diff --git a/programs/aes/aescrypt2.c b/programs/aes/aescrypt2.c
index f17c641..b0c1fea 100644
--- a/programs/aes/aescrypt2.c
+++ b/programs/aes/aescrypt2.c
@@ -22,7 +22,7 @@
 /* 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
+#define _POSIX_C_SOURCE 200112L
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"