diff --git a/src/api/err.c b/src/api/err.c
index 30348a91..1593e70a 100644
--- a/src/api/err.c
+++ b/src/api/err.c
@@ -42,6 +42,7 @@
 
 #include "lwip/errno.h"
 
+#if !NO_SYS
 /** Table to quickly map an lwIP error (err_t) to a socket error
   * by using -err as an index */
 static const int err_to_errno_table[] = {
@@ -63,6 +64,7 @@ static const int err_to_errno_table[] = {
   ENOTCONN,      /* ERR_CLSD       -15     Connection closed.       */
   EIO            /* ERR_ARG        -16     Illegal argument.        */
 };
+#endif /* !NO_SYS */
 
 #ifdef LWIP_DEBUG
 
@@ -103,6 +105,7 @@ lwip_strerr(err_t err)
 
 #endif /* LWIP_DEBUG */
 
+#if !NO_SYS
 int
 err_to_errno(err_t err)
 {
@@ -111,3 +114,4 @@ err_to_errno(err_t err)
   }
   return err_to_errno_table[-err];
 }
+#endif /* !NO_SYS */
diff --git a/src/include/lwip/err.h b/src/include/lwip/err.h
index 136e6b2c..84e528d1 100644
--- a/src/include/lwip/err.h
+++ b/src/include/lwip/err.h
@@ -108,7 +108,9 @@ extern const char *lwip_strerr(err_t err);
 #define lwip_strerr(x) ""
 #endif /* LWIP_DEBUG */
 
+#if !NO_SYS
 int err_to_errno(err_t err);
+#endif /* !NO_SYS */
 
 #ifdef __cplusplus
 }
diff --git a/src/include/lwip/errno.h b/src/include/lwip/errno.h
index b6a897b7..e2d996c6 100644
--- a/src/include/lwip/errno.h
+++ b/src/include/lwip/errno.h
@@ -173,6 +173,13 @@
 extern int errno;
 #endif
 
+#else /* LWIP_PROVIDE_ERRNO */
+
+/* Define LWIP_ERRNO_INCLUDE to <errno.h> to include the error defines here */
+#ifdef LWIP_ERRNO_INCLUDE
+#include LWIP_ERRNO_INCLUDE
+#endif /* LWIP_ERRNO_INCLUDE */
+
 #endif /* LWIP_PROVIDE_ERRNO */
 
 #ifdef __cplusplus