diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/socklen_t.h b/libbuild2-autoconf/libbuild2/autoconf/checks/socklen_t.h
new file mode 100644
index 0000000..cef454d
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/socklen_t.h
@@ -0,0 +1,31 @@
+// socklen_t : BUILD2_AUTOCONF_LIBC_VERSION
+
+#ifndef BUILD2_AUTOCONF_LIBC_VERSION
+#  error BUILD2_AUTOCONF_LIBC_VERSION appears to be conditionally included
+#endif
+
+#undef socklen_t
+
+/* Since 4.xBSD, SunOS
+ * The Single UNIX Specification, Version 2
+ *   <sys/socket.h> makes available a type, socklen_t, which is
+ *   an unsigned opaque integral type of length of at least 32 bits.
+ *   To forestall portability problems, it is recommended that
+ *   applications should not use values larger than 232 - 1.
+ */
+#if defined(__linux__)                  || \
+    defined(__FreeBSD__)                || \
+    defined(__OpenBSD__)                || \
+    defined(__NetBSD__)                 || \
+    defined(BUILD2_AUTOCONF_MACOS)      || \
+    (defined(__sun) && defined(__SVR4)) || \
+    defined(__CYGWIN__)
+#   include <sys/socket.h>
+    /* If available, we do nothing. */
+#elif defined(_WIN32)
+#   include <ws2tcpip.h>
+    /* If available, we do nothing. */
+#else
+    /* Else define it to unsigned int (suggested fallback by libevent). */
+#   define socklen_t unsigned int
+#endif