diff --git a/compiler/fn.v b/compiler/fn.v
index 7fb50facd7..84fab3610d 100644
--- a/compiler/fn.v
+++ b/compiler/fn.v
@@ -518,6 +518,13 @@ fn (p mut Parser) fn_call(f Fn, method_ph int, receiver_var, receiver_type strin
 		p.error('function `$f.name` is private')
 	}
 	p.calling_c = f.is_c
+	if f.is_c && !p.builtin_pkg {
+		if f.name == 'free' {
+			p.error('use `free()` instead of `C.free()`') 
+		} else if f.name == 'malloc' {
+			p.error('use `malloc()` instead of `C.malloc()`') 
+		} 
+	} 
 	cgen_name := p.table.cgen_name(f)
 	// if p.pref.is_prof {
 	// p.cur_fn.called_fns << cgen_name
diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index c5fdfe5824..342c886331 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -93,11 +93,6 @@ pub fn free(ptr voidptr) {
 	C.free(ptr)
 }
 
-fn _strlen(s byteptr) int {
-	return C.strlen(s)
-}
-
-
 fn memdup(src voidptr, sz int) voidptr {
 	mem := malloc(sz)
 	return C.memcpy(mem, src, sz)
diff --git a/vlib/builtin/int.v b/vlib/builtin/int.v
index ad08158df9..117737781a 100644
--- a/vlib/builtin/int.v
+++ b/vlib/builtin/int.v
@@ -7,25 +7,25 @@ module builtin
 pub fn (d double) str() string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
 	C.sprintf(buf, '%f', d)
-	return tos(buf, _strlen(buf))
+	return tos(buf, strlen(buf))
 }
 
 pub fn (d f64) str() string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
 	C.sprintf(buf, '%f', d)
-	return tos(buf, _strlen(buf))
+	return tos(buf, strlen(buf))
 }
 
 pub fn (d f32) str() string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
 	C.sprintf(buf, '%f', d)
-	return tos(buf, _strlen(buf))
+	return tos(buf, strlen(buf))
 }
 
 pub fn ptr_str(ptr voidptr) string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
 	C.sprintf(buf, '%p', ptr)
-	return tos(buf, _strlen(buf))
+	return tos(buf, strlen(buf))
 }
 
 // fn (nn i32) str() string {
diff --git a/vlib/os/os.v b/vlib/os/os.v
index 507a444e78..0523c0bbd2 100644
--- a/vlib/os/os.v
+++ b/vlib/os/os.v
@@ -105,24 +105,21 @@ fn parse_windows_cmd_line(cmd byteptr) []string {
 
 // read_file reads the file in `path` and returns the contents.
 pub fn read_file(path string) ?string { 
-	mut res := ''
 	mut mode := 'rb' 
 	cpath := path.cstr()
 	fp := C.fopen(cpath, mode.cstr()) 
 	if isnil(fp) {
 		return error('failed to open file "$path"')
-		//panic('failed to open file "$path"')
 	}
 	C.fseek(fp, 0, SEEK_END)
 	fsize := C.ftell(fp)
-	// C.fseek(fp, 0, SEEK_SET)  // same as C.rewind(fp) below
+	// C.fseek(fp, 0, SEEK_SET)  // same as `C.rewind(fp)` below
 	C.rewind(fp)
 	mut str := malloc(fsize + 1)
 	C.fread(str, fsize, 1, fp)
 	C.fclose(fp)
 	str[fsize] = 0
-	res = tos(str, fsize)
-	return res
+	return string(str, fsize)
 }
 
 // file_size returns the size of the file located in `path`.
@@ -463,7 +460,7 @@ pub fn get_raw_line() string {
 		if nr_chars == 0 {
 			return ''
 		}
-		return tos(buf, nr_chars)
+		return string(buf, nr_chars)
 	} 
 }
 
@@ -480,6 +477,15 @@ pub fn user_os() string {
 	$if freebsd {
 		return 'freebsd' 
 	} 
+	$if openbsd {
+		return 'openbsd' 
+	} 
+	$if netbsd {
+		return 'netbsd' 
+	} 
+	$if dragonfly {
+		return 'dragonfly' 
+	} 
 	return 'unknown'
 }
 
@@ -501,7 +507,7 @@ pub fn home_dir() string {
 	return home
 }
 
-// write_file writes text data to a file in `path`. 
+// write_file writes `text` data to a file in `path`. 
 pub fn write_file(path, text string) {
 	f := os.create(path) or {
 		return 
@@ -536,11 +542,11 @@ pub fn executable() string {
 		if count < 0 {
 			panic('error reading /proc/self/exe to get exe path')
 		}
-		return tos(result, count)
+		return string(result, count)
 	}
 	$if windows {
 		ret := int(C.GetModuleFileName( 0, result, MAX_PATH ))
-		return tos( result, ret)
+		return string( result, ret)
 	}
 	$if mac {
 		pid := C.getpid() 
@@ -567,14 +573,14 @@ pub fn executable() string {
 		if count < 0 {
 			panic('error reading /proc/curproc/exe to get exe path')
 		}
-		return tos(result, count)
+		return string(result, count)
 	} 
 	$if dragonfly {
 		count := int(C.readlink('/proc/curproc/file', result, MAX_PATH ))
 		if count < 0 {
 			panic('error reading /proc/curproc/file to get exe path')
 		}
-		return tos(result, count)
+		return string(result, count)
 	} 
 	return '.' 
 }
diff --git a/vlib/strings/builder.v b/vlib/strings/builder.v
index 11f34c6127..59a5f32ab8 100644
--- a/vlib/strings/builder.v
+++ b/vlib/strings/builder.v
@@ -38,5 +38,5 @@ pub fn (b Builder) cut(n int) {
 }
 
 pub fn (b mut Builder) free() {
-	C.free(b.buf.data)
+	free(b.buf.data)
 }
diff --git a/vlib/strings/strings.v b/vlib/strings/strings.v
index ce32c38aa9..919e831cd7 100644
--- a/vlib/strings/strings.v
+++ b/vlib/strings/strings.v
@@ -5,9 +5,10 @@ pub fn repeat(c byte, n int) string {
 		return ''
 	}
 	mut arr := malloc(n + 1)
+	//mut arr := [byte(0); n + 1] 
 	for i := 0; i < n; i++ {
 		arr[i] = c
 	}
 	arr[n] = `\0`
-	return tos(arr, n)
+	return string(arr, n)
 }