diff --git a/compiler/scanner.v b/compiler/scanner.v
index 4df09115cd..30c86fc0e6 100644
--- a/compiler/scanner.v
+++ b/compiler/scanner.v
@@ -45,7 +45,7 @@ fn new_scanner(file_path string) &Scanner {
 		if c_text[0] == 0xEF && c_text[1] == 0xBB && c_text[2] == 0xBF {
 			// skip three BOM bytes
 			offset_from_begin := 3
-			raw_text = tos(c_text[offset_from_begin], C.strlen(c_text) - offset_from_begin)
+			raw_text = tos(c_text[offset_from_begin], vstrlen(c_text) - offset_from_begin)
 		}
 	}
 
diff --git a/vlib/builtin/builtin.v b/vlib/builtin/builtin.v
index b16c8e57c9..686f4bca93 100644
--- a/vlib/builtin/builtin.v
+++ b/vlib/builtin/builtin.v
@@ -17,17 +17,19 @@ fn on_panic(f fn (int) int) {
 	// TODO
 }
 
+fn C.backtrace(voidptr, int) int
+
 pub fn print_backtrace_skipping_top_frames(skipframes int) {
 	$if mac {
 		buffer := [100]byteptr
-		nr_ptrs := C.backtrace(buffer, 100)
-		C.backtrace_symbols_fd(&buffer[skipframes], nr_ptrs-skipframes, 1)
+		nr_ptrs := C.backtrace(*voidptr(buffer), 100)
+		C.backtrace_symbols_fd(*voidptr(&buffer[skipframes]), nr_ptrs-skipframes, 1)
 		return
 	}
 	$if linux {
 		if C.backtrace_symbols_fd != 0 {
 			buffer := [100]byteptr
-			nr_ptrs := C.backtrace(buffer, 100)
+			nr_ptrs := C.backtrace(*voidptr(buffer), 100)
 			C.backtrace_symbols_fd(&buffer[skipframes], nr_ptrs-skipframes, 1)
 			return
 		}else{
diff --git a/vlib/builtin/int.v b/vlib/builtin/int.v
index 00ceb116c8..d6d7a97cb7 100644
--- a/vlib/builtin/int.v
+++ b/vlib/builtin/int.v
@@ -9,20 +9,20 @@ module builtin
 
 pub fn (d f64) str() string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
-	C.sprintf(buf, '%f', d)
-	return tos(buf, strlen(buf))
+	C.sprintf(*char(buf), '%f', d)
+	return tos(buf, vstrlen(buf))
 }
 
 pub fn (d f32) str() string {
 	buf := malloc(sizeof(double) * 5 + 1)// TODO
-	C.sprintf(buf, '%f', d)
-	return tos(buf, strlen(buf))
+	C.sprintf(*char(buf), '%f', d)
+	return tos(buf, vstrlen(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))
+	C.sprintf(*char(buf), '%p', ptr)
+	return tos(buf, vstrlen(buf))
 }
 
 // compare floats using C epsilon
@@ -160,7 +160,7 @@ pub fn (n int) hex() string {
 		11
 	}
 	hex := malloc(len) // 0x + \n
-	count := int(C.sprintf(hex, '0x%x', n))
+	count := int(C.sprintf(*char(hex), '0x%x', n))
 	return tos(hex, count)
 }
 
@@ -171,7 +171,7 @@ pub fn (n i64) hex() string {
 		19
 	}
 	hex := malloc(len)
-	count := int(C.sprintf(hex, '0x%x', n))
+	count := int(C.sprintf(*char(hex), '0x%llx', n))
 	return tos(hex, count)
 }
 
diff --git a/vlib/builtin/map.v b/vlib/builtin/map.v
index 17bd44561c..0720770ba1 100644
--- a/vlib/builtin/map.v
+++ b/vlib/builtin/map.v
@@ -211,7 +211,7 @@ pub fn (m mut map) delete(key string) {
 	m.size--
 }
 
-pub fn (m map) exists(key string) bool {
+pub fn (m map) exists(key string) {
 	panic('map.exists(key) was removed from the language. Use `key in map` instead.')
 }
 
diff --git a/vlib/builtin/string.v b/vlib/builtin/string.v
index 76e2a48f16..ac715a4561 100644
--- a/vlib/builtin/string.v
+++ b/vlib/builtin/string.v
@@ -22,6 +22,10 @@ pub:
 // For C strings only
 fn C.strlen(s byteptr) int
 
