diff --git a/libc/bionic/pthread_attr.cpp b/libc/bionic/pthread_attr.cpp
index fdf2965..d597c7e 100644
--- a/libc/bionic/pthread_attr.cpp
+++ b/libc/bionic/pthread_attr.cpp
@@ -94,18 +94,21 @@
   return 0;
 }
 
-int pthread_attr_setstackaddr(pthread_attr_t*, void*) {
+#if !defined(__LP64__)
+// TODO: this exists only for backward binary compatibility on 32 bit platforms.
+extern "C" int pthread_attr_setstackaddr(pthread_attr_t*, void*) {
   // This was removed from POSIX.1-2008, and is not implemented on bionic.
   // Needed for ABI compatibility with the NDK.
   return ENOSYS;
 }
 
-int pthread_attr_getstackaddr(const pthread_attr_t* attr, void** stack_addr) {
+extern "C" int pthread_attr_getstackaddr(const pthread_attr_t* attr, void** stack_addr) {
   // This was removed from POSIX.1-2008.
   // Needed for ABI compatibility with the NDK.
   *stack_addr = (char*)attr->stack_base + attr->stack_size;
   return 0;
 }
+#endif // !defined(__LP64__)
 
 int pthread_attr_setstack(pthread_attr_t* attr, void* stack_base, size_t stack_size) {
   if ((stack_size & (PAGE_SIZE - 1) || stack_size < PTHREAD_STACK_MIN)) {