{
+	qse_mbs_t* str = (qse_mbs_t*)ctx;
+	str->val.ptr[str->val.len++] = c;
+	return 1;
+}
+
+static int put_wchar_nocheck (qse_wchar_t* c, void* ctx)
+{
+	qse_wcs_t* str = (qse_wcs_t*)ctx;
+	str->val.ptr[str->val.len++] = c;
+	return 1; 
+}
+
 static int wcs_to_mbs (
 	const qse_wchar_t* wcs, qse_size_t* wcslen,
 	qse_mchar_t* mbs, qse_size_t* mbslen, void* ctx)
@@ -73,7 +87,8 @@ static int mbs_to_wcs (
 #undef fmtout_t
 #undef fmtout
 #undef put_char_null
-#undef put_char
+#undef put_char_check
+#undef put_char_nocheck
 #undef conv_char
 #undef str_t
 #undef str_open
@@ -104,7 +119,9 @@ static int mbs_to_wcs (
 #undef str_trm
 #undef str_pac
 #undef str_fmt
+#undef str_vfmt
 #undef str_fcat
+#undef str_vfcat
 
 #define char_t qse_mchar_t
 #define xstr_t qse_mxstr_t
@@ -117,7 +134,8 @@ static int mbs_to_wcs (
 #define fmtout_t qse_mfmtout_t
 #define fmtout qse_mfmtout
 #define put_char_null put_mchar_null
-#define put_char put_mchar
+#define put_char_check put_mchar_check
+#define put_char_nocheck put_mchar_nocheck
 #define conv_char wcs_to_mbs
 #define str_t qse_mbs_t
 #define str_open qse_mbs_open 
@@ -148,7 +166,9 @@ static int mbs_to_wcs (
 #define str_trm qse_mbs_trm 
 #define str_pac qse_mbs_pac 
 #define str_fmt qse_mbs_fmt 
+#define str_vfmt qse_mbs_vfmt 
 #define str_fcat qse_mbs_fcat 
+#define str_vfcat qse_mbs_vfcat 
 #include "str-dyn.h"
 
 /* -------------------------------------------------------- */
@@ -164,7 +184,8 @@ static int mbs_to_wcs (
 #undef fmtout_t
 #undef fmtout
 #undef put_char_null
-#undef put_char
+#undef put_char_check
+#undef put_char_nocheck
 #undef conv_char
 #undef str_t
 #undef str_open
@@ -195,7 +216,9 @@ static int mbs_to_wcs (
 #undef str_trm
 #undef str_pac
 #undef str_fmt
+#undef str_vfmt
 #undef str_fcat
+#undef str_vfcat
 
 #define char_t qse_wchar_t
 #define xstr_t qse_wxstr_t
@@ -208,7 +231,8 @@ static int mbs_to_wcs (
 #define fmtout_t qse_wfmtout_t
 #define fmtout qse_wfmtout
 #define put_char_null put_wchar_null
-#define put_char put_wchar
+#define put_char_check put_wchar_check
+#define put_char_nocheck put_wchar_nocheck
 #define conv_char mbs_to_wcs
 #define str_t qse_wcs_t
 #define str_open qse_wcs_open 
@@ -239,5 +263,7 @@ static int mbs_to_wcs (
 #define str_trm qse_wcs_trm 
 #define str_pac qse_wcs_pac 
 #define str_fmt qse_wcs_fmt 
+#define str_vfmt qse_wcs_vfmt 
 #define str_fcat qse_wcs_fcat 
+#define str_vfcat qse_wcs_vfcat 
 #include "str-dyn.h"
diff --git a/qse/lib/cmn/str-dyn.h b/qse/lib/cmn/str-dyn.h
index 9320c6d8..ec0f5249 100644
--- a/qse/lib/cmn/str-dyn.h
+++ b/qse/lib/cmn/str-dyn.h
@@ -426,13 +426,52 @@ qse_size_t str_pac (str_t* str)
 }
 
 
-qse_size_t str_fmt (str_t* str, const char_t* fmt, ...)
+
+qse_size_t str_vfcat (str_t* str, const char_t* fmt, va_list ap)
 {
-	return (qse_size_t)-1;
+	va_list orgap;
+	fmtout_t fo;
+	int x;
+	qse_size_t old_len;
+
+	old_len = str->val.len;
+
+	fo.limit = QSE_TYPE_MAX(qse_size_t) - 1;
+	fo.ctx = str;
+	fo.put = str->val.ptr? put_char_check: put_char_null;
+	fo.conv = conv_char;
+
+	va_copy (orgap, ap);
+	x = fmtout (fmt, &fo, ap);
+
+	if (x <= -1)
+	{
+		str->val.len = old_len;
+		return (qse_size_t)-1;
+	}
+	
+	if (str->val.ptr == QSE_NULL || str->val.len - old_len < fo.count)
+	{
+		str->val.len = old_len;
+
+		/* resizing is required */
+		x = resize_for_ncat (str, fo.count);
+
+		if (x <= -1) return (qse_size_t)-1;
+		if (x >= 1)
+		{
+			fo.put = put_char_nocheck;
+			x = fmtout (fmt, &fo, orgap);
+		}
+	}
+
+	str->val.ptr[str->val.len] = T('\0');
+	return str->val.len;
 }
 
 qse_size_t str_fcat (str_t* str, const char_t* fmt, ...)
 {
+#if 0
 	va_list ap;
 	fmtout_t fo;
 	int x;
@@ -475,4 +514,53 @@ qse_size_t str_fcat (str_t* str, const char_t* fmt, ...)
 
 	str->val.ptr[str->val.len] = T('\0');
 	return str->val.len;
+#endif
+	qse_size_t x;
+	va_list ap;
+
+	va_start (ap, fmt);
+	x =  str_vfcat (str, fmt, ap);
+	va_end (ap);
+
+	return x;
+}
+
+qse_size_t str_vfmt (str_t* str, const char_t* fmt, va_list ap)
+{
+	int x;
+	fmtout_t fo;
+	va_list orgap;
+
+	fo.limit = QSE_TYPE_MAX(qse_size_t) - 1;
+	fo.ctx = str;
+	fo.put = put_char_null;
+	fo.conv = conv_char;
+
+	va_copy (orgap, ap);
+	if (fmtout (fmt, &fo, ap) <= -1) return (qse_size_t)-1;
+
+	str_clear (str);
+	x = resize_for_ncat (str, fo.count);
+
+	if (x <= -1) return (qse_size_t)-1;
+	if (x >= 1)
+	{
+		fo.put = put_char_nocheck;
+		x = fmtout (fmt, &fo, orgap);
+	}
+
+	str->val.ptr[str->val.len] = T('\0');
+	return str->val.len;
+}
+
+qse_size_t str_fmt (str_t* str, const char_t* fmt, ...)
+{
+	qse_size_t x;
+	va_list ap;
+
+	va_start (ap, fmt);
+	x = str_vfmt (str, fmt, ap);
+	va_end (ap);
+
+	return x;
 }