diff --git a/src/buffer.c b/src/buffer.c
index 531e4b9..5ecb638 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -61,8 +61,9 @@
 #endif
 
 /*
- * Open current buffer, that is: open the memfile and read the file into memory
- * return FAIL for failure, OK otherwise
+ * Open current buffer, that is: open the memfile and read the file into
+ * memory.
+ * Return FAIL for failure, OK otherwise.
  */
     int
 open_buffer(read_stdin, eap)
diff --git a/src/eval.c b/src/eval.c
index 446df8e..3c238fc 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -11869,6 +11869,9 @@
 	"perl",
 #endif
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+	"persistent_undo",
+#endif
 #ifdef FEAT_PYTHON
 #ifndef DYNAMIC_PYTHON
 	"python",
diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 1ef885a..f41e0f4 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -773,6 +773,8 @@
 			RANGE|DFLALL|EXTRA|NEEDARG|CMDWIN),
 EX(CMD_rubyfile,	"rubyfile",	ex_rubyfile,
 			RANGE|FILE1|NEEDARG|CMDWIN),
+EX(CMD_rundo,		"rundo",	ex_rundo,
+			NEEDARG|EXTRA|XFILE),
 EX(CMD_rviminfo,	"rviminfo",	ex_viminfo,
 			BANG|FILE1|TRLBAR|CMDWIN),
 EX(CMD_substitute,	"substitute",	do_sub,
@@ -1061,6 +1063,8 @@
 			BANG|FILE1|ARGOPT|DFLALL|TRLBAR),
 EX(CMD_wsverb,		"wsverb",	ex_wsverb,
 			EXTRA|NOTADR|NEEDARG),
+EX(CMD_wundo,		"wundo",	ex_wundo,
+			BANG|NEEDARG|EXTRA|XFILE),
 EX(CMD_wviminfo,	"wviminfo",	ex_viminfo,
 			BANG|FILE1|TRLBAR|CMDWIN),
 EX(CMD_xit,		"xit",		ex_exit,
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index ff39040..4097f1d 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -243,6 +243,10 @@
 # define ex_spellinfo		ex_ni
 # define ex_spellrepall		ex_ni
 #endif
+#ifndef FEAT_PERSISTENT_UNDO
+# define ex_rundo		ex_ni
+# define ex_wundo		ex_ni
+#endif
 #ifndef FEAT_MZSCHEME
 # define ex_mzscheme		ex_script_ni
 # define ex_mzfile		ex_ni
@@ -298,6 +302,10 @@
 static void	ex_at __ARGS((exarg_T *eap));
 static void	ex_bang __ARGS((exarg_T *eap));
 static void	ex_undo __ARGS((exarg_T *eap));
+#ifdef FEAT_PERSISTENT_UNDO
+static void	ex_wundo __ARGS((exarg_T *eap));
+static void	ex_rundo __ARGS((exarg_T *eap));
+#endif
 static void	ex_redo __ARGS((exarg_T *eap));
 static void	ex_later __ARGS((exarg_T *eap));
 static void	ex_redir __ARGS((exarg_T *eap));
@@ -8452,6 +8460,28 @@
 	u_undo(1);
 }
 
+#ifdef FEAT_PERSISTENT_UNDO
+    void
+ex_wundo(eap)
+    exarg_T *eap;
+{
+    char_u hash[UNDO_HASH_SIZE];
+
+    u_compute_hash(hash);
+    u_write_undo(eap->arg, eap->forceit, curbuf, hash);
+}
+
+    void
+ex_rundo(eap)
+    exarg_T *eap;
+{
+    char_u hash[UNDO_HASH_SIZE];
+
+    u_compute_hash(hash);
+    u_read_undo(eap->arg, hash);
+}
+#endif
+
 /*
  * ":redo".
  */
diff --git a/src/feature.h b/src/feature.h
index 6b8f600..61fe334 100644
--- a/src/feature.h
+++ b/src/feature.h
@@ -1275,3 +1275,11 @@
 	    || defined(FEAT_BIG)
 # define FEAT_AUTOCHDIR
 #endif
+
+/*
+ * +persistent_undo	'undofile', 'undodir' options, :wundo and :rundo, and
+ * implementation.
+ */
+#ifdef FEAT_NORMAL
+# define FEAT_PERSISTENT_UNDO
+#endif
diff --git a/src/fileio.c b/src/fileio.c
index 7a697ee..b7c86af 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -253,6 +253,10 @@
     char_u	*cryptkey = NULL;
     int		did_ask_for_key = FALSE;
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+    context_sha256_T sha_ctx;
+    int		read_undo_file = FALSE;
+#endif
     int		split = 0;		/* number of split lines */
 #define UNKNOWN	 0x0fffffff		/* file size is unknown */
     linenr_T	linecnt;
@@ -1178,6 +1182,12 @@
 #ifdef FEAT_MBYTE
 	conv_restlen = 0;
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+	read_undo_file = (newfile && curbuf->b_ffname != NULL && curbuf->b_p_udf
+				&& !filtering && !read_stdin && !read_buffer);
+	if (read_undo_file)
+	    sha256_start(&sha_ctx);
+#endif
     }
 
     while (!error && !got_int)
@@ -2133,6 +2143,10 @@
 			    error = TRUE;
 			    break;
 			}
+#ifdef FEAT_PERSISTENT_UNDO
+			if (read_undo_file)
+			    sha256_update(&sha_ctx, line_start, len);
+#endif
 			++lnum;
 			if (--read_count == 0)
 			{
@@ -2197,6 +2211,10 @@
 			    error = TRUE;
 			    break;
 			}
+#ifdef FEAT_PERSISTENT_UNDO
+			if (read_undo_file)
+			    sha256_update(&sha_ctx, line_start, len);
+#endif
 			++lnum;
 			if (--read_count == 0)
 			{
@@ -2237,11 +2255,17 @@
 	if (set_options)
 	    curbuf->b_p_eol = FALSE;
 	*ptr = NUL;
-	if (ml_append(lnum, line_start,
-			(colnr_T)(ptr - line_start + 1), newfile) == FAIL)
+	len = (colnr_T)(ptr - line_start + 1);
+	if (ml_append(lnum, line_start, len, newfile) == FAIL)
 	    error = TRUE;
 	else
+	{
+#ifdef FEAT_PERSISTENT_UNDO
+	    if (read_undo_file)
+		sha256_update(&sha_ctx, line_start, len);
+#endif
 	    read_no_eol_lnum = ++lnum;
+	}
     }
 
     if (set_options)
@@ -2555,6 +2579,19 @@
      */
     write_no_eol_lnum = read_no_eol_lnum;
 
+#ifdef FEAT_PERSISTENT_UNDO
+    /*
+     * When opening a new file locate undo info and read it.
+     */
+    if (read_undo_file)
+    {
+	char_u	hash[UNDO_HASH_SIZE];
+
+	sha256_finish(&sha_ctx, hash);
+	u_read_undo(NULL, hash);
+    }
+#endif
+
 #ifdef FEAT_AUTOCMD
     if (!read_stdin && !read_buffer)
     {
@@ -3038,6 +3075,10 @@
     vim_acl_T	    acl = NULL;		/* ACL copied from original file to
 					   backup or new file */
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+    int		    write_undo_file = FALSE;
+    context_sha256_T sha_ctx;
+#endif
 
     if (fname == NULL || *fname == NUL)	/* safety check */
 	return FAIL;
@@ -4344,6 +4385,14 @@
     write_info.bw_start_lnum = start;
 #endif
 
