diff --git a/include/commodity.h b/include/commodity.h
index e3545785..38ef0990 100644
--- a/include/commodity.h
+++ b/include/commodity.h
@@ -45,6 +45,7 @@ struct comstr {
     short ef_type;
     short com_uid;
     unsigned com_seqno;
+    unsigned com_generation;
     time_t com_timestamp;
     natid com_owner;
     /* end of part matching struct empobj */
diff --git a/include/empobj.h b/include/empobj.h
index 713f2f4a..3906c13a 100644
--- a/include/empobj.h
+++ b/include/empobj.h
@@ -58,6 +58,7 @@ struct empobj {
     short ef_type;
     short uid;
     unsigned seqno;
+    unsigned generation;
     time_t timestamp;
     /* end of part matching struct emptypedstr */
     natid own;		/* valid if EFF_OWNER is in table's flags */
diff --git a/include/file.h b/include/file.h
index 9aa6f389..0ffa5b0c 100644
--- a/include/file.h
+++ b/include/file.h
@@ -88,6 +88,7 @@ struct emptypedstr {
     short ef_type;
     short uid;
     unsigned seqno;
+    unsigned generation;
     time_t timestamp;
 };
 
@@ -203,6 +204,8 @@ enum {
 
 extern struct castr *ef_cadef(int);
 extern int ef_read(int, int, void *);
+extern void ef_make_stale(void);
+extern void ef_mark_fresh(int, void *);
 extern void *ef_ptr(int, int);
 extern char *ef_nameof(int);
 extern time_t ef_mtime(int);
diff --git a/include/game.h b/include/game.h
index 9f77dc0f..b10426fc 100644
--- a/include/game.h
+++ b/include/game.h
@@ -41,6 +41,7 @@ struct gamestr {
     short ef_type;
     short game_uid;
     unsigned game_seqno;
+    unsigned game_generation;
     time_t game_timestamp;
     /* end of part matching struct empobj */
     char game_upd_disable;	/* updates disabled? */
diff --git a/include/land.h b/include/land.h
index 6f5232ff..99f9c301 100644
--- a/include/land.h
+++ b/include/land.h
@@ -52,6 +52,7 @@ struct lndstr {
     short ef_type;
     short lnd_uid;		/* unit id (land unit) */
     unsigned lnd_seqno;
+    unsigned lnd_generation;
     time_t lnd_timestamp;	/* Last time this unit was touched */
     natid lnd_own;		/* owner's country num */
     coord lnd_x;		/* x location in abs coords */
diff --git a/include/loan.h b/include/loan.h
index f4f900f7..fbfe1568 100644
--- a/include/loan.h
+++ b/include/loan.h
@@ -45,6 +45,7 @@ struct lonstr {
     short ef_type;
     short l_uid;
     unsigned l_seqno;
+    unsigned l_generation;
     time_t l_timestamp;
     /* end of part matching struct empobj */
     natid l_loner;		/* loan shark */
diff --git a/include/lost.h b/include/lost.h
index d30f8e85..021f2633 100644
--- a/include/lost.h
+++ b/include/lost.h
@@ -42,6 +42,7 @@ struct loststr {
     short ef_type;
     short lost_uid;
     unsigned lost_seqno;
+    unsigned lost_generation;
     time_t lost_timestamp;	/* When it was lost */
     natid lost_owner;		/* Who lost it */
     /* end of part matching struct empobj */
diff --git a/include/nat.h b/include/nat.h
index 5749a1bd..e4c135f9 100644
--- a/include/nat.h
+++ b/include/nat.h
@@ -73,6 +73,7 @@ struct realmstr {
     short ef_type;
     short r_uid;		/* realm table index */
     unsigned r_seqno;
+    unsigned r_generation;
     time_t r_timestamp;		/* Last time this realm was touched */
     natid r_cnum;		/* country number */
     /* end of part matching struct empobj */
@@ -86,6 +87,7 @@ struct natstr {
     short ef_type;
     short nat_uid;		/* equals nat_cnum */
     unsigned nat_seqno;
+    unsigned nat_generation;
