diff --git a/tests/suites/main_test.function b/tests/suites/main_test.function
index a1ba610..db1c88a 100644
--- a/tests/suites/main_test.function
+++ b/tests/suites/main_test.function
@@ -19,6 +19,12 @@
  *  This file is part of Mbed TLS (https://tls.mbed.org)
  */
 
+#if defined(__unix__) || (defined(__APPLE__) && defined(__MACH__))
+#if !defined(_POSIX_C_SOURCE)
+#define _POSIX_C_SOURCE 1 // for fileno() from <stdio.h>
+#endif
+#endif
+
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include <mbedtls/config.h>
 #else