+#ifdef FEAT_PERSISTENT_UNDO
+    write_undo_file = (buf->b_p_udf && overwriting && !append
+					      && !filtering && reset_changed);
+    if (write_undo_file)
+	/* Prepare for computing the hash value of the text. */
+	sha256_start(&sha_ctx);
+#endif
+
     write_info.bw_len = bufsize;
 #ifdef HAS_BW_FLAGS
     write_info.bw_flags = wb_flags;
@@ -4358,6 +4407,10 @@
 	 * Keep it fast!
 	 */
 	ptr = ml_get_buf(buf, lnum, FALSE) - 1;
+#ifdef FEAT_PERSISTENT_UNDO
+	if (write_undo_file)
+	    sha256_update(&sha_ctx, ptr + 1, STRLEN(ptr + 1) + 1);
+#endif
 	while ((c = *++ptr) != NUL)
 	{
 	    if (c == NL)
@@ -4886,6 +4939,20 @@
     }
     msg_scroll = msg_save;
 
+#ifdef FEAT_PERSISTENT_UNDO
+    /*
+     * When writing the whole file and 'undofile' is set, also write the undo
+     * file.
+     */
+    if (retval == OK && write_undo_file)
+    {
+	char_u	    hash[UNDO_HASH_SIZE];
+
+	sha256_finish(&sha_ctx, hash);
+	u_write_undo(NULL, FALSE, buf, hash);
+    }
+#endif
+
 #ifdef FEAT_AUTOCMD
 #ifdef FEAT_EVAL
     if (!should_abort(retval))
diff --git a/src/memline.c b/src/memline.c
index b3c1727..c3fdba3 100644
--- a/src/memline.c
+++ b/src/memline.c
@@ -245,9 +245,6 @@
 #ifdef FEAT_BYTEOFF
 static void ml_updatechunk __ARGS((buf_T *buf, long line, long len, int updtype));
 #endif
-#ifdef HAVE_READLINK
-static int resolve_symlink __ARGS((char_u *fname, char_u *buf));
-#endif
 
 /*
  * Open a new memline for "buf".
@@ -3559,7 +3556,7 @@
     }
 }
 
-#ifdef HAVE_READLINK
+#if defined(HAVE_READLINK) || defined(PROTO)
 /*
  * Resolve a symlink in the last component of a file name.
  * Note that f_resolve() does it for every part of the path, we don't do that
@@ -3567,7 +3564,7 @@
  * If it worked returns OK and the resolved link in "buf[MAXPATHL]".
  * Otherwise returns FAIL.
  */
-    static int
+    int
 resolve_symlink(fname, buf)
     char_u	*fname;
     char_u	*buf;
@@ -3862,7 +3859,7 @@
  * Returns the name in allocated memory or NULL.
  *
  * Note: If BASENAMELEN is not correct, you will get error messages for
- *	 not being able to open the swapfile
+ *	 not being able to open the swap or undo file
  * Note: May trigger SwapExists autocmd, pointers may change!
  */
     static char_u *
@@ -3886,29 +3883,29 @@
 # define CREATE_DUMMY_FILE
     FILE	*dummyfd = NULL;
 
-/*
- * If we start editing a new file, e.g. "test.doc", which resides on an MSDOS
- * compatible filesystem, it is possible that the file "test.doc.swp" which we
- * create will be exactly the same file. To avoid this problem we temporarily
- * create "test.doc".
- * Don't do this when the check below for a 8.3 file name is used.
- */
+    /*
+     * If we start editing a new file, e.g. "test.doc", which resides on an
+     * MSDOS compatible filesystem, it is possible that the file
+     * "test.doc.swp" which we create will be exactly the same file. To avoid
+     * this problem we temporarily create "test.doc".  Don't do this when the
+     * check below for a 8.3 file name is used.
+     */
     if (!(buf->b_p_sn || buf->b_shortname) && buf->b_fname != NULL
 					     && mch_getperm(buf->b_fname) < 0)
 	dummyfd = mch_fopen((char *)buf->b_fname, "w");
 #endif
 
-/*
- * Isolate a directory name from *dirp and put it in dir_name.
- * First allocate some memory to put the directory name in.
- */
+    /*
+     * Isolate a directory name from *dirp and put it in dir_name.
+     * First allocate some memory to put the directory name in.
+     */
     dir_name = alloc((unsigned)STRLEN(*dirp) + 1);
     if (dir_name != NULL)
 	(void)copy_option_part(dirp, dir_name, 31000, ",");
 
-/*
- * we try different names until we find one that does not exist yet
- */
+    /*
+     * we try different names until we find one that does not exist yet
+     */
     if (dir_name == NULL)	    /* out of memory */
 	fname = NULL;
     else
diff --git a/src/option.c b/src/option.c
index 6c0618a..a4af18a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -177,6 +177,9 @@
 #define PV_TS		OPT_BUF(BV_TS)
 #define PV_TW		OPT_BUF(BV_TW)
 #define PV_TX		OPT_BUF(BV_TX)
+#ifdef FEAT_PERSISTENT_UNDO
+# define PV_UDF		OPT_BUF(BV_UDF)
+#endif
 #define PV_WM		OPT_BUF(BV_WM)
 
 /*
@@ -362,6 +365,9 @@
 static long	p_ts;
 static long	p_tw;
 static int	p_tx;
+#ifdef FEAT_PERSISTENT_UNDO
+static int	p_udf;
+#endif
 static long	p_wm;
 #ifdef FEAT_KEYMAP
 static char_u	*p_keymap;
@@ -2586,6 +2592,22 @@
     {"ttytype",	    "tty",  P_STRING|P_EXPAND|P_NODEFAULT|P_NO_MKRC|P_VI_DEF|P_RALL,
 			    (char_u *)&T_NAME, PV_NONE,
 			    {(char_u *)"", (char_u *)0L} SCRIPTID_INIT},
+    {"undodir",     "udir", P_STRING|P_EXPAND|P_COMMA|P_NODUP|P_SECURE|P_VI_DEF,
+#ifdef FEAT_PERSISTENT_UNDO
+			    (char_u *)&p_udir, PV_NONE,
+			    {(char_u *)".", (char_u *)0L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)0L, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
+    {"undofile",    "udf",  P_BOOL|P_VI_DEF|P_VIM,
+#ifdef FEAT_PERSISTENT_UNDO
+			    (char_u *)&p_udf, PV_UDF,
+#else
+			    (char_u *)NULL, PV_NONE,
+#endif
+			    {(char_u *)FALSE, (char_u *)0L} SCRIPTID_INIT},
     {"undolevels",  "ul",   P_NUM|P_VI_DEF,
 			    (char_u *)&p_ul, PV_NONE,
 			    {
@@ -9404,6 +9426,9 @@
 	case PV_TS:	return (char_u *)&(curbuf->b_p_ts);
 	case PV_TW:	return (char_u *)&(curbuf->b_p_tw);
 	case PV_TX:	return (char_u *)&(curbuf->b_p_tx);
+#ifdef FEAT_PERSISTENT_UNDO
+	case PV_UDF:	return (char_u *)&(curbuf->b_p_udf);
+#endif
 	case PV_WM:	return (char_u *)&(curbuf->b_p_wm);
 #ifdef FEAT_KEYMAP
 	case PV_KMAP:	return (char_u *)&(curbuf->b_p_keymap);
@@ -9774,6 +9799,9 @@
 #if defined(FEAT_BEVAL) && defined(FEAT_EVAL)
 	    buf->b_p_bexpr = empty_option;
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+	    buf->b_p_udf = p_udf;
+#endif
 
