diff --git a/libc/bionic/pthread_create.cpp b/libc/bionic/pthread_create.cpp
index 7e74dac..e4abee9 100644
--- a/libc/bionic/pthread_create.cpp
+++ b/libc/bionic/pthread_create.cpp
@@ -158,6 +158,7 @@
   //   thread stack (including guard page)
   stack_top -= sizeof(pthread_internal_t);
   pthread_internal_t* thread = reinterpret_cast<pthread_internal_t*>(stack_top);
+  attr->stack_size = stack_top - reinterpret_cast<uint8_t*>(attr->stack_base);
 
   // No need to check stack_top alignment. The size of pthread_internal_t is 16-bytes aligned,
   // and user allocated stack is guaranteed by pthread_attr_setstack.