diff --git a/expected/wasm32-wasi/predefined-macros.txt b/expected/wasm32-wasi/predefined-macros.txt
index ef6de5c..efe4195 100644
--- a/expected/wasm32-wasi/predefined-macros.txt
+++ b/expected/wasm32-wasi/predefined-macros.txt
@@ -1628,6 +1628,7 @@
 #define O_SEARCH (0x08000000)
 #define O_SYNC __WASI_FDFLAG_SYNC
 #define O_TRUNC (__WASI_O_TRUNC << 12)
+#define O_TTY_INIT (0)
 #define O_WRONLY (0x10000000)
 #define PACKETSZ NS_PACKETSZ
 #define PACKET_ADD_MEMBERSHIP 1
diff --git a/libc-bottom-half/headers/public/__header_fcntl.h b/libc-bottom-half/headers/public/__header_fcntl.h
index af720ea..c5d5a0c 100644
--- a/libc-bottom-half/headers/public/__header_fcntl.h
+++ b/libc-bottom-half/headers/public/__header_fcntl.h
@@ -20,7 +20,17 @@
 #define O_SEARCH   (0x08000000)
 #define O_WRONLY   (0x10000000)
 
+/*
+ * O_CLOEXEC is defined to be zero, as WASI has no exec-style functions.
+ */
 #define O_CLOEXEC  (0)
+
+/*
+ * O_TTY_INIT is defined to be zero, meaning that WASI implementations are
+ * expected to always initialize a terminal the first time it's opened.
+ */
+#define O_TTY_INIT (0)
+
 #define O_NOCTTY   (0)
 
 #define O_RDWR (O_RDONLY | O_WRONLY)