diff --git a/crypto.h b/crypto.h
index 6f7e79e..f434618 100644
--- a/crypto.h
+++ b/crypto.h
@@ -337,14 +337,14 @@
 				 unsigned char *buf);
 
 
-dtls_handshake_parameters_t *dtls_handshake_new();
+dtls_handshake_parameters_t *dtls_handshake_new(void);
 
 void dtls_handshake_free(dtls_handshake_parameters_t *handshake);
 
-dtls_security_parameters_t *dtls_security_new();
+dtls_security_parameters_t *dtls_security_new(void);
 
 void dtls_security_free(dtls_security_parameters_t *security);
-void crypto_init();
+void crypto_init(void);
 
 #endif /* _DTLS_CRYPTO_H_ */
 
diff --git a/hmac.h b/hmac.h
index d5e72a8..81a058d 100644
--- a/hmac.h
+++ b/hmac.h
@@ -49,10 +49,10 @@
 #endif /* WITH_SHA256 */
 
 #ifndef WITH_CONTIKI
-static inline void dtls_hmac_storage_init()
+static inline void dtls_hmac_storage_init(void)
 { }
 #else
-void dtls_hmac_storage_init();
+void dtls_hmac_storage_init(void);
 #endif
 
 /**
diff --git a/peer.h b/peer.h
index 49dac5a..30a8bf4 100644
--- a/peer.h
+++ b/peer.h
@@ -106,7 +106,7 @@
   peer->security_params[0] = security;
 }
 
-void peer_init();
+void peer_init(void);
 
 /**
  * Creates a new peer for given @p session. The current configuration