diff --git a/make/source.mk b/make/source.mk
index 36f16c3b51..c766d6e518 100644
--- a/make/source.mk
+++ b/make/source.mk
@@ -13,6 +13,7 @@ COMMON_SRC = \
             common/maths.c \
             common/printf.c \
             common/streambuf.c \
+            common/string_light.c \
             common/time.c \
             common/typeconversion.c \
             config/config_eeprom.c \
diff --git a/src/main/common/string_light.c b/src/main/common/string_light.c
index 1546f733c4..b60e3cdb6e 100755
--- a/src/main/common/string_light.c
+++ b/src/main/common/string_light.c
@@ -16,46 +16,52 @@
  */
 
 #include <limits.h>
+#include <ctype.h>
+#include <string.h>
 
-#include "string_light.h"
 #include "typeconversion.h"
 
-int sl_isalnum(int c)
+int isalnum(int c)
 {
-    return sl_isdigit(c) || sl_isupper(c) || sl_islower(c);
+    return isdigit(c) || isupper(c) || islower(c);
 }
 
-int sl_isdigit(int c)
+int isdigit(int c)
 {
     return (c >= '0' && c <= '9');
 }
 
-int sl_isupper(int c)
+int isupper(int c)
 {
     return (c >= 'A' && c <= 'Z');
 }
 
-int sl_islower(int c)
+int islower(int c)
 {
     return (c >= 'a' && c <= 'z');
 }
 
-int sl_tolower(int c)
+int isspace(int c)
 {
-    return sl_isupper(c) ? (c) - 'A' + 'a' : c;
+    return (c == ' ' || c == '\t' || c == '\n' || c == '\r' || c == '\f' || c == '\v');
 }
 
-int sl_toupper(int c)
+int tolower(int c)
 {
-    return sl_islower(c) ? (c) - 'a' + 'A' : c;
+    return isupper(c) ? (c) - 'A' + 'a' : c;
 }
 
-int sl_strcasecmp(const char * s1, const char * s2)
+int toupper(int c)
 {
-    return sl_strncasecmp(s1, s2, INT_MAX);
+    return islower(c) ? (c) - 'a' + 'A' : c;
 }
 
-int sl_strncasecmp(const char * s1, const char * s2, int n)
+int strcasecmp(const char * s1, const char * s2)
+{
+    return strncasecmp(s1, s2, (size_t)-1);
+}
+
+int strncasecmp(const char * s1, const char * s2, size_t n)
 {
     const unsigned char * ucs1 = (const unsigned char *) s1;
     const unsigned char * ucs2 = (const unsigned char *) s2;
@@ -63,8 +69,8 @@ int sl_strncasecmp(const char * s1, const char * s2, int n)
     int d = 0;
 
     for ( ; n != 0; n--) {
-        const int c1 = sl_tolower(*ucs1++);
-        const int c2 = sl_tolower(*ucs2++);
+        const int c1 = tolower(*ucs1++);
+        const int c2 = tolower(*ucs2++);
         if (((d = c1 - c2) != 0) || (c2 == '\0')) {
             break;
         }
diff --git a/src/main/common/string_light.h b/src/main/common/string_light.h
deleted file mode 100755
index 005a6c6b12..0000000000
--- a/src/main/common/string_light.h
+++ /dev/null
@@ -1,28 +0,0 @@
-/*
- * This file is part of Cleanflight.
- *
- * Cleanflight is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * Cleanflight is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#pragma once
-
-int sl_isalnum(int c);
-int sl_isdigit(int c);
-int sl_isupper(int c);
-int sl_islower(int c);
-int sl_tolower(int c);
-int sl_toupper(int c);
-
-int sl_strcasecmp(const char * s1, const char * s2);
-int sl_strncasecmp(const char * s1, const char * s2, int n);
diff --git a/src/main/ctype.h b/src/main/ctype.h
new file mode 100644
index 0000000000..a2dde62e3d
--- /dev/null
+++ b/src/main/ctype.h
@@ -0,0 +1,33 @@
+/*
+ * Replacemnet for system header file <ctype.h> to avoid macro definitions
+ * Functions are implemented in common/string_light.c
+ */
+
+#ifdef __cplusplus
+// use original implementation for C++
+# include_next <ctype.h>
+#endif
+
+#ifndef _CTYPE_H_
+#define _CTYPE_H_
+
+#ifndef _EXFUN
+# define _EXFUN(name,proto) name proto
+#endif
+
+int _EXFUN(isalnum, (int __c));
+int _EXFUN(isalpha, (int __c));
+int _EXFUN(iscntrl, (int __c));
+int _EXFUN(isdigit, (int __c));
+int _EXFUN(isgraph, (int __c));
+int _EXFUN(islower, (int __c));
+int _EXFUN(isprint, (int __c));
+int _EXFUN(ispunct, (int __c));
+int _EXFUN(isspace, (int __c));
+int _EXFUN(isupper, (int __c));
+int _EXFUN(isxdigit,(int __c));
+int _EXFUN(tolower, (int __c));
+int _EXFUN(toupper, (int __c));
+int _EXFUN(isblank, (int __c));
+
+#endif /* _CTYPE_H_ */