 	    /*
 	     * Don't copy the options set by ex_help(), use the saved values,
diff --git a/src/option.h b/src/option.h
index b49c0c4..f0ebc8d 100644
--- a/src/option.h
+++ b/src/option.h
@@ -815,6 +815,7 @@
 # define TTYM_JSBTERM		0x10
 # define TTYM_PTERM		0x20
 #endif
+EXTERN char_u	*p_udir;	/* 'undodir' */
 EXTERN long	p_ul;		/* 'undolevels' */
 EXTERN long	p_uc;		/* 'updatecount' */
 EXTERN long	p_ut;		/* 'updatetime' */
@@ -1004,6 +1005,7 @@
     , BV_TS
     , BV_TW
     , BV_TX
+    , BV_UDF
     , BV_WM
     , BV_COUNT	    /* must be the last one */
 };
diff --git a/src/os_mac.h b/src/os_mac.h
index 314ec32..f1ed965 100644
--- a/src/os_mac.h
+++ b/src/os_mac.h
@@ -204,7 +204,7 @@
 #endif
 
 #ifndef DFLT_VDIR
-#  define DFLT_VDIR	"$VIM/vimfiles/view"	/* default for 'viewdir' */
+# define DFLT_VDIR	"$VIM/vimfiles/view"	/* default for 'viewdir' */
 #endif
 
 #define DFLT_ERRORFILE		"errors.err"
diff --git a/src/proto/memline.pro b/src/proto/memline.pro
index de75a7d..4d50671 100644
--- a/src/proto/memline.pro
+++ b/src/proto/memline.pro
@@ -25,6 +25,7 @@
 void ml_setmarked __ARGS((linenr_T lnum));
 linenr_T ml_firstmarked __ARGS((void));
 void ml_clearmarked __ARGS((void));
+int resolve_symlink __ARGS((char_u *fname, char_u *buf));
 char_u *makeswapname __ARGS((char_u *fname, char_u *ffname, buf_T *buf, char_u *dir_name));
 char_u *get_file_in_dir __ARGS((char_u *fname, char_u *dname));
 void ml_setflags __ARGS((buf_T *buf));
diff --git a/src/proto/sha256.pro b/src/proto/sha256.pro
index a6d6be7..e9a3d3c 100644
--- a/src/proto/sha256.pro
+++ b/src/proto/sha256.pro
@@ -1,4 +1,7 @@
 /* sha256.c */
+void sha256_start __ARGS((context_sha256_T *ctx));
+void sha256_update __ARGS((context_sha256_T *ctx, char_u *input, uint32_t length));
+void sha256_finish __ARGS((context_sha256_T *ctx, char_u digest[32]));
 char_u *sha256_key __ARGS((char_u *buf));
 int sha256_self_test __ARGS((void));
 void sha2_seed __ARGS((char_u header[], int header_len));
diff --git a/src/proto/spell.pro b/src/proto/spell.pro
index f497dc6..8ab544d 100644
--- a/src/proto/spell.pro
+++ b/src/proto/spell.pro
@@ -2,11 +2,14 @@
 int spell_check __ARGS((win_T *wp, char_u *ptr, hlf_T *attrp, int *capcol, int docount));
 int spell_move_to __ARGS((win_T *wp, int dir, int allwords, int curline, hlf_T *attrp));
 void spell_cat_line __ARGS((char_u *buf, char_u *line, int maxlen));
+int get2c __ARGS((FILE *fd));
+int get3c __ARGS((FILE *fd));
+int get4c __ARGS((FILE *fd));
 char_u *did_set_spelllang __ARGS((buf_T *buf));
 void spell_free_all __ARGS((void));
 void spell_reload __ARGS((void));
 int spell_check_msm __ARGS((void));
-void put_bytes __ARGS((FILE *fd, long_u nr, int len));
+int put_bytes __ARGS((FILE *fd, long_u nr, int len));
 void ex_mkspell __ARGS((exarg_T *eap));
 void ex_spell __ARGS((exarg_T *eap));
 void spell_add_word __ARGS((char_u *word, int len, int bad, int idx, int undo));
diff --git a/src/proto/undo.pro b/src/proto/undo.pro
index e712f03..e5f28be 100644
--- a/src/proto/undo.pro
+++ b/src/proto/undo.pro
@@ -5,6 +5,9 @@
 int u_inssub __ARGS((linenr_T lnum));
 int u_savedel __ARGS((linenr_T lnum, long nlines));
 int undo_allowed __ARGS((void));
+void u_compute_hash __ARGS((char_u *hash));
+void u_read_undo __ARGS((char_u *name, char_u *hash));
+void u_write_undo __ARGS((char_u *name, int forceit, buf_T *buf, char_u *hash));
 void u_undo __ARGS((int count));
 void u_redo __ARGS((int count));
 void undo_time __ARGS((long step, int sec, int absolute));
diff --git a/src/sha256.c b/src/sha256.c
index 9372d5f..048ce75 100644
--- a/src/sha256.c
+++ b/src/sha256.c
@@ -20,18 +20,9 @@
 
 #include "vim.h"
 
-#ifdef FEAT_CRYPT
+#if defined(FEAT_CRYPT) || defined(FEAT_PERSISTENT_UNDO)
 
-typedef struct {
-  UINT32_T total[2];
-  UINT32_T state[8];
-  char_u   buffer[64];
-} context_sha256_T;
-
-static void sha256_starts __ARGS((context_sha256_T *ctx));
 static void sha256_process __ARGS((context_sha256_T *ctx, char_u data[64]));
-static void sha256_update __ARGS((context_sha256_T *ctx, char_u *input, UINT32_T length));
-static void sha256_finish __ARGS((context_sha256_T *ctx, char_u digest[32]));
 static char_u *sha256_bytes __ARGS((char_u *buf, int buflen));
 static unsigned int get_some_time __ARGS((void));
 
@@ -52,8 +43,8 @@
     (b)[(i) + 3] = (char_u)((n)      );   \
 }
 
-    static void
-sha256_starts(ctx)
+     void
+sha256_start(ctx)
     context_sha256_T *ctx;
 {
     ctx->total[0] = 0;
@@ -203,7 +194,7 @@
     ctx->state[7] += H;
 }
 
-    static void
+    void
 sha256_update(ctx, input, length)
     context_sha256_T *ctx;
     char_u	     *input;
@@ -250,7 +241,7 @@
     0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
 };
 
-    static void
+    void
 sha256_finish(ctx, digest)
     context_sha256_T *ctx;
     char_u           digest[32];
@@ -296,7 +287,7 @@
 
     sha256_self_test();
 
-    sha256_starts(&ctx);
+    sha256_start(&ctx);
     sha256_update(&ctx, buf, buflen);
     sha256_finish(&ctx, sha256sum);
     for (j = 0; j < 32; j++)
@@ -368,7 +359,7 @@
 	}
 	else
 	{
-	    sha256_starts(&ctx);
+	    sha256_start(&ctx);
 	    memset(buf, 'a', 1000);
 	    for (j = 0; j < 1000; j++)
 		sha256_update(&ctx, (char_u *)buf, 1000);
@@ -416,7 +407,7 @@
 
     for (i = 0; i < (int)sizeof(random_data) - 1; i++)
 	random_data[i] = (char_u)((get_some_time() ^ rand()) & 0xff);
-    sha256_starts(&ctx);
+    sha256_start(&ctx);
     sha256_update(&ctx, (char_u *)random_data, sizeof(random_data));
     sha256_finish(&ctx, sha256sum);
 
