Index: uspace/lib/c/include/string.h
===================================================================
--- uspace/lib/c/include/string.h	(revision 3b3fcf369f7972634c32240f6c2c7e342deadefe)
+++ uspace/lib/c/include/string.h	(revision 68f11ddca33887bba2a17f8d05bf3a01b01a01a5)
@@ -36,5 +36,6 @@
 #define LIBC_STRING_H_
 
-#if defined(_HELENOS_SOURCE) && !defined(_REALLY_WANT_STRING_H)
+#if defined(_HELENOS_SOURCE) && !defined(_REALLY_WANT_STRING_H) && \
+    !defined(_LIBC_SOURCE)
 #error Please use str.h and mem.h instead
 #endif
@@ -63,4 +64,10 @@
 extern size_t strlen(const char *);
 
+#if defined(_XOPEN_SOURCE) || defined(_GNU_SOURCE) || defined(_LIBC_SOURCE)
+extern size_t strnlen(const char *, size_t);
+extern char *strdup(const char *);
+extern char *strndup(const char *, size_t);
+#endif
+
 #endif