     time_t nat_timestamp;
     natid nat_cnum;		/* our country number */
     /* end of part matching struct empobj */
diff --git a/include/news.h b/include/news.h
index 24554f09..01a47826 100644
--- a/include/news.h
+++ b/include/news.h
@@ -49,6 +49,7 @@ struct nwsstr {
     short ef_type;
     short nws_uid;
     unsigned nws_seqno;
+    unsigned nws_generation;
     time_t nws_timestamp;
     /* end of part matching struct empobj */
     natid nws_ano;		/* "actor" country # */
diff --git a/include/nuke.h b/include/nuke.h
index 231f0a04..bb1efd53 100644
--- a/include/nuke.h
+++ b/include/nuke.h
@@ -45,6 +45,7 @@ struct nukstr {
     short ef_type;
     short nuk_uid;
     unsigned nuk_seqno;
+    unsigned nuk_generation;
     time_t nuk_timestamp;	/* Last time this nuke was touched */
     natid nuk_own;
     coord nuk_x, nuk_y;		/* current loc of device */
diff --git a/include/plane.h b/include/plane.h
index 8e37d0d2..5e809e2d 100644
--- a/include/plane.h
+++ b/include/plane.h
@@ -49,6 +49,7 @@ struct plnstr {
     short ef_type;
     short pln_uid;		/* plane unit id */
     unsigned pln_seqno;
+    unsigned pln_generation;
     time_t pln_timestamp;	/* Last time this plane was touched */
     natid pln_own;		/* owning country */
     coord pln_x;		/* plane x-y */
diff --git a/include/sect.h b/include/sect.h
index 336b71f1..f242b22f 100644
--- a/include/sect.h
+++ b/include/sect.h
@@ -47,6 +47,7 @@ struct sctstr {
     short ef_type;
     short sct_uid;		/* equals XYOFFSET(sct_x, sct_y) */
     unsigned sct_seqno;
+    unsigned sct_generation;
     time_t sct_timestamp;	/* Last time this sector was written to */
     natid sct_own;		/* owner's country num */
     coord sct_x;		/* x coord of sector */
diff --git a/include/ship.h b/include/ship.h
index 21e749f5..a8762d57 100644
--- a/include/ship.h
+++ b/include/ship.h
@@ -66,6 +66,7 @@ struct shpstr {
     short ef_type;
     short shp_uid;		/* unit id (ship #) */
     unsigned shp_seqno;
+    unsigned shp_generation;
     time_t shp_timestamp;	/* Last time this ship was touched. */
     natid shp_own;		/* owner's country num */
     coord shp_x;		/* x location in abs coords */
diff --git a/include/trade.h b/include/trade.h
index 6fc9919e..26449ae8 100644
--- a/include/trade.h
+++ b/include/trade.h
@@ -45,6 +45,7 @@ struct trdstr {
     short ef_type;
     short trd_uid;
     unsigned trd_seqno;
+    unsigned trd_generation;
     time_t trd_timestamp;
     natid trd_owner;
     /* end of part matching struct empobj */
diff --git a/include/treaty.h b/include/treaty.h
index 0e164eef..e70c2f98 100644
--- a/include/treaty.h
+++ b/include/treaty.h
@@ -42,6 +42,7 @@ struct trtstr {
     short ef_type;
     short trt_uid;
     unsigned trt_seqno;
+    unsigned trt_generation;
     time_t trt_timestamp;
     /* end of part matching struct empobj */
     natid trt_cna;		/* proposer */
diff --git a/src/lib/common/file.c b/src/lib/common/file.c
index 908b024b..16633eee 100644
--- a/src/lib/common/file.c
+++ b/src/lib/common/file.c
@@ -52,9 +52,12 @@ static int do_read(struct empfile *, void *, int, int);
 static int do_write(struct empfile *, void *, int, int);
 static unsigned get_seqno(struct empfile *, int);
 static void new_seqno(struct empfile *, void *);
+static void must_be_fresh(struct empfile *, void *);
 static void do_blank(struct empfile *, void *, int, int);
 static int ef_check(int);
 
