todd had his panties in a wad about backwards compatibility so I lifted
the ksh history load code out of ksh to dump it in a text file.  Compile
like:
"cc kshconv.c -o kshconv"

then run it like:
"./kshconv -i ~/.hist -o texthist"

Conversion code:
=================================== 8< ===================================
#include <stdio.h>
#include <stdlib.h>
#include <err.h>
#include <sys/types.h>
#include <sys/mman.h>

#define HMAGIC1         0xab
#define HMAGIC2         0xcd
#define COMMAND         0xff

extern char             *__progname;

typedef enum state {
        shdr,           /* expecting a header */
        sline,          /* looking for a null byte to end the line */
        sn1,            /* bytes 1 to 4 of a line no */
        sn2, sn3, sn4
} State;

void
histload(FILE *f, unsigned char *base, int bytes)
{
        State           state;
        int             lno = 0;
        unsigned char   *line = NULL;

        for (state = shdr; bytes-- > 0; base++) {
                switch (state) {
                case shdr:
                        if (*base == COMMAND)
                                state = sn1;
                        break;
                case sn1:
                        lno = (((*base)&0xff)<<24);
                        state = sn2;
                        break;
                case sn2:
                        lno |= (((*base)&0xff)<<16);
                        state = sn3;
                        break;
                case sn3:
                        lno |= (((*base)&0xff)<<8);
                        state = sn4;
                        break;
                case sn4:
                        lno |= (*base)&0xff;
                        line = base+1;
                        state = sline;
                        break;
                case sline:
                        if (*base == '\0') {
                                fprintf(f, "%s\n", line);
                                state = shdr;
                        }
                }
        }
}
int
main(int argc, char *argv[])
{
        int             c;
        char            *in;
        char            *out;
        FILE            *fin;
        FILE            *fout;
        off_t           hsize;
        unsigned char   *base;

        while ((c = getopt(argc, argv, "i:o:")) != -1) {
                switch (c) {
                case 'i':
                        in = optarg;
                        break;
                case 'o':
                        out = optarg;
                        break;
                default:
                        errx(1, "%s -i file -o file", __progname);
                }
        }

        if (in == NULL || out == NULL)
                errx(1, "%s -i file -o file", __progname);

        if ((fin = fopen(in, "r")) == NULL)
                err(1, "%s", in);
        if ((fout = fopen(out, "w+")) == NULL)
                err(1, "%s", out);

        hsize = lseek(fileno(fin), 0L, SEEK_END);
        if (hsize == 0)
                errx(1, "nothing to do");
        else if (hsize > 0) {
                /*
                 * we have some data
                 */
                base = (unsigned char *)mmap(0, hsize, PROT_READ,
                    MAP_FILE|MAP_PRIVATE, fileno(fin), 0);
                /*
                 * check on its validity
                 */
                if (base == MAP_FAILED || *base != HMAGIC1 || base[1] != 
HMAGIC2) {
                        if (base != MAP_FAILED)
                                munmap((caddr_t)base, hsize);
                        goto done;
                }

                histload(fout, base+2, hsize-2);
                munmap((caddr_t)base, hsize);
        }
done:
        fclose(fin);
        fclose(fout);
}
=================================== >8 ===================================

V5 of diff with millert's suggestion to remove mmap.h.

Index: alloc.c
===================================================================
RCS file: /cvs/src/bin/ksh/alloc.c,v
retrieving revision 1.8
diff -u -p -r1.8 alloc.c
--- alloc.c     21 Jul 2008 17:30:08 -0000      1.8
+++ alloc.c     30 Aug 2011 18:05:47 -0000
@@ -62,7 +62,7 @@ alloc(size_t size, Area *ap)
 {
        struct link *l;
 