@@ -424,4 +415,4 @@
 	header[i] = sha256sum[i % sizeof(sha256sum)];
 }
 
-#endif /* FEAT_CRYPT */
+#endif /* FEAT_CRYPT || FEAT_PERSISTENT_UNDO */
diff --git a/src/spell.c b/src/spell.c
index 47b86ad..1c3fd0f 100644
--- a/src/spell.c
+++ b/src/spell.c
@@ -854,9 +854,6 @@
 static void int_wordlist_spl __ARGS((char_u *fname));
 static void spell_load_cb __ARGS((char_u *fname, void *cookie));
 static slang_T *spell_load_file __ARGS((char_u *fname, char_u *lang, slang_T *old_lp, int silent));
-static int get2c __ARGS((FILE *fd));
-static int get3c __ARGS((FILE *fd));
-static int get4c __ARGS((FILE *fd));
 static time_t get8c __ARGS((FILE *fd));
 static char_u *read_cnt_string __ARGS((FILE *fd, int cnt_bytes, int *lenp));
 static char_u *read_string __ARGS((FILE *fd, int cnt));
@@ -2988,7 +2985,7 @@
 /*
  * Read 2 bytes from "fd" and turn them into an int, MSB first.
  */
-    static int
+    int
 get2c(fd)
     FILE	*fd;
 {
@@ -3002,7 +2999,7 @@
 /*
  * Read 3 bytes from "fd" and turn them into an int, MSB first.
  */
-    static int
+    int
 get3c(fd)
     FILE	*fd;
 {
@@ -3017,7 +3014,7 @@
 /*
  * Read 4 bytes from "fd" and turn them into an int, MSB first.
  */
-    static int
+    int
 get4c(fd)
     FILE	*fd;
 {
@@ -8018,7 +8015,7 @@
 /*
  * Write a number to file "fd", MSB first, in "len" bytes.
  */
-    void
+    int
 put_bytes(fd, nr, len)
     FILE    *fd;
     long_u  nr;
@@ -8027,7 +8024,9 @@
     int	    i;
 
     for (i = len - 1; i >= 0; --i)
-	putc((int)(nr >> (i * 8)), fd);
+	if (putc((int)(nr >> (i * 8)), fd) == EOF)
+	    return FAIL;
+    return OK;
 }
 
 #ifdef _MSC_VER
diff --git a/src/structs.h b/src/structs.h
index a34fa66..a7632b8 100644
--- a/src/structs.h
+++ b/src/structs.h
@@ -1465,6 +1465,9 @@
     char_u	*b_p_dict;	/* 'dictionary' local value */
     char_u	*b_p_tsr;	/* 'thesaurus' local value */
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+    int		b_p_udf;	/* 'undofile' */
+#endif
 
     /* end of buffer options */
 
@@ -2392,3 +2395,9 @@
 #define CPT_KIND    2	/* "kind" */
 #define CPT_INFO    3	/* "info" */
 #define CPT_COUNT   4	/* Number of entries */
+
+typedef struct {
+  UINT32_T total[2];
+  UINT32_T state[8];
+  char_u   buffer[64];
+} context_sha256_T;
diff --git a/src/testdir/Makefile b/src/testdir/Makefile
index 6a8e85f..53e0854 100644
--- a/src/testdir/Makefile
+++ b/src/testdir/Makefile
@@ -69,7 +69,7 @@
 		  fi \
 		else echo $* NO OUTPUT >>test.log; \
 		fi"
-	#-rm -rf X* test.ok viminfo
+	-rm -rf X* test.ok viminfo
 
 test49.out: test49.vim
 
diff --git a/src/testdir/test61.in b/src/testdir/test61.in
index 7ce72e6..909d123 100644
--- a/src/testdir/test61.in
+++ b/src/testdir/test61.in
@@ -50,6 +50,53 @@
 obbbb:set ul=100
 :undojoin
 occccu:.w >>test.out
+:"
+:" Test 'undofile': first a simple one-line change.
+:set nocp ul=100 undofile
+:e! Xtestfile
+ggdGithis is one line:set ul=100
+:s/one/ONE/
+:set ul=100
+:w
+:bwipe!
+:e Xtestfile
+u:.w >>test.out
+:"
+:" Test 'undofile', change in original file fails check
+:set noundofile
+:e! Xtestfile
+:s/line/Line/
+:w
+:set undofile
+:bwipe!
+:e Xtestfile
+u:.w >>test.out
+:"
+:" Test 'undofile', add 10 lines, delete 6 lines, undo 3
+:set undofile
+ggdGione
+two
+three
+four
+five
+six
+seven
+eight
+nine
+ten:set ul=100
+3Gdd:set ul=100
+dd:set ul=100
+dd:set ul=100
+dd:set ul=100
+dd:set ul=100
+dd:set ul=100
+:w
+:bwipe!
+:e Xtestfile
+uuu:w >>test.out
+:"
+:" Rename the undo file so that it gets cleaned up.
+:call rename(".Xtestfile.un~", "Xtestundo")
 :qa!
 ENDTEST
 
diff --git a/src/testdir/test61.ok b/src/testdir/test61.ok
index 020dd53..ea88c07 100644
--- a/src/testdir/test61.ok
+++ b/src/testdir/test61.ok
@@ -22,3 +22,12 @@
 123456abc
 aaaa
 aaaa
+this is one line
+this is ONE Line
+one
+two
+six
+seven
+eight
+nine
+ten
diff --git a/src/undo.c b/src/undo.c
index 4035466..b4945d1 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -99,6 +99,14 @@
 static void u_freebranch __ARGS((buf_T *buf, u_header_T *uhp, u_header_T **uhpp));
 static void u_freeentries __ARGS((buf_T *buf, u_header_T *uhp, u_header_T **uhpp));
 static void u_freeentry __ARGS((u_entry_T *, long));
+#ifdef FEAT_PERSISTENT_UNDO
+static void unserialize_pos __ARGS((pos_T *pos, FILE *fp));
+static void unserialize_visualinfo __ARGS((visualinfo_T *info, FILE *fp));
+static char_u *u_get_undo_file_name __ARGS((char_u *, int reading));
+static int serialize_uep __ARGS((u_entry_T *uep, FILE *fp));
+static void serialize_pos __ARGS((pos_T pos, FILE *fp));
+static void serialize_visualinfo __ARGS((visualinfo_T info, FILE *fp));
+#endif
 
 #ifdef U_USE_MALLOC
 # define U_FREE_LINE(ptr) vim_free(ptr)
@@ -119,6 +127,8 @@
  */
 static int	undo_undoes = FALSE;
 
+static int	lastmark = 0;
+
 #ifdef U_DEBUG
 /*
  * Check the undo structures for being valid.  Print a warning when something
@@ -652,6 +662,795 @@
     return FAIL;
 }
 
