diff --git a/libc/bionic/__set_errno.cpp b/libc/bionic/__set_errno.cpp
new file mode 100644
index 0000000..5b249c8
--- /dev/null
+++ b/libc/bionic/__set_errno.cpp
@@ -0,0 +1,52 @@
+/*
+ * Copyright (C) 2008 The Android Open Source Project
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *  * Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
+ * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <errno.h>
+
+// These functions are called from our assembler syscall stubs.
+// C/C++ code should just assign 'errno' instead.
+
+// TODO: should be __LIBC_HIDDEN__, but already exported by NDK :-(
+// TODO: this isn't used on ARM.
+extern "C" int __set_errno(int n) {
+  errno = n;
+  return -1;
+}
+
+// TODO: this is only used on ARM, but is exported by NDK on all platforms :-(
+extern "C" __LIBC_HIDDEN__ int __set_syscall_errno(int n) {
+  // Some syscalls, mmap() for example, have valid return
+  // values that are "negative".  Since errno values are not
+  // greater than 131 on Linux, we will just consider
+  // anything significantly out of range as not-an-error.
+  if(n > -256) {
+    return __set_errno(-n);
+  } else {
+    return n;
+  }
+}