-       l = malloc(sizeof(struct link) + size);
+       l = calloc(1, sizeof(struct link) + size);
        if (l == NULL)
                internal_errorf(1, "unable to allocate memory");
        l->next = ap->freelist;
Index: history.c
===================================================================
RCS file: /cvs/src/bin/ksh/history.c,v
retrieving revision 1.39
diff -u -p -r1.39 history.c
--- history.c   19 May 2010 17:36:08 -0000      1.39
+++ history.c   1 Sep 2011 13:49:13 -0000
@@ -11,8 +11,7 @@
  *     a)      the original in-memory history  mechanism
  *     b)      a more complicated mechanism done by  p...@hillside.co.uk
  *             that more closely follows the real ksh way of doing
- *             things. You need to have the mmap system call for this
- *             to work on your system
+ *             things.
  */
 
 #include "sh.h"
@@ -20,21 +19,11 @@
 
 #ifdef HISTORY
 # include <sys/file.h>
-# include <sys/mman.h>
 
-/*
- *     variables for handling the data file
- */
-static int     histfd;
-static int     hsize;
-
-static int hist_count_lines(unsigned char *, int);
-static int hist_shrink(unsigned char *, int);
-static unsigned char *hist_skip_back(unsigned char *,int *,int);
-static void histload(Source *, unsigned char *, int);
-static void histinsert(Source *, int, unsigned char *);
-static void writehistfile(int, char *);
-static int sprinkle(int);
+static void    writehistfile(FILE *);
+static FILE    *history_open(int *);
+static int     history_load(FILE *, Source *);
+static void    history_close(FILE *);
 
 static int     hist_execute(char *);
 static int     hist_replace(char **, const char *, const char *, int);
@@ -45,8 +34,8 @@ static void   histbackup(void);
 static char   **current;       /* current position in history[] */
 static char    *hname;         /* current name of history file */
 static int     hstarted;       /* set after hist_init() called */
-static Source  *hist_source;
-
+static Source  *hist_source;
+static struct stat last_sb;
 
 int
 c_fc(char **wp)
@@ -529,15 +518,10 @@ sethistfile(const char *name)
        /* if the name is the same as the name we have */
        if (hname && strcmp(hname, name) == 0)
                return;
-
        /*
         * its a new name - possibly
         */
-       if (histfd) {
-               /* yes the file is open */
-               (void) close(histfd);
-               histfd = 0;
-               hsize = 0;
+       if (hname) {
                afree(hname, APERM);
                hname = NULL;
                /* let's reset the history */
@@ -577,18 +561,26 @@ init_histvec(void)
 void
 histsave(int lno, const char *cmd, int dowrite)
 {
-       char **hp;
-       char *c, *cp;
+       char            **hp;
+       char            *c, *cp;
+       int             changed;
+       FILE            *f = NULL;
+
+       if (dowrite) {
+               f = history_open(&changed);
+               if (f && changed) {
+                       /* reset history */
+                       histptr = history - 1;
+                       hist_source->line = 0;
+                       history_load(f, hist_source);
+               }
+       }
 
        c = str_save(cmd, APERM);
        if ((cp = strchr(c, '\n')) != NULL)
                *cp = '\0';
 
-       if (histfd && dowrite)
-               writehistfile(lno, c);
-
        hp = histptr;
-
        if (++hp >= history + histsize) { /* remove oldest command */
                afree((void*)*history, APERM);
                for (hp = history; hp < history + histsize - 1; hp++)
@@ -596,371 +588,125 @@ histsave(int lno, const char *cmd, int d
        }
        *hp = c;
        histptr = hp;
-}
-
-/*
- *     Write history data to a file nominated by HISTFILE
- *     if HISTFILE is unset then history still happens, but
- *     the data is not written to a file
- *     All copies of ksh looking at the file will maintain the
- *     same history. This is ksh behaviour.
- *
- *     This stuff uses mmap()
- *     if your system ain't got it - then you'll have to undef HISTORYFILE
- */
 
-/*
- *     Open a history file
- *     Format is:
- *     Bytes 1, 2: HMAGIC - just to check that we are dealing with
- *                 the correct object
- *     Then follows a number of stored commands
- *     Each command is
- *     <command byte><command number(4 bytes)><bytes><null>
- */
-#define HMAGIC1                0xab
-#define HMAGIC2                0xcd
-#define COMMAND                0xff
+       if (dowrite && f) {
+               writehistfile(f);
+               history_close(f);
+       }
+}
 
