diff --git a/dos/string.h b/dos/string.h
index f648de2..a502132 100644
--- a/dos/string.h
+++ b/dos/string.h
@@ -5,12 +5,13 @@
 #ifndef _STRING_H
 #define _STRING_H
 
+#include <stddef.h>
+
 /* Standard routines */
 #define memcpy(a,b,c)	__builtin_memcpy(a,b,c)
 #define memmove(a,b,c)	__builtin_memmove(a,b,c)
 #define memset(a,b,c)	__builtin_memset(a,b,c)
 #define strcpy(a,b)	__builtin_strcpy(a,b)
-#define strlen(a)	__builtin_strlen(a)
 
 /* This only returns true or false */
 static inline int memcmp(const void *__m1, const void *__m2, unsigned int __n)
@@ -21,6 +22,13 @@ static inline int memcmp(const void *__m1, const void *__m2, unsigned int __n)
     return rv;
 }
 
+static inline size_t strlen(const char *s)
+{
+    size_t len = 0;
+    while (*s++) len++;
+    return len;
+}
+
 extern char *strchr(const char *s, int c);
 
 #endif /* _STRING_H */