diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index 4acf38d..36dabe9 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -19,6 +19,11 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 
+/* Enable definition of fileno() even when compiling with -std=c99. Must be
+ * set before config.h, which pulls in glibc's features.h indirectly.
+ * Harmless on other platforms. */
+#define _POSIX_C_SOURCE 1
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
 #else