--- a/build_defs.bzl
+++ b/build_defs.bzl
@@ -188,6 +188,12 @@ def xnnpack_std_c_defines():
     We need this for things like `struct timespec` that are not in c99, but
     are part of POSIX 1003.1b-1993."""
 
-    return ["_DARWIN_C_SOURCE=1", "_POSIX_C_SOURCE=199309L"]
+    # On FreeBSD the default visibility already exposes all needed symbols;
+    # setting _POSIX_C_SOURCE hides __BSD_VISIBLE and breaks MAP_ANONYMOUS,
+    # lrintf, FP_NAN and other C99/BSD extensions.
+    return ["_DARWIN_C_SOURCE=1"] + select({
+        "@platforms//os:freebsd": [],
+        "//conditions:default": ["_POSIX_C_SOURCE=199309L"],
+    })
 
 def xnnpack_test_deps_for_library():