diff --git a/libc/include/pthread.h b/libc/include/pthread.h
index b4a48f0..3262bd1 100644
--- a/libc/include/pthread.h
+++ b/libc/include/pthread.h
@@ -133,7 +133,7 @@
 
 __BEGIN_DECLS
 
-int pthread_atfork(void (*)(void), void (*)(void), void (*)(void)) __INTRODUCED_IN(21);
+int pthread_atfork(void (*)(void), void (*)(void), void (*)(void)) __INTRODUCED_IN(12);
 
 int pthread_attr_destroy(pthread_attr_t* _Nonnull);
 int pthread_attr_getdetachstate(const pthread_attr_t* _Nonnull, int* _Nonnull);