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diff --git a/utils/async_netdb.h b/utils/async_netdb.h
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+/* vi: set tw=78: */
+
+/* async_netdb.h, Copyright (c) Dave Odell <dmo2118@gmail.com>
+ *
+ * Permission to use, copy, modify, distribute, and sell this software and its
+ * documentation for any purpose is hereby granted without fee, provided that
+ * the above copyright notice appear in all copies and that both that
+ * copyright notice and this permission notice appear in supporting
+ * documentation.  No representations are made about the suitability of this
+ * software for any purpose.  It is provided "as is" without express or
+ * implied warranty.
+ */
+
+#ifndef ASYNC_NETDB_H
+#define ASYNC_NETDB_H
+
+#include "thread_util.h"
+
+#include <netdb.h>
+#include <netinet/in.h>
+#include <sys/socket.h>
+#include <unistd.h>
+
+/*
+   Both async_name_from_addr_* and async_addr_from_name_* follow basic pattern
+   for io_thread clients as described in thread_util.h:
+   1. async_*_from_*_start
+   2. async_*_from_*_is_done (repeat as necessary)
+   3a. async_*_from_*_finish to retrieve the results, or:
+   3b. async_*_from_*_cancel to abort the lookup.
+
+   On systems that can't do asynchronous host lookups, the *_finish functions
+   do the actual lookup.
+ */
+
+#ifndef NI_MAXHOST
+/*
+   From the glibc man page for getnameinfo(3):
+      Since glibc 2.8, these definitions are exposed only if one of the
+      feature test macros _BSD_SOURCE, _SVID_SOURCE, or _GNU_SOURCE is
+      defined.
+ */
+# define NI_MAXHOST 1025
+#endif
+
+#if HAVE_PTHREAD && HAVE_GETADDRINFO
+/*
+   If threads or getaddrinfo() are unavailable, then the older gethostbyname()
+   and gethostbyaddr() functions are used, and IPv6 is disabled.
+ */
+# define ASYNC_NETDB_USE_GAI 1
+#endif
+
+#if ASYNC_NETDB_USE_GAI
+
+/* Without using union, gcc-6 warns for
+   breaking strict aliasing rules
+ */
+typedef union {
+	struct sockaddr_storage x_sockaddr_storage;
+	struct sockaddr_in x_sockaddr_in;
+	struct sockaddr_in6 x_sockaddr_in6;
+} async_netdb_sockaddr_storage_t;
+
+int _async_netdb_is_done (struct io_thread *io);
+
+#else
+
+/* Because the definition for the above case is now union,
+   the definition for this case must also be union...
+*/
+typedef union {
+	struct sockaddr_in x_sockaddr_in;
+} async_netdb_sockaddr_storage_t;
+
+# ifndef EAI_SYSTEM
+/* The EAI_* codes are specified specifically as preprocessor macros, so
+   the #ifdef here should always work...
+   http://pubs.opengroup.org/onlinepubs/009604499/basedefs/netdb.h.html */
+
+#   define ASYNC_NETDB_FAKE_EAI 1
+
+/* Even without addrinfo, the EAI_* error codes are used. The numbers are from
+   Linux's netdb.h. */
+#   define EAI_NONAME -2
+#   define EAI_AGAIN  -3
+#   define EAI_FAIL   -4
+#   define EAI_MEMORY -10
+#   define EAI_SYSTEM -11
+
+const char *_async_netdb_strerror (int errcode);
+
+#   define gai_strerror(errcode) _async_netdb_strerror (errcode)
+# endif
+
+# define _async_netdb_is_done(io) 1
+
+#endif
+
+/* In non-threaded mode, _async_name_from_addr_param is used in place of
+   async_name_from_addr. */
+struct _async_name_from_addr_param
+{
+  socklen_t addrlen;
+  async_netdb_sockaddr_storage_t addr;
+};
+
+typedef struct async_name_from_addr
+{
+  /*
+     Stupid memory trick, thwarted: The host string could be at the beginning
+     of this structure, and the memory block that contains this struct could
+     be resized and returned directly in async_name_from_addr_finish. But...
+
+     There is no aligned_realloc. In fact, aligned_realloc is a bit of a
+     problem, mostly because:
+     1. realloc() is the only way to resize a heap-allocated memory block.
