diff --git a/programs/ssl/ssl_mail_client.c b/programs/ssl/ssl_mail_client.c
index 08ff025956..b7458cd212 100644
--- a/programs/ssl/ssl_mail_client.c
+++ b/programs/ssl/ssl_mail_client.c
@@ -23,6 +23,7 @@
  * be set before config.h, which pulls in glibc's features.h indirectly.
  * Harmless on other platforms. */
 #define _POSIX_C_SOURCE 200112L
+#define _XOPEN_SOURCE 600
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"