+#ifdef FEAT_PERSISTENT_UNDO
+
+# define UF_START_MAGIC	    0xfeac	/* magic at start of undofile */
+# define UF_HEADER_MAGIC    0x5fd0	/* magic at start of header */
+# define UF_END_MAGIC	    0xe7aa	/* magic after last header */
+# define UF_VERSION	    1		/* 2-byte undofile version number */
+
+/*
+ * Compute the hash for the current buffer text into hash[UNDO_HASH_SIZE].
+ */
+    void
+u_compute_hash(hash)
+    char_u *hash;
+{
+    context_sha256_T	ctx;
+    linenr_T		lnum;
+    char_u		*p;
+
+    sha256_start(&ctx);
+    for (lnum = 1; lnum < curbuf->b_ml.ml_line_count; ++lnum)
+    {
+	p = ml_get(lnum);
+	sha256_update(&ctx, p, STRLEN(p) + 1);
+    }
+    sha256_finish(&ctx, hash);
+}
+
+/*
+ * Unserialize the pos_T at the current position in fp.
+ */
+    static void
+unserialize_pos(pos, fp)
+    pos_T *pos;
+    FILE  *fp;
+{
+    pos->lnum = get4c(fp);
+    pos->col = get4c(fp);
+#ifdef FEAT_VIRTUALEDIT
+    pos->coladd = get4c(fp);
+#else
+    (void)get4c(fp);
+#endif
+}
+
+/*
+ * Unserialize the visualinfo_T at the current position in fp.
+ */
+    static void
+unserialize_visualinfo(info, fp)
+    visualinfo_T    *info;
+    FILE	    *fp;
+{
+    unserialize_pos(&info->vi_start, fp);
+    unserialize_pos(&info->vi_end, fp);
+    info->vi_mode = get4c(fp);
+    info->vi_curswant = get4c(fp);
+}
+
+/*
+ * Return an allocated string of the full path of the target undofile.
+ * When "reading" is TRUE find the file to read, go over all directories in
+ * 'undodir'.
+ * When "reading" is FALSE use the first name where the directory exists.
+ */
+    static char_u *
+u_get_undo_file_name(buf_ffname, reading)
+    char_u	*buf_ffname;
+    int		reading;
+{
+    char_u	*dirp;
+    char_u	dir_name[IOSIZE + 1];
+    char_u	*munged_name = NULL;
+    char_u	*undo_file_name = NULL;
+    int		dir_len;
+    char_u	*p;
+    struct stat st;
+    char_u	*ffname = buf_ffname;
+#ifdef HAVE_READLINK
+    char_u	fname_buf[MAXPATHL];
+#endif
+
+    if (ffname == NULL)
+	return NULL;
+
+#ifdef HAVE_READLINK
+    /* Expand symlink in the file name, so that we put the undo file with the
+     * actual file instead of with the symlink. */
+    if (resolve_symlink(ffname, fname_buf) == OK)
+	ffname = fname_buf;
+#endif
+
+    /* Loop over 'undodir'.  When reading find the first file that exists.
+     * When not reading use the first directory that exists or ".". */
+    dirp = p_udir;
+    while (*dirp != NUL)
+    {
+        dir_len = copy_option_part(&dirp, dir_name, IOSIZE, ",");
+	if (dir_len == 1 && dir_name[0] == '.')
+	{
+	    /* Use same directory as the ffname,
+	     * "dir/name" -> "dir/.name.un~" */
+	    undo_file_name = vim_strnsave(ffname, STRLEN(ffname) + 5);
+	    if (undo_file_name == NULL)
+		break;
+	    p = gettail(undo_file_name);
+	    mch_memmove(p + 1, p, STRLEN(p) + 1);
+	    *p = '.';
+	    STRCAT(p, ".un~");
+	}
+	else
+	{
+	    dir_name[dir_len] = NUL;
+	    if (mch_isdir(dir_name))
+	    {
+		if (munged_name == NULL)
+		{
+		    munged_name = vim_strsave(ffname);
+		    if (munged_name == NULL)
+			return NULL;
+		    for (p = munged_name; *p != NUL; mb_ptr_adv(p))
+			if (vim_ispathsep(*p))
+			    *p = '%';
+		}
+		undo_file_name = concat_fnames(dir_name, munged_name, TRUE);
+	    }
+	}
+
+	/* When reading check if the file exists. */
+	if (undo_file_name != NULL && (!reading
+			       || mch_stat((char *)undo_file_name, &st) >= 0))
+	    break;
+	vim_free(undo_file_name);
+	undo_file_name = NULL;
+    }
+
+    vim_free(munged_name);
+    return undo_file_name;
+}
+
+/*
+ * Load the undo tree from an undo file.
+ * If "name" is not NULL use it as the undo file name.  This also means being
+ * a bit more verbose.
+ * Otherwise use curbuf->b_ffname to generate the undo file name.
+ * "hash[UNDO_HASH_SIZE]" must be the hash value of the buffer text.
+ */
+    void
+u_read_undo(name, hash)
+    char_u *name;
+    char_u *hash;
+{
+    char_u	*file_name;
+    FILE	*fp;
+    long	magic, version, str_len;
+    char_u	*line_ptr = NULL;
+    linenr_T	line_lnum;
+    colnr_T	line_colnr;
+    linenr_T	line_count;
+    int		uep_len;
+    int		line_len;
+    int		num_head;
+    long	old_header_seq, new_header_seq, cur_header_seq;
+    long	seq_last, seq_cur;
+    short	old_idx = -1, new_idx = -1, cur_idx = -1;
+    long	num_read_uhps = 0;
+    time_t	seq_time;
+    int		i, j;
+    int		c;
+    short	found_first_uep = 0;
+    char_u	**array;
+    char_u	*line;
+    u_entry_T	*uep, *last_uep, *nuep;
+    u_header_T	*uhp;
+    u_header_T	**uhp_table = NULL;
+    char_u	read_hash[UNDO_HASH_SIZE];
+
+    if (name == NULL)
+    {
+        file_name = u_get_undo_file_name(curbuf->b_ffname, TRUE);
+	if (file_name == NULL)
+	    return;
+    }
+    else
+        file_name = name;
+
+    if (p_verbose > 0)
+	smsg((char_u *)_("Reading undo file: %s"), file_name);
+    fp = mch_fopen((char *)file_name, "r");
+    if (fp == NULL)
+    {
+        if (name != NULL || p_verbose > 0)
+            EMSG2(_("E822: Cannot open undo file for reading: %s"), file_name);
+	goto error;
+    }
+
+    /* Begin overall file information */
+    magic = get2c(fp);
+    if (magic != UF_START_MAGIC)
+    {
+        EMSG2(_("E823: Corrupted undo file: %s"), file_name);
+        goto error;
+    }
+    version = get2c(fp);
+    if (version != UF_VERSION)
+    {
+        EMSG2(_("E824: Incompatible undo file: %s"), file_name);
+        goto error;
+    }
+
+    fread(read_hash, UNDO_HASH_SIZE, 1, fp);
+    line_count = (linenr_T)get4c(fp);
+    if (memcmp(hash, read_hash, UNDO_HASH_SIZE) != 0
+				  || line_count != curbuf->b_ml.ml_line_count)
+    {
+        if (p_verbose > 0 || name != NULL)
+        {
+            verbose_enter();
+            give_warning((char_u *)_("Undo file contents changed"), TRUE);
+            verbose_leave();
+        }
+        goto error;
+    }
+
+    /* Begin undo data for U */
+    str_len = get4c(fp);
+    if (str_len < 0)
+        goto error;
+    else if (str_len > 0)
+    {
+        if ((line_ptr = U_ALLOC_LINE(str_len)) == NULL)
+            goto error;
+        for (i = 0; i < str_len; i++)
+            line_ptr[i] = (char_u)getc(fp);
+        line_ptr[i] = NUL;
+    }
