-extern bool digest_create(digest_t *, const void *indata, size_t inlen, void *outdata);
-extern bool digest_verify(digest_t *, const void *indata, size_t inlen, const void *digestdata);
-extern bool digest_set_key(digest_t *, const void *key, size_t len);
+extern bool digest_create(digest_t *, const void *indata, size_t inlen, void *outdata) __attribute__ ((__warn_unused_result__));
+extern bool digest_verify(digest_t *, const void *indata, size_t inlen, const void *digestdata) __attribute__ ((__warn_unused_result__));
+extern bool digest_set_key(digest_t *, const void *key, size_t len) __attribute__ ((__warn_unused_result__));