diff --git a/src/libstrongswan/plugins/sshkey/sshkey_builder.c b/src/libstrongswan/plugins/sshkey/sshkey_builder.c
index 569b0b738..652663108 100644
--- a/src/libstrongswan/plugins/sshkey/sshkey_builder.c
+++ b/src/libstrongswan/plugins/sshkey/sshkey_builder.c
@@ -13,6 +13,7 @@
  * for more details.
  */
 
+#define _GNU_SOURCE /* for fmemopen() */
 #include <unistd.h>
 #include <stdio.h>
 #include <errno.h>