+    line_lnum = (linenr_T)get4c(fp);
+    line_colnr = (colnr_T)get4c(fp);
+
+    /* Begin general undo data */
+    old_header_seq = get4c(fp);
+    new_header_seq = get4c(fp);
+    cur_header_seq = get4c(fp);
+    num_head = get4c(fp);
+    seq_last = get4c(fp);
+    seq_cur = get4c(fp);
+    seq_time = get4c(fp);
+
+    /* uhp_table will store the freshly created undo headers we allocate
+     * until we insert them into curbuf. The table remains sorted by the
+     * sequence numbers of the headers. */
+    uhp_table = (u_header_T **)U_ALLOC_LINE(num_head * sizeof(u_header_T *));
+    if (uhp_table == NULL)
+        goto error;
+    vim_memset(uhp_table, 0, num_head * sizeof(u_header_T *));
+
+    c = get2c(fp);
+    while (c == UF_HEADER_MAGIC)
+    {
+        found_first_uep = 0;
+        uhp = (u_header_T *)U_ALLOC_LINE((unsigned)sizeof(u_header_T));
+        if (uhp == NULL)
+            goto error;
+        vim_memset(uhp, 0, sizeof(u_header_T));
+        /* We're not actually trying to store pointers here. We're just storing
+         * IDs so we can swizzle them into pointers later - hence the type
+	 * cast. */
+        uhp->uh_next = (u_header_T *)(long)get4c(fp);
+        uhp->uh_prev = (u_header_T *)(long)get4c(fp);
+        uhp->uh_alt_next = (u_header_T *)(long)get4c(fp);
+        uhp->uh_alt_prev = (u_header_T *)(long)get4c(fp);
+        uhp->uh_seq = get4c(fp);
+        if (uhp->uh_seq <= 0)
+        {
+            EMSG2(_("E825: Undo file corruption: invalid uh_seq.: %s"),
+								   file_name);
+            U_FREE_LINE(uhp);
+            goto error;
+        }
+        uhp->uh_walk = 0;
+        unserialize_pos(&uhp->uh_cursor, fp);
+#ifdef FEAT_VIRTUALEDIT
+        uhp->uh_cursor_vcol = get4c(fp);
+#else
+        (void)get4c(fp);
+#endif
+        uhp->uh_flags = get2c(fp);
+        for (i = 0; i < NMARKS; ++i)
+            unserialize_pos(&uhp->uh_namedm[i], fp);
+#ifdef FEAT_VISUAL
+        unserialize_visualinfo(&uhp->uh_visual, fp);
+#else
+	{
+	    visualinfo_T info;
+	    unserialize_visualinfo(&info, fp);
+	}
+#endif
+        uhp->uh_time = get4c(fp);
+
+        /* Unserialize uep list. The first 4 bytes is the length of the
+         * entire uep in bytes minus the length of the strings within.
+         * -1 is a sentinel value meaning no more ueps.*/
+        last_uep = NULL;
+        while ((uep_len = get4c(fp)) != -1)
+        {
+            uep = (u_entry_T *)U_ALLOC_LINE((unsigned)sizeof(u_entry_T));
+            vim_memset(uep, 0, sizeof(u_entry_T));
+            if (uep == NULL)
+                goto error;
+            uep->ue_top = get4c(fp);
+            uep->ue_bot = get4c(fp);
+            uep->ue_lcount = get4c(fp);
+            uep->ue_size = get4c(fp);
+            uep->ue_next = NULL;
+            array = (char_u **)U_ALLOC_LINE(
+				 (unsigned)(sizeof(char_u *) * uep->ue_size));
+            for (i = 0; i < uep->ue_size; i++)
+            {
+                line_len = get4c(fp);
+                /* U_ALLOC_LINE provides an extra byte for the NUL terminator.*/
+                line = (char_u *)U_ALLOC_LINE(
+				      (unsigned) (sizeof(char_u) * line_len));
+                if (line == NULL)
+                    goto error;
+                for (j = 0; j < line_len; j++)
+                {
+                    line[j] = getc(fp);
+                }
+                line[j] = '\0';
+                array[i] = line;
+            }
+            uep->ue_array = array;
+            if (found_first_uep == 0)
+            {
+                uhp->uh_entry = uep;
+                found_first_uep = 1;
+            }
+            else
+            {
+                last_uep->ue_next = uep;
+            }
+            last_uep = uep;
+        }
+
+        /* Insertion sort the uhp into the table by its uh_seq. This is
+         * required because, while the number of uhps is limited to
+         * num_heads, and the uh_seq order is monotonic with respect to
+         * creation time, the starting uh_seq can be > 0 if any undolevel
+         * culling was done at undofile write time, and there can be uh_seq
+         * gaps in the uhps.
+         */
+        for (i = num_read_uhps - 1; i >= -1; i--)
+        {
+            /* if i == -1, we've hit the leftmost side of the table, so insert
+             * at uhp_table[0]. */
+            if (i == -1 || uhp->uh_seq > uhp_table[i]->uh_seq)
+            {
+                /* If we've had to move from the rightmost side of the table,
+                 * we have to shift everything to the right by one spot. */
+                if (i < num_read_uhps - 1)
+                {
+                    memmove(uhp_table + i + 2, uhp_table + i + 1,
+                            (num_read_uhps - i) * sizeof(u_header_T *));
+                }
+                uhp_table[i + 1] = uhp;
+                break;
+            }
+            else if (uhp->uh_seq == uhp_table[i]->uh_seq)
+            {
+                EMSG2(_("E826 Undo file corruption: duplicate uh_seq: %s"),
+								   file_name);
+                goto error;
+            }
+        }
+        num_read_uhps++;
+	c = get2c(fp);
+    }
+
+    if (c != UF_END_MAGIC)
+    {
+	EMSG2(_("E827: Undo file corruption; no end marker: %s"), file_name);
+	goto error;
+    }
+
+    /* We've organized all of the uhps into a table sorted by uh_seq. Now we
+     * iterate through the table and swizzle each sequence number we've
+     * stored in uh_foo into a pointer corresponding to the header with that
+     * sequence number. Then free curbuf's old undo structure, give curbuf
+     * the updated {old,new,cur}head pointers, and then free the table. */
+    for (i = 0; i < num_head; i++)
+    {
+        uhp = uhp_table[i];
+        if (uhp == NULL)
+            continue;
+        for (j = 0; j < num_head; j++)
+        {
+            if (uhp_table[j] == NULL)
+                continue;
+            if (uhp_table[j]->uh_seq == (long)uhp->uh_next)
+                uhp->uh_next = uhp_table[j];
+            if (uhp_table[j]->uh_seq == (long)uhp->uh_prev)
+                uhp->uh_prev = uhp_table[j];
+            if (uhp_table[j]->uh_seq == (long)uhp->uh_alt_next)
