Index: test/asan/lit.cfg
===================================================================
--- test/asan/lit.cfg
+++ test/asan/lit.cfg
@@ -77,6 +77,9 @@
 asan_dynamic_flags = []
 if config.asan_dynamic:
   asan_dynamic_flags = ["-shared-libasan"]
+  # On FreeBSD, we need to add -pthread to ensure pthread functions are available.
+  if platform.system() == 'FreeBSD':
+    asan_dynamic_flags += ['-pthread']
   # On Windows, we need to simulate "clang-cl /MD" on the clang driver side.
   if platform.system() == 'Windows':
     asan_dynamic_flags += ["-D_MT", "-D_DLL", "-Wl,-nodefaultlib:libcmt,-defaultlib:msvcrt,-defaultlib:oldnames"]