diff --git a/README.markdown b/README.markdown
index 8b12369a..f6426303 100644
--- a/README.markdown
+++ b/README.markdown
@@ -77,6 +77,9 @@ Before doing anything else with the library, call:
 
     sodium_init();
 
+This function is not thread-safe. No other Sodium functions should be
+called until it successfully returns.
+
 And if you need to release memory and other resources possibly
 allocated by the library, call:
 
diff --git a/src/libsodium/sodium/core.c b/src/libsodium/sodium/core.c
index f354f1fa..84fad58e 100644
--- a/src/libsodium/sodium/core.c
+++ b/src/libsodium/sodium/core.c
@@ -2,14 +2,15 @@
 #include "core.h"
 #include "crypto_onetimeauth.h"
 
-static char initialized;
+static _Bool initialized;
 
 int
 sodium_init(void)
 {
-    if (__sync_lock_test_and_set(&initialized, 1) != 0) {
+    if (initialized != 0) {
         return 1;
     }
+    initialized = 1;
     if (crypto_onetimeauth_pick_best_implementation() == NULL) {
         return -1;
     }