--- orig/smtp.c 2018-11-16 12:25:40.294880870 +0100 +++ patched/smtp.c 2018-11-16 12:26:05.291016726 +0100 @@ -53,7 +53,7 @@ * - test with more mail servers... * */ - +#define _POSIX_C_SOURCE 200809L #include "lwip/apps/smtp.h" #if LWIP_TCP && LWIP_CALLBACK_API