+                uhp->uh_alt_next = uhp_table[j];
+            if (uhp_table[j]->uh_seq == (long)uhp->uh_alt_prev)
+                uhp->uh_alt_prev = uhp_table[j];
+        }
+        if (old_header_seq > 0 && old_idx < 0 && uhp->uh_seq == old_header_seq)
+            old_idx = i;
+        if (new_header_seq > 0 && new_idx < 0 && uhp->uh_seq == new_header_seq)
+            new_idx = i;
+        if (cur_header_seq > 0 && cur_idx < 0 && uhp->uh_seq == cur_header_seq)
+            cur_idx = i;
+    }
+    u_blockfree(curbuf);
+    curbuf->b_u_oldhead = old_idx < 0 ? 0 : uhp_table[old_idx];
+    curbuf->b_u_newhead = new_idx < 0 ? 0 : uhp_table[new_idx];
+    curbuf->b_u_curhead = cur_idx < 0 ? 0 : uhp_table[cur_idx];
+    curbuf->b_u_line_ptr = line_ptr;
+    curbuf->b_u_line_lnum = line_lnum;
+    curbuf->b_u_line_colnr = line_colnr;
+    curbuf->b_u_numhead = num_head;
+    curbuf->b_u_seq_last = seq_last;
+    curbuf->b_u_seq_cur = seq_cur;
+    curbuf->b_u_seq_time = seq_time;
+    U_FREE_LINE(uhp_table);
+#ifdef U_DEBUG
+    u_check(TRUE);
+#endif
+    if (name != NULL)
+	smsg((char_u *)_("Finished reading undo file %s"), file_name);
+    goto theend;
+
+error:
+    if (line_ptr != NULL)
+        U_FREE_LINE(line_ptr);
+    if (uhp_table != NULL)
+    {
+        for (i = 0; i < num_head; i++)
+        {
+            if (uhp_table[i] != NULL)
+            {
+                uep = uhp_table[i]->uh_entry;
+                while (uep != NULL)
+                {
+                    nuep = uep->ue_next;
+                    u_freeentry(uep, uep->ue_size);
+                    uep = nuep;
+                }
+                U_FREE_LINE(uhp_table[i]);
+            }
+        }
+        U_FREE_LINE(uhp_table);
+    }
+
+theend:
+    if (fp != NULL)
+        fclose(fp);
+    if (file_name != name)
+	vim_free(file_name);
+    return;
+}
+
+/*
+ * Serialize "uep" to "fp".
+ */
+    static int
+serialize_uep(uep, fp)
+    u_entry_T	*uep;
+    FILE	*fp;
+{
+    int i;
+    int uep_len;
+    int *entry_lens;
+
+    if (uep->ue_size > 0)
+        entry_lens = (int *)alloc(uep->ue_size * sizeof(int));
+
+    /* Define uep_len to be the size of the entire uep minus the size of its
+     * component strings, in bytes. The sizes of the component strings
+     * are written before each individual string.
+     * We have 4 entries each of 4 bytes, plus ue_size * 4 bytes
+     * of string size information. */
+
+    uep_len = uep->ue_size * 4;
+    /* Collect sizing information for later serialization. */
+    for (i = 0; i < uep->ue_size; i++)
+    {
+        entry_lens[i] = (int)STRLEN(uep->ue_array[i]);
+        uep_len += entry_lens[i];
+    }
+    put_bytes(fp, (long_u)uep_len, 4);
+    put_bytes(fp, (long_u)uep->ue_top, 4);
+    put_bytes(fp, (long_u)uep->ue_bot, 4);
+    put_bytes(fp, (long_u)uep->ue_lcount, 4);
+    put_bytes(fp, (long_u)uep->ue_size, 4);
+    for (i = 0; i < uep->ue_size; i++)
+    {
+        if (put_bytes(fp, (long_u)entry_lens[i], 4) == FAIL)
+	    return FAIL;
+        fprintf(fp, "%s", uep->ue_array[i]);
+    }
+    if (uep->ue_size > 0)
+        vim_free(entry_lens);
+    return OK;
+}
+
+/*
+ * Serialize "pos" to "fp".
+ */
+    static void
+serialize_pos(pos, fp)
+    pos_T pos;
+    FILE  *fp;
+{
+    put_bytes(fp, (long_u)pos.lnum, 4);
+    put_bytes(fp, (long_u)pos.col, 4);
+#ifdef FEAT_VIRTUALEDIT
+    put_bytes(fp, (long_u)pos.coladd, 4);
+#else
+    put_bytes(fp, (long_u)0, 4);
+#endif
+}
+
+/*
+ * Serialize "info" to "fp".
+ */
+    static void
+serialize_visualinfo(info, fp)
+    visualinfo_T    info;
+    FILE	    *fp;
+{
+    serialize_pos(info.vi_start, fp);
+    serialize_pos(info.vi_end, fp);
+    put_bytes(fp, (long_u)info.vi_mode, 4);
+    put_bytes(fp, (long_u)info.vi_curswant, 4);
+}
+
+static char_u e_not_open[] = N_("E828: Cannot open undo file for writing: %s");
+
+/*
+ * Write the undo tree in an undo file.
+ * When "name" is not NULL, use it as the name of the undo file.
+ * Otherwise use buf->b_ffname to generate the undo file name.
+ * "buf" must never be null, buf->b_ffname is used to obtain the original file
+ * permissions.
+ * "forceit" is TRUE for ":wundo!", FALSE otherwise.
+ * "hash[UNDO_HASH_SIZE]" must be the hash value of the buffer text.
+ */
+    void
+u_write_undo(name, forceit, buf, hash)
+    char_u	*name;
+    int		forceit;
+    buf_T	*buf;
+    char_u	*hash;
+{
+    u_header_T	*uhp;
+    u_entry_T	*uep;
+    char_u	*file_name;
+    int		str_len, i, uep_len, mark;
+    int		fd;
+    FILE	*fp = NULL;
+    int		perm;
+    int		write_ok = FALSE;
+#ifdef UNIX
+    struct stat	st_old;
+    struct stat	st_new;
+#endif
+
+    if (name == NULL)
+    {
+        file_name = u_get_undo_file_name(buf->b_ffname, FALSE);
+	if (file_name == NULL)
+	    return;
+    }
+    else
+        file_name = name;
+
+#ifdef UNIX
+    if (mch_stat((char *)buf->b_ffname, &st_old) >= 0)
+	perm = st_old.st_mode;
+    else
+	perm = 0600;
+#else
+    perm = mch_getperm(buf->b_ffname);
+    if (perm < 0)
+	perm = 0600;
+#endif
+    /* set file protection same as original file, but strip s-bit */
+    perm = perm & 0777;
+
+    /* If the undo file exists, verify that it actually is an undo file, and
+     * delete it. */
+    if (mch_getperm(file_name) >= 0)
+    {
+	if (name == NULL || !forceit)
+	{
+	    /* Check we can read it and it's an undo file. */
+	    fd = mch_open((char *)file_name, O_RDONLY|O_EXTRA, 0);
+	    if (fd < 0)
+	    {
+		if (name != NULL || p_verbose > 0)
+		    smsg((char_u *)_("Will not overwrite with undo file, cannot read: %s"),
+								   file_name);
+		goto theend;
+	    }
+	    else
+	    {
+		char_u buf[2];
+
+		vim_read(fd, buf, 2);
+		close(fd);
+		if ((buf[0] << 8) + buf[1] != UF_START_MAGIC)
+		{
+		    if (name != NULL || p_verbose > 0)
+			smsg((char_u *)_("Will not overwrite, this is not an undo file: %s"),
+								   file_name);
+		    goto theend;
+		}
+	    }
+	}
+	mch_remove(file_name);
+    }
+
+    fd = mch_open((char *)file_name,
+			    O_CREAT|O_EXTRA|O_WRONLY|O_EXCL|O_NOFOLLOW, perm);
+    (void)mch_setperm(file_name, perm);
+    if (fd < 0)
+    {
+        EMSG2(_(e_not_open), file_name);
+	goto theend;
+    }
