diff --git a/libc/bionic/libgen.cpp b/libc/bionic/libgen.cpp
index d8df494..b98f504 100644
--- a/libc/bionic/libgen.cpp
+++ b/libc/bionic/libgen.cpp
@@ -36,22 +36,7 @@
 
 #include "private/ThreadLocalBuffer.h"
 
-GLOBAL_INIT_THREAD_LOCAL_BUFFER(basename);
-GLOBAL_INIT_THREAD_LOCAL_BUFFER(dirname);
-
-char* basename(const char* path) {
-  LOCAL_INIT_THREAD_LOCAL_BUFFER(char*, basename, MAXPATHLEN);
-  int rc = basename_r(path, basename_tls_buffer, basename_tls_buffer_size);
-  return (rc < 0) ? NULL : basename_tls_buffer;
-}
-
-char* dirname(const char* path) {
-  LOCAL_INIT_THREAD_LOCAL_BUFFER(char*, dirname, MAXPATHLEN);
-  int rc = dirname_r(path, dirname_tls_buffer, dirname_tls_buffer_size);
-  return (rc < 0) ? NULL : dirname_tls_buffer;
-}
-
-int basename_r(const char* path, char* buffer, size_t buffer_size) {
+__LIBC64_HIDDEN__ int basename_r(const char* path, char* buffer, size_t buffer_size) {
   const char* startp = NULL;
   const char* endp = NULL;
   int len;
@@ -103,7 +88,7 @@
   return result;
 }
 
-int dirname_r(const char* path, char* buffer, size_t buffer_size) {
+__LIBC64_HIDDEN__ int dirname_r(const char* path, char* buffer, size_t buffer_size) {
   const char* endp = NULL;
   int len;
   int result;
@@ -161,3 +146,18 @@
   }
   return result;
 }
+
+GLOBAL_INIT_THREAD_LOCAL_BUFFER(basename);
+GLOBAL_INIT_THREAD_LOCAL_BUFFER(dirname);
+
+char* basename(const char* path) {
+  LOCAL_INIT_THREAD_LOCAL_BUFFER(char*, basename, MAXPATHLEN);
+  int rc = basename_r(path, basename_tls_buffer, basename_tls_buffer_size);
+  return (rc < 0) ? NULL : basename_tls_buffer;
+}
+
+char* dirname(const char* path) {
+  LOCAL_INIT_THREAD_LOCAL_BUFFER(char*, dirname, MAXPATHLEN);
+  int rc = dirname_r(path, dirname_tls_buffer, dirname_tls_buffer_size);
+  return (rc < 0) ? NULL : dirname_tls_buffer;
+}
diff --git a/libc/include/libgen.h b/libc/include/libgen.h
index 4caf8b9..e89328e 100644
--- a/libc/include/libgen.h
+++ b/libc/include/libgen.h
@@ -38,29 +38,21 @@
 /*
  * <string.h> gets you the GNU basename.
  * <libgen.h> the POSIX one.
- * Note that our "POSIX" one has the wrong argument cv-qualifiers.
+ * Note that our "POSIX" one has the wrong argument cv-qualifiers, but doesn't
+ * modify its input and uses thread-local storage for the result if necessary.
  */
 extern char* basename(const char*);
 #define __bionic_using_posix_basename
 #endif
 
-/* our version of dirname/basename don't modify the input path */
-extern char*  dirname (const char*  path);
+/* This has the wrong argument cv-qualifiers, but doesn't modify its input and uses thread-local storage for the result if necessary. */
+extern char* dirname(const char*);
 
-/* special thread-safe Bionic versions
- *
- * if 'buffer' is NULL, 'bufflen' is ignored and the length of the result is returned
- * otherwise, place result in 'buffer'
- *
- * at most bufflen-1 characters written, plus a terminating zero
- *
- * return length of result, or -1 in case of error, with errno set to:
- *
- *    ERANGE:        buffer is too short
- *    ENAMETOOLONG:  the result is too long for a valid path
- */
-extern int    dirname_r(const char*  path, char*  buffer, size_t  bufflen);
-extern int    basename_r(const char*  path, char*  buffer, size_t  bufflen);
+#if !defined(__LP64__)
+/* These non-standard functions are not needed on Android; basename and dirname use thread-local storage. */
+extern int dirname_r(const char*, char*, size_t);
+extern int basename_r(const char*, char*, size_t);
+#endif
 
 __END_DECLS
 
diff --git a/tests/libgen_test.cpp b/tests/libgen_test.cpp
index d0402db..e9a5d5c 100644
--- a/tests/libgen_test.cpp
+++ b/tests/libgen_test.cpp
@@ -63,7 +63,7 @@
   TestDirname("/", "/");
 }
 
-#if defined(__BIONIC__)
+#if defined(__BIONIC__) && !defined(__LP64__)
 static void TestBasename(const char* in, const char* expected_out, int expected_rc,
                          char* buf, size_t buf_size, int expected_errno) {
   errno = 0;
@@ -88,7 +88,7 @@
 #endif // __BIONIC__
 
 TEST(libgen, basename_r) {
-#if defined(__BIONIC__)
+#if defined(__BIONIC__) && !defined(__LP64__)
   char buf[256];
   TestBasename("", ".",  1, NULL, 0, 0);
   TestBasename("", ".", -1, buf, 0, ERANGE);
@@ -107,7 +107,7 @@
 }
 
 TEST(libgen, dirname_r) {
-#if defined(__BIONIC__)
+#if defined(__BIONIC__) && !defined(__LP64__)
   char buf[256];
   TestDirname("", ".",  1, NULL, 0, 0);
   TestDirname("", ".", -1, buf, 0, ERANGE);