diff --git a/libc/private/bionic_macros.h b/libc/private/bionic_macros.h
index a3a3ece..979a704 100644
--- a/libc/private/bionic_macros.h
+++ b/libc/private/bionic_macros.h
@@ -78,4 +78,20 @@
 #define BIONIC_STOP_UNWIND asm volatile(".cfi_undefined $ra")
 #endif
 
+// The arraysize(arr) macro returns the # of elements in an array arr.
+// The expression is a compile-time constant, and therefore can be
+// used in defining new arrays, for example.  If you use arraysize on
+// a pointer by mistake, you will get a compile-time error.
+//
+// One caveat is that arraysize() doesn't accept any array of an
+// anonymous type or a type defined inside a function.
+//
+// This template function declaration is used in defining arraysize.
+// Note that the function doesn't need an implementation, as we only
+// use its type.
+template <typename T, size_t N>
+char (&ArraySizeHelper(T (&array)[N]))[N];  // NOLINT(readability/casting)
+
+#define arraysize(array) (sizeof(ArraySizeHelper(array)))
+
 #endif // _BIONIC_MACROS_H_