+static unsigned ef_generation;
+
 /*
  * Open the file-backed table TYPE (EF_SECTOR, ...).
  * HOW are flags to control operation.  Naturally, immutable flags are
@@ -338,6 +341,7 @@ ef_read(int type, int id, void *into)
 	cachep = ep->cache + (id - ep->baseid) * ep->size;
     }
     memcpy(into, cachep, ep->size);
+    ef_mark_fresh(type, into);
 
     if (ep->postread)
 	ep->postread(id, into);
@@ -500,9 +504,11 @@ ef_write(int type, int id, void *from)
 	if (ep->onresize && ep->onresize(type) < 0)
 	    return 0;
     }
-    if (id >= ep->baseid && id < ep->baseid + ep->cids)
+    if (id >= ep->baseid && id < ep->baseid + ep->cids) {
 	cachep = ep->cache + (id - ep->baseid) * ep->size;
-    else
+	if (cachep != from)
+	    must_be_fresh(ep, from);
+    } else
 	cachep = NULL;
     if (ep->prewrite)
 	ep->prewrite(id, cachep, from);
@@ -586,6 +592,35 @@ new_seqno(struct empfile *ep, void *buf)
     elt->seqno = old_seqno + 1;
 }
 
+void
+ef_make_stale(void)
+{
+    ef_generation++;
+}
+
+void
+ef_mark_fresh(int type, void *buf)
+{
+    struct empfile *ep;
+
+    if (ef_check(type) < 0)
+	return;
+    ep = &empfile[type];
+    if (!(ep->flags & EFF_TYPED))
+	return;
+    ((struct emptypedstr *)buf)->generation = ef_generation;
+}
+
+static void
+must_be_fresh(struct empfile *ep, void *buf)
+{
+    struct emptypedstr *elt = buf;
+
+    if (!(ep->flags & EFF_TYPED))
+	return;
+    CANT_HAPPEN(elt->generation != ef_generation);
+}
+
 /*
  * Extend table TYPE by COUNT elements.
  * Any pointers obtained from ef_ptr() become invalid.
@@ -663,6 +698,7 @@ ef_blank(int type, int id, void *buf)
 	elt = buf;
 	elt->seqno = get_seqno(ep, elt->uid);
     }
+    ef_mark_fresh(type, buf);
 }
 
 /*
diff --git a/src/lib/empthread/io.c b/src/lib/empthread/io.c
index 5bbce8d9..3fcf164b 100644
--- a/src/lib/empthread/io.c
+++ b/src/lib/empthread/io.c
@@ -51,6 +51,7 @@
 #include <unistd.h>
 #include "empio.h"
 #include "empthread.h"
+#include "file.h"
 #include "ioqueue.h"
 #include "misc.h"
 #include "queue.h"
@@ -123,6 +124,9 @@ io_input(struct iop *iop, int waitforinput)
     int cc;
     int res;
 
+    if (waitforinput)
+	ef_make_stale();
+
     /* Not a read IOP */
     if ((iop->flags & IO_READ) == 0) {
 	errno = EBADF;
@@ -183,6 +187,9 @@ io_output(struct iop *iop, int waitforoutput)
     int n;
     int remain;
 
+    if (waitforoutput)
+	ef_make_stale();
+
     /* If there is no output waiting. */
     if (!io_outputwaiting(iop))
 	return 0;
@@ -264,6 +271,9 @@ io_write(struct iop *iop, char *buf, int nbytes, int doWait)
 {
     int len;
 
+    if (doWait)
+	ef_make_stale();
+
     if ((iop->flags & IO_WRITE) == 0)
 	return -1;
     ioq_append(iop->output, buf, nbytes);
@@ -285,6 +295,8 @@ io_output_all(struct iop *iop)
 {
     int n;
 
+    ef_make_stale();
+
     /*
      * Mustn't block a player thread while update is pending, or else
      * a malicous player could delay the update indefinitely
diff --git a/src/lib/empthread/lwp.c b/src/lib/empthread/lwp.c
index 03bf4a64..66015a78 100644
--- a/src/lib/empthread/lwp.c
+++ b/src/lib/empthread/lwp.c
@@ -37,6 +37,7 @@
 #include <signal.h>
 #include <time.h>
 #include "empthread.h"
+#include "file.h"
 #include "misc.h"
 
 /* Flags that were passed to empth_init() */
@@ -65,6 +66,7 @@ empth_create(void (*entry)(void *), int size, int flags,
 {
     if (!flags)
 	flags = empth_flags;
+    ef_make_stale();
     return lwpCreate(1, entry, size, flags, name, 0, 0, ud);
 }
 
@@ -89,18 +91,21 @@ empth_set_name(empth_t *thread, char *name)
 void
 empth_exit(void)
 {
+    ef_make_stale();
     lwpExit();
 }
 
 void
 empth_yield(void)
 {
+    ef_make_stale();
     lwpYield();
 }
 
 int
 empth_select(int fd, int flags, struct timeval *timeout)
 {
+    ef_make_stale();
     return lwpSleepFd(fd, flags, timeout);
 }
 
@@ -113,6 +118,7 @@ empth_wakeup(empth_t *a)
 int
 empth_sleep(time_t until)
 {
+    ef_make_stale();
     return lwpSleepUntil(until);
 }
 
@@ -123,6 +129,7 @@ empth_wait_for_signal(void)
     int sig, err;
     time_t now;
 
+    ef_make_stale();
     sigemptyset(&set);
     sigaddset(&set, SIGHUP);
     sigaddset(&set, SIGINT);
@@ -153,12 +160,14 @@ empth_rwlock_destroy(empth_rwlock_t *rwlock)
 void
 empth_rwlock_wrlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     lwp_rwlock_wrlock(rwlock);
 }
 
 void
 empth_rwlock_rdlock(empth_rwlock_t *rwlock)
 {
+    ef_make_stale();
     lwp_rwlock_rdlock(rwlock);
 }
 
diff --git a/src/lib/subs/check.c b/src/lib/subs/check.c
index f20f9617..6623a3ef 100644
--- a/src/lib/subs/check.c
+++ b/src/lib/subs/check.c
@@ -52,7 +52,11 @@ obj_changed(struct empobj *obj, size_t sz)
     get_empobj(obj->ef_type, obj->uid, &old);
     memcpy(&tobj, obj, sz);
     old.gen.timestamp = tobj.gen.timestamp = 0;
-    return memcmp(&tobj, &old, sz);
+    old.gen.generation = tobj.gen.generation = 0;
+    if (memcmp(&tobj, &old, sz))
+	return 1;
+    ef_mark_fresh(obj->ef_type, obj);
+    return 0;
 }
 
 int