+    if (p_verbose > 0)
+	smsg((char_u *)_("Writing undo file: %s"), file_name);
+
+#ifdef UNIX
+    /*
+     * Try to set the group of the undo file same as the original file. If
+     * this fails, set the protection bits for the group same as the
+     * protection bits for others.
+     */
+    if (mch_stat((char *)file_name, &st_new) >= 0
+	    && st_new.st_gid != st_old.st_gid
+# ifdef HAVE_FCHOWN  /* sequent-ptx lacks fchown() */
+	    && fchown(fd, (uid_t)-1, st_old.st_gid) != 0
+# endif
+       )
+	mch_setperm(file_name, (perm & 0707) | ((perm & 07) << 3));
+# ifdef HAVE_SELINUX
+    mch_copy_sec(buf->b_ffname, file_name);
+# endif
+#endif
+
+    fp = fdopen(fd, "w");
+    if (fp == NULL)
+    {
+        EMSG2(_(e_not_open), file_name);
+	close(fd);
+	mch_remove(file_name);
+	goto theend;
+    }
+
+    /* Start writing, first overall file information */
+    put_bytes(fp, (long_u)UF_START_MAGIC, 2);
+    put_bytes(fp, (long_u)UF_VERSION, 2);
+
+    /* Write a hash of the buffer text, so that we can verify it is still the
+     * same when reading the buffer text. */
+    if (fwrite(hash, (size_t)UNDO_HASH_SIZE, (size_t)1, fp) != 1)
+	goto write_error;
+    put_bytes(fp, (long_u)buf->b_ml.ml_line_count, 4);
+
+    /* Begin undo data for U */
+    str_len = buf->b_u_line_ptr != NULL ? STRLEN(buf->b_u_line_ptr) : 0;
+    put_bytes(fp, (long_u)str_len, 4);
+    if (str_len > 0 && fwrite(buf->b_u_line_ptr, (size_t)str_len,
+							  (size_t)1, fp) != 1)
+	goto write_error;
+
+    put_bytes(fp, (long_u)buf->b_u_line_lnum, 4);
+    put_bytes(fp, (long_u)buf->b_u_line_colnr, 4);
+
+    /* Begin general undo data */
+    uhp = buf->b_u_oldhead;
+    put_bytes(fp, (long_u)(uhp != NULL ? uhp->uh_seq : 0), 4);
+
+    uhp = buf->b_u_newhead;
+    put_bytes(fp, (long_u)(uhp != NULL ? uhp->uh_seq : 0), 4);
+
+    uhp = buf->b_u_curhead;
+    put_bytes(fp, (long_u)(uhp != NULL ? uhp->uh_seq : 0), 4);
+
+    put_bytes(fp, (long_u)buf->b_u_numhead, 4);
+    put_bytes(fp, (long_u)buf->b_u_seq_last, 4);
+    put_bytes(fp, (long_u)buf->b_u_seq_cur, 4);
+    put_bytes(fp, (long_u)buf->b_u_seq_time, 4);
+
+    /* Iteratively serialize UHPs and their UEPs from the top down.  */
+    mark = ++lastmark;
+    uhp = buf->b_u_oldhead;
+    while (uhp != NULL)
+    {
+        /* Serialize current UHP if we haven't seen it */
+        if (uhp->uh_walk != mark)
+        {
+	    if (put_bytes(fp, (long_u)UF_HEADER_MAGIC, 2) == FAIL)
+		goto write_error;
+
+            put_bytes(fp, (long_u)((uhp->uh_next != NULL)
+					      ? uhp->uh_next->uh_seq : 0), 4);
+            put_bytes(fp, (long_u)((uhp->uh_prev != NULL)
+					      ? uhp->uh_prev->uh_seq : 0), 4);
+            put_bytes(fp, (long_u)((uhp->uh_alt_next != NULL)
+					  ? uhp->uh_alt_next->uh_seq : 0), 4);
+            put_bytes(fp, (long_u)((uhp->uh_alt_prev != NULL)
+					  ? uhp->uh_alt_prev->uh_seq : 0), 4);
+            put_bytes(fp, uhp->uh_seq, 4);
+            serialize_pos(uhp->uh_cursor, fp);
+#ifdef FEAT_VIRTUALEDIT
+            put_bytes(fp, (long_u)uhp->uh_cursor_vcol, 4);
+#else
+            put_bytes(fp, (long_u)0, 4);
+#endif
+            put_bytes(fp, (long_u)uhp->uh_flags, 2);
+            /* Assume NMARKS will stay the same. */
+            for (i = 0; i < NMARKS; ++i)
+            {
+                serialize_pos(uhp->uh_namedm[i], fp);
+            }
+#ifdef FEAT_VISUAL
+            serialize_visualinfo(uhp->uh_visual, fp);
+#endif
+            put_bytes(fp, (long_u)uhp->uh_time, 4);
+
+            uep = uhp->uh_entry;
+            while (uep != NULL)
+            {
+                if (serialize_uep(uep, fp) == FAIL)
+		    goto write_error;
+                uep = uep->ue_next;
+            }
+            /* Sentinel value: no more ueps */
+            uep_len = -1;
+            put_bytes(fp, (long_u)uep_len, 4);
+            uhp->uh_walk = mark;
+        }
+
+        /* Now walk through the tree - algorithm from undo_time */
+        if (uhp->uh_prev != NULL && uhp->uh_prev->uh_walk != mark)
+            uhp = uhp->uh_prev;
+        else if (uhp->uh_alt_next != NULL && uhp->uh_alt_next->uh_walk != mark)
+            uhp = uhp->uh_alt_next;
+        else if (uhp->uh_next != NULL && uhp->uh_alt_prev == NULL
+					     && uhp->uh_next->uh_walk != mark)
+            uhp = uhp->uh_next;
+        else if (uhp->uh_alt_prev != NULL)
+            uhp = uhp->uh_alt_prev;
+        else
+            uhp = uhp->uh_next;
+    }
+
+    if (put_bytes(fp, (long_u)UF_END_MAGIC, 2) == OK)
+	write_ok = TRUE;
+
+write_error:
+    fclose(fp);
+    if (!write_ok)
+        EMSG2(_("E829: write error in undo file: %s"), file_name);
+
+#if defined(MACOS_CLASSIC) || defined(WIN3264)
+    (void)mch_copy_file_attribute(buf->b_ffname, file_name);
+#endif
+#ifdef HAVE_ACL
+    {
+	vim_acl_T	    acl;
+
+	/* For systems that support ACL: get the ACL from the original file. */
+	acl = mch_get_acl(buf->b_ffname);
+	mch_set_acl(file_name, acl);
+    }
+#endif
+
+theend:
+    if (file_name != name)
+	vim_free(file_name);
+}
+
+#endif /* FEAT_PERSISTENT_UNDO */
+
+
 /*
  * If 'cpoptions' contains 'u': Undo the previous undo or redo (vi compatible).
  * If 'cpoptions' does not contain 'u': Always undo.
@@ -757,8 +1556,6 @@
     u_undo_end(undo_undoes, FALSE);
 }
 
-static int lastmark = 0;
-
 /*
  * Undo or redo over the timeline.
  * When "step" is negative go back in time, otherwise goes forward in time.
@@ -927,7 +1724,7 @@
 
 	if (absolute)
 	{
-	    EMSGN(_("Undo number %ld not found"), step);
+	    EMSGN(_("E830: Undo number %ld not found"), step);
 	    return;
 	}
 
diff --git a/src/version.c b/src/version.c
index fe82987..04a33b8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -426,6 +426,11 @@
 #else
 	"-perl",
 #endif
+#ifdef FEAT_PERSISTENT_UNDO
+	"+persistent_undo",
+#else
+	"-persistent_undo",
+#endif
 #ifdef FEAT_PRINTER
 # ifdef FEAT_POSTSCRIPT
 	"+postscript",
diff --git a/src/vim.h b/src/vim.h
index d320d3a..0447273 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1398,6 +1398,9 @@
  */
 #define MAXMAPLEN   50
 
+/* Size in bytes of the hash used in the undo file. */
+#define UNDO_HASH_SIZE 32
+
 #ifdef HAVE_FCNTL_H
 # include <fcntl.h>
 #endif