-void
-hist_init(Source *s)
+static FILE *
+history_open(int *changed)
 {
-       unsigned char   *base;
-       int     lines;
-       int     fd;
-
-       if (Flag(FTALKING) == 0)
-               return;
-
-       hstarted = 1;
-
-       hist_source = s;
-
-       hname = str_val(global("HISTFILE"));
-       if (hname == NULL)
-               return;
-       hname = str_save(hname, APERM);
+       int             fd;
+       FILE            *f = NULL;
+       struct stat     sb;
 
-  retry:
-       /* we have a file and are interactive */
-       if ((fd = open(hname, O_RDWR|O_CREAT|O_APPEND, 0600)) < 0)
-               return;
-
-       histfd = savefd(fd);
-       if (histfd != fd)
+       if ((fd = open(hname, O_RDWR | O_CREAT | O_EXLOCK, 0600)) == -1)
+               return (NULL);
+       f = fdopen(fd, "r+");
+       if (f == NULL) {
                close(fd);
+               goto bad;
+       }
 
-       (void) flock(histfd, LOCK_EX);
+       if (fstat(fileno(f), &sb) == -1)
+               goto bad;
+       if (timespeccmp(&sb.st_mtim, &last_sb.st_mtim, ==))
+               *changed = 0;
+       else
+               *changed = 1;
 
-       hsize = lseek(histfd, 0L, SEEK_END);
+       return (f);
+bad:
+       if (f)
+               fclose(f);
 
-       if (hsize == 0) {
-               /* add magic */
-               if (sprinkle(histfd)) {
-                       hist_finish();
-                       return;
-               }
-       }
-       else if (hsize > 0) {
-               /*
-                * we have some data
-                */
-               base = (unsigned char *)mmap(0, hsize, PROT_READ,
-                   MAP_FILE|MAP_PRIVATE, histfd, 0);
-               /*
-                * check on its validity
-                */
-               if (base == MAP_FAILED || *base != HMAGIC1 || base[1] != 
HMAGIC2) {
-                       if (base != MAP_FAILED)
-                               munmap((caddr_t)base, hsize);
-                       hist_finish();
-                       if (unlink(hname) != 0)
-                               return;
-                       goto retry;
-               }
-               if (hsize > 2) {
-                       lines = hist_count_lines(base+2, hsize-2);
-                       if (lines > histsize) {
-                               /* we need to make the file smaller */
-                               if (hist_shrink(base, hsize))
-                                       if (unlink(hname) != 0)
-                                               return;
-                               munmap((caddr_t)base, hsize);
-                               hist_finish();
-                               goto retry;
-                       }
-               }
-               histload(hist_source, base+2, hsize-2);
-               munmap((caddr_t)base, hsize);
-       }
-       (void) flock(histfd, LOCK_UN);
-       hsize = lseek(histfd, 0L, SEEK_END);
+       return (NULL);
 }
 
-typedef enum state {
-       shdr,           /* expecting a header */
-       sline,          /* looking for a null byte to end the line */
-       sn1,            /* bytes 1 to 4 of a line no */
-       sn2, sn3, sn4
-} State;
+static void
+history_close(FILE *f)
+{
+       fflush(f);
+       fstat(fileno(f), &last_sb);
+       fclose(f);
+}
 
 static int
-hist_count_lines(unsigned char *base, int bytes)
+history_load(FILE *f, Source *s)
 {
-       State state = shdr;
-       int lines = 0;
+       char            *p, line[LINE + 1];
+       uint32_t        i;
 