+pub fn vstrlen(s byteptr) int {
+	return C.strlen(*char(s))
+}	
+
 fn todo() { }
 
 // Converts a C string to a V string.
@@ -50,7 +54,7 @@ fn tos2(s byteptr) string {
 	if isnil(s) {
 		panic('tos2: nil string')
 	}
-	len := C.strlen(s)
+	len := vstrlen(s)
 	res := tos(s, len)
 	return res
 }
diff --git a/vlib/builtin/utf8.v b/vlib/builtin/utf8.v
index 975ad1c03b..c3352bc0a2 100644
--- a/vlib/builtin/utf8.v
+++ b/vlib/builtin/utf8.v
@@ -4,8 +4,8 @@
 
 module builtin
 
-pub fn utf8_char_len(b byte) int { 
-	return (( 0xe5000000 >> (( b >> 3 ) & 0x1e )) & 3 ) + 1 
+pub fn utf8_char_len(b byte) int {
+	return (( 0xe5000000 >> (( b >> 3 ) & 0x1e )) & 3 ) + 1
 }
 
 // Convert utf32 to utf8
@@ -157,12 +157,11 @@ fn utf8_len(c byte) int {
 pub fn utf8_getchar() int {
   c := int(C.getchar())
   len := utf8_len(~c)
-
   if c < 0 {
     return 0
-  } else if (len == 0) {
+  } else if len == 0 {
     return c
-  } else if (len == 1) {
+  } else if len == 1 {
     return -1
   } else {
     mut uc := int(c & ((1 << (7 - len)) - 1))
@@ -171,7 +170,7 @@ pub fn utf8_getchar() int {
       if c2 != -1 && (c2 >> 6) == 2 {
         uc <<= 6
         uc |= int((c2 & 63))
-      }  else if (c2 == -1) {
+      }  else if c2 == -1 {
         return 0
       } else {
         return -1
diff --git a/vlib/os/os.v b/vlib/os/os.v
index 31fef7e5c1..74d1e0b5dd 100644
--- a/vlib/os/os.v
+++ b/vlib/os/os.v
@@ -138,7 +138,7 @@ pub fn read_lines(path string) []string {
 
 	mut buf_index := 0
 	for C.fgets(buf + buf_index, buf_len - buf_index, fp) != 0 {
-		len := C.strlen(buf)
+		len := vstrlen(buf)
 		if len == buf_len - 1 && buf[len - 1] != 10 {
 			buf_len *= 2
 			buf = C.realloc(buf, buf_len)
@@ -307,7 +307,7 @@ pub fn exec(cmd string) ?Result {
 	buf := [1000]byte
 	mut res := ''
 	for C.fgets(buf, 1000, f) != 0 {
-		res += tos(buf, strlen(buf))
+		res += tos(buf, vstrlen(buf))
 	}
 	res = res.trim_space()
 	exit_code := pclose(f)
@@ -709,7 +709,7 @@ pub fn realpath(fpath string) string {
 		res = int( C.realpath( fpath.str, fullpath ) )
 	}
 	if res != 0 {
-		return string(fullpath, strlen(fullpath))
+		return string(fullpath, vstrlen(fullpath))
 	}
 	return fpath
 }
diff --git a/vlib/os/os_nix.v b/vlib/os/os_nix.v
index 8077ce849a..f06c9b7733 100644
--- a/vlib/os/os_nix.v
+++ b/vlib/os/os_nix.v
@@ -22,7 +22,7 @@ pub fn get_error_msg(code int) string {
 	if _ptr_text == 0 {
 		return ''
 	}
-	return tos(_ptr_text, C.strlen(_ptr_text))
+	return tos(_ptr_text, vstrlen(_ptr_text))
 }
 
 pub fn ls(path string) []string {
diff --git a/vlib/os/os_win.v b/vlib/os/os_win.v
index b41fba8870..2ec909f78d 100644
--- a/vlib/os/os_win.v
+++ b/vlib/os/os_win.v
@@ -186,5 +186,5 @@ pub fn get_error_msg(code int) string {
     if _ptr_text == 0 { // compare with null
         return ''
     }
-    return tos(_ptr_text, C.strlen(_ptr_text))
+    return tos(_ptr_text, vstrlen(_ptr_text))
 }