+     2. realloc() moves memory.
+     3. The location that realloc() moves memory to won't be aligned.
+   */
+
+  struct _async_name_from_addr_param param;
+  struct io_thread io;
+
+  char host[NI_MAXHOST];
+  int gai_error;
+  int errno_error;
+
+} *async_name_from_addr_t;
+
+async_name_from_addr_t async_name_from_addr_start (Display *dpy,
+                                                   const struct sockaddr *addr,
+                                                   socklen_t addrlen);
+/*
+   Starts an asynchronous name-from-address lookup.
+   dpy:     An X11 Display with a .useThreads resource.
+   addr:    An address. Like various socket functions (e.g. bind(2),
+            connect(2), sendto(2)), this isn't really a struct sockaddr so
+            much as a "subtype" of sockaddr, like sockaddr_in, or
+            sockaddr_in6, or whatever.
+   addrlen: The (actual) length of *addr.
+   Returns NULL if the request couldn't be created (due to low memory).
+ */
+
+#define async_name_from_addr_is_done(self) _async_netdb_is_done (&(self)->io)
+
+#if ASYNC_NETDB_USE_GAI
+void async_name_from_addr_cancel (async_name_from_addr_t self);
+#else
+# define async_name_from_addr_cancel(self) (free (self))
+#endif
+
+int async_name_from_addr_finish (async_name_from_addr_t self,
+                                 char **host, int *errno_error);
+/*
+   Gets the result of an asynchronous name-from-address lookup. If the lookup
+   operation is still in progress, or if the system can't do async lookups,
+   this will block. This cleans up the lookup operation; do not use 'self'
+   after calling this function.
+   self:        The lookup operation.
+   host:        If no error, the name of the host. Free this with free(3).
+   errno_error: The value of errno if EAI_SYSTEM is returned. Can be NULL.
+   Returns 0 on success, otherwise an error from getnameinfo(3).
+ */
+
+/* In non-threaded mode, async_addr_from_name contains different stuff. */
+typedef struct async_addr_from_name
+{
+#if ASYNC_NETDB_USE_GAI
+  struct io_thread io;
+
+  int gai_error;
+  int errno_error;
+
+  struct addrinfo *res;
+#else
+  char dont_complain_about_empty_structs;
+#endif
+} *async_addr_from_name_t;
+
+async_addr_from_name_t async_addr_from_name_start (Display *dpy,
+                                                   const char *host);
+/*
+   Starts an asynchronous address-from-name lookup.
+   dpy:  An X11 display.
+   host: The hostname to look up.
+   Returns NULL if the request couldn't be created (due to low memory).
+ */
+
+#define async_addr_from_name_is_done(self) _async_netdb_is_done (&(self)->io)
+
+#if ASYNC_NETDB_USE_GAI
+void async_addr_from_name_cancel (async_addr_from_name_t self);
+#else
+# define async_addr_from_name_cancel(self) (thread_free (self));
+#endif
+
+/* sockaddr must be sizeof(async_netdb_sockaddr_storage_t) in size. */
+int async_addr_from_name_finish (async_addr_from_name_t self, void *addr,
+                                 socklen_t *addrlen, int *errno_error);
+/*
+   Returns the address from an asynchronous address-from-name operation. If
+   the lookup is still in progress, or the system can't do an asynchronous
+   lookup, this blocks. This cleans up the lookup operation; do not use 'self'
+   after calling this function.
+   self:        The lookup operation.
+   addr:        A sockaddr. This must be as large as or larger than
+                sizeof(async_netdb_sockaddr_storage_t). (Hint: just use an
+                instance of async_netdb_sockaddr_storage_t itself here.)
+   addrlen:     The length of the obtained sockaddr.
+   errno_error: The value of errno if EAI_SYSTEM is returned. Can be NULL.
+   Returns 0 on success, or an error from getaddrinfo(3).
+ */
+
+#endif
+
+/* Local Variables:      */
+/* mode: c               */
+/* fill-column: 78       */
+/* c-file-style: "gnu"   */
+/* c-basic-offset: 2     */
+/* indent-tabs-mode: nil */
+/* End:                  */