-       while (bytes--) {
-               switch (state) {
-               case shdr:
-                       if (*base == COMMAND)
-                               state = sn1;
+       /* just read it all; will auto resize history upon next command */
+       for (i = 1; ; i++) {
+               p = fgets(line, sizeof line, f);
+               if (p == NULL || feof(f) || ferror(f))
                        break;
-               case sn1:
-                       state = sn2; break;
-               case sn2:
-                       state = sn3; break;
-               case sn3:
-                       state = sn4; break;
-               case sn4:
-                       state = sline; break;
-               case sline:
-                       if (*base == '\0')
-                               lines++, state = shdr;
+               if ((p = strchr(line, '\n')) == NULL) {
+                       bi_errorf("history file is corrupt");
+                       return (1);
                }
-               base++;
+               *p = '\0';
+
+               s->line = i;
+               s->cmd_offset = i;
+               histsave(i, (char *)line, 0);
        }
-       return lines;
+
+       return (0);
 }
 
-/*
- *     Shrink the history file to histsize lines
- */
-static int
-hist_shrink(unsigned char *oldbase, int oldbytes)
+void
+hist_init(Source *s)
 {
-       int fd;
-       char    nfile[1024];
-       struct  stat statb;
-       unsigned char *nbase = oldbase;
-       int nbytes = oldbytes;
-
-       nbase = hist_skip_back(nbase, &nbytes, histsize);
-       if (nbase == NULL)
-               return 1;
-       if (nbase == oldbase)
-               return 0;
-
-       /*
-        *      create temp file
-        */
-       (void) shf_snprintf(nfile, sizeof(nfile), "%s.%d", hname, procpid);
-       if ((fd = open(nfile, O_CREAT | O_TRUNC | O_WRONLY, 0600)) < 0)
-               return 1;
+       FILE            *f = NULL;
+       int             changed;
 
-       if (sprinkle(fd)) {
-               close(fd);
-               unlink(nfile);
-               return 1;
-       }
-       if (write(fd, nbase, nbytes) != nbytes) {
-               close(fd);
-               unlink(nfile);
-               return 1;
-       }
-       /*
-        *      worry about who owns this file
-        */
-       if (fstat(histfd, &statb) >= 0)
-               fchown(fd, statb.st_uid, statb.st_gid);
-       close(fd);
+       if (Flag(FTALKING) == 0)
+               return;
 
-       /*
-        *      rename
-        */
-       if (rename(nfile, hname) < 0)
-               return 1;
-       return 0;
-}
+       hstarted = 1;
 
+       hist_source = s;
 
-/*
- *     find a pointer to the data `no' back from the end of the file
- *     return the pointer and the number of bytes left
- */
-static unsigned char *
-hist_skip_back(unsigned char *base, int *bytes, int no)
-{
-       int lines = 0;
-       unsigned char *ep;
+       hname = str_val(global("HISTFILE"));
+       if (hname == NULL)
+               return;
+       hname = str_save(hname, APERM);
 
-       for (ep = base + *bytes; --ep > base; ) {
-               /* this doesn't really work: the 4 byte line number that is
-                * encoded after the COMMAND byte can itself contain the
-                * COMMAND byte....
-                */
-               for (; ep > base && *ep != COMMAND; ep--)
-                       ;
-               if (ep == base)
-                       break;
-               if (++lines == no) {
-                       *bytes = *bytes - ((char *)ep - (char *)base);
-                       return ep;
-               }
-       }
-       return NULL;
-}
+       f = history_open(&changed);
+       if (f == NULL)
+               return;
 
-/*
- *     load the history structure from the stored data
- */
-static void
-histload(Source *s, unsigned char *base, int bytes)
-{
-       State state;
-       int     lno = 0;
-       unsigned char   *line = NULL;
-
-       for (state = shdr; bytes-- > 0; base++) {
-               switch (state) {
-               case shdr:
-                       if (*base == COMMAND)
-                               state = sn1;
-                       break;
-               case sn1:
-                       lno = (((*base)&0xff)<<24);
-                       state = sn2;
-                       break;
-               case sn2:
-                       lno |= (((*base)&0xff)<<16);
-                       state = sn3;
-                       break;
-               case sn3:
-                       lno |= (((*base)&0xff)<<8);
-                       state = sn4;
-                       break;
-               case sn4:
-                       lno |= (*base)&0xff;
-                       line = base+1;
-                       state = sline;
-                       break;
-               case sline:
-                       if (*base == '\0') {
-                               /* worry about line numbers */
-                               if (histptr >= history && lno-1 != s->line) {
-                                       /* a replacement ? */
-                                       histinsert(s, lno, line);
-                               }
-                               else {
-                                       s->line = lno;
-                                       s->cmd_offset = lno;
-                                       histsave(lno, (char *)line, 0);
-                               }
-                               state = shdr;
-                       }
-               }
-       }
+       history_load(f, s);
+       history_close(f);
 }
 
