+# if defined(OPENSSL_RAND_SEED_LIBRANDOM)
+# error "librandom not (yet) supported"
+# endif
+
+/*
+ * Try the various seeding methods in turn, exit when successful.
+ *
+ * TODO(DRBG): If more than one entropy source is available, is it
+ * preferable to stop as soon as enough entropy has been collected
+ * (as favored by @rsalz) or should one rather be defensive and add
+ * more entropy than requested and/or from different sources?
+ *
+ * Currently, the user can select multiple entropy sources in the
+ * configure step, yet in practice only the first available source
+ * will be used. A more flexible solution has been requested, but
+ * currently it is not clear how this can be achieved without
+ * overengineering the problem. There are many parameters which
+ * could be taken into account when selecting the order and amount
+ * of input from the different entropy sources (trust, quality,
+ * possibility of blocking).
+ */
+size_t rand_pool_acquire_entropy(RAND_POOL *pool)
+{
+# ifdef OPENSSL_RAND_SEED_NONE
+ return rand_pool_entropy_available(pool);
+# else
+ size_t bytes_needed;
+ size_t entropy_available = 0;
+ unsigned char *buffer;
+
+# ifdef OPENSSL_RAND_SEED_GETRANDOM
+ bytes_needed = rand_pool_bytes_needed(pool, 8 /*entropy_per_byte*/);
+ buffer = rand_pool_add_begin(pool, bytes_needed);
+ if (buffer != NULL) {
+ size_t bytes = 0;
+
+ if (getrandom(buffer, bytes_needed, 0) == (int)bytes_needed)
+ bytes = bytes_needed;
+
+ rand_pool_add_end(pool, bytes, 8 * bytes);
+ entropy_available = rand_pool_entropy_available(pool);
+ }
+ if (entropy_available > 0)
+ return entropy_available;