-/*
- *     Insert a line into the history at a specified number
- */
 static void
-histinsert(Source *s, int lno, unsigned char *line)
+writehistfile(FILE *f)
 {
-       char **hp;
+       int             i;
+       char            *cmd;
 
-       if (lno >= s->line-(histptr-history) && lno <= s->line) {
-               hp = &histptr[lno-s->line];
-               if (*hp)
-                       afree((void*)*hp, APERM);
-               *hp = str_save((char *)line, APERM);
-       }
-}
+       if (ftruncate(fileno(f), 0) == -1)
+               return;
+       rewind(f);
 
-/*
- *     write a command to the end of the history file
- *     This *MAY* seem easy but it's also necessary to check
- *     that the history file has not changed in size.
- *     If it has - then some other shell has written to it
- *     and we should read those commands to update our history
- */
-static void
-writehistfile(int lno, char *cmd)
-{
-       int     sizenow;
-       unsigned char   *base;
-       unsigned char   *new;
-       int     bytes;
-       unsigned char   hdr[5];
-
-       (void) flock(histfd, LOCK_EX);
-       sizenow = lseek(histfd, 0L, SEEK_END);
-       if (sizenow != hsize) {
-               /*
-                *      Things have changed
-                */
-               if (sizenow > hsize) {
-                       /* someone has added some lines */
-                       bytes = sizenow - hsize;
-                       base = (unsigned char *)mmap(0, sizenow,
-                           PROT_READ, MAP_FILE|MAP_PRIVATE, histfd, 0);
-                       if (base == MAP_FAILED)
-                               goto bad;
-                       new = base + hsize;
-                       if (*new != COMMAND) {
-                               munmap((caddr_t)base, sizenow);
-                               goto bad;
-                       }
-                       hist_source->line--;
-                       histload(hist_source, new, bytes);
-                       hist_source->line++;
-                       lno = hist_source->line;
-                       munmap((caddr_t)base, sizenow);
-                       hsize = sizenow;
-               } else {
-                       /* it has shrunk */
-                       /* but to what? */
-                       /* we'll give up for now */
-                       goto bad;
-               }
+       for (i = 0; i < histsize; i++) {
+               cmd = history[i];
+               if (cmd == NULL)
+                       break;
+               if (fprintf(f, "%s\n", cmd) == -1)
+                       return;
        }
-       /*
-        *      we can write our bit now
-        */
-       hdr[0] = COMMAND;
-       hdr[1] = (lno>>24)&0xff;
-       hdr[2] = (lno>>16)&0xff;
-       hdr[3] = (lno>>8)&0xff;
-       hdr[4] = lno&0xff;
-       (void) write(histfd, hdr, 5);
-       (void) write(histfd, cmd, strlen(cmd)+1);
-       hsize = lseek(histfd, 0L, SEEK_END);
-       (void) flock(histfd, LOCK_UN);
-       return;
-bad:
-       hist_finish();
 }
 
 void
 hist_finish(void)
 {
-       (void) flock(histfd, LOCK_UN);
-       (void) close(histfd);
-       histfd = 0;
 }
-
-/*
- *     add magic to the history file
- */
-static int
-sprinkle(int fd)
-{
-       static unsigned char mag[] = { HMAGIC1, HMAGIC2 };
-
-       return(write(fd, mag, 2) != 2);
-}
-
 #else /* HISTORY */
 
 /* No history to be compiled in: dummy routines to avoid lots more ifdefs */

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