Should this be "if (fname != NULL)"?  Some other callers of
adjustname() check for a NULL pointer.  strlen(NULL) would crash by
SIGSEGV; strlen("") would return 0, because "" is a non-NULL pointer
to an ASCII NUL '\0'.

I agree. Amended diff below.

Index: def.h
===================================================================
RCS file: /cvs/src/usr.bin/mg/def.h,v
retrieving revision 1.168
diff -u -p -r1.168 def.h
--- def.h       1 Mar 2021 10:51:14 -0000       1.168
+++ def.h       19 Mar 2021 06:02:05 -0000
@@ -363,6 +363,7 @@ int          ask_makedir(void);

 /* dired.c */
 struct buffer  *dired_(char *);
+int             dired_jump(int, int);
 int             do_dired(char *);

 /* file.c X */
Index: dired.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/dired.c,v
retrieving revision 1.98
diff -u -p -r1.98 dired.c
--- dired.c     5 Mar 2021 16:16:53 -0000       1.98
+++ dired.c     19 Mar 2021 06:02:05 -0000
@@ -53,6 +53,7 @@ static int     d_refreshbuffer(int, int);
 static int      d_filevisitalt(int, int);
 static int      d_gotofile(int, int);
 static void     reaper(int);
+static int      gotofile(char*);
 static struct buffer   *refreshbuffer(struct buffer *);
 static int      createlist(struct buffer *);
 static void     redelete(struct buffer *);
@@ -1091,13 +1092,38 @@ createlist(struct buffer *bp)
 }

 int
+dired_jump(int f, int n)
+{
+       char             dname[NFILEN], *fname;
+       struct buffer   *bp;
+       int              ret;
+
+       if (getbufcwd(dname, sizeof(dname)) != TRUE)
+               return (FALSE);
+
+       fname = curbp->b_fname;
+
+       if ((bp = dired_(dname)) == NULL)
+               return (FALSE);
+       curbp = bp;
+
+       ret = showbuffer(bp, curwp, WFFULL | WFMODE);
+       if (ret != TRUE)
+               return ret;
+
+       fname = adjustname(fname, TRUE);
+       if (fname != NULL)
+               gotofile(fname);
+
+       return (TRUE);
+}
+
+int
 d_gotofile(int f, int n)
 {
-       struct line     *lp, *nlp;
        size_t           lenfpath;
-       char             fpath[NFILEN], fname[NFILEN];
-       char            *p, *fpth, *fnp = NULL;
-       int              tmp;
+       char             fpath[NFILEN];
+       char            *fpth, *fnp = NULL;

        if (getbufcwd(fpath, sizeof(fpath)) != TRUE)
                fpath[0] = '\0';
@@ -1114,8 +1140,18 @@ d_gotofile(int f, int n)
                ewprintf("No file to find");  /* Current directory given so  */
                return (TRUE);                  /* return at present location. 
*/
        }
+       return gotofile(fpth);
+}
+
+int
+gotofile(char *fpth)
+{
+       struct line     *lp, *nlp;
+       char             fname[NFILEN];
+       char            *p;
+       int              tmp;
+
        (void)xbasename(fname, fpth, NFILEN);
-       curbp = curwp->w_bufp;
        tmp = 0;
        for (lp = bfirstlp(curbp); lp != curbp->b_headp; lp = nlp) {
                tmp++;
Index: funmap.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/funmap.c,v
retrieving revision 1.59
diff -u -p -r1.59 funmap.c
--- funmap.c    11 Jul 2019 18:20:18 -0000      1.59
+++ funmap.c    19 Mar 2021 06:02:05 -0000
@@ -98,6 +98,7 @@ static struct funmap functnames[] = {
        {desckey, "describe-key-briefly", 1},
        {diffbuffer, "diff-buffer-with-file", 0},
        {digit_argument, "digit-argument", 1},
+       {dired_jump, "dired-jump", 1},
        {lowerregion, "downcase-region", 0},
        {lowerword, "downcase-word", 0},
        {showversion, "emacs-version", 0},
Index: keymap.c
===================================================================
RCS file: /cvs/src/usr.bin/mg/keymap.c,v
retrieving revision 1.58
diff -u -p -r1.58 keymap.c
--- keymap.c    29 Dec 2015 19:44:32 -0000      1.58
+++ keymap.c    19 Mar 2021 06:02:05 -0000
@@ -129,7 +129,8 @@ static PF cXcB[] = {
        ctrlg                   /* ^G */
 };

-static PF cXcL[] = {
+static PF cXcJ[] = {
+       dired_jump,             /* ^J */
        lowerregion,            /* ^L */
        rescan,                 /* ^M */
        rescan,                 /* ^N */
@@ -198,7 +199,7 @@ struct KEYMAPE (6) cXmap = {
                        CCHR('B'), CCHR('G'), cXcB, NULL
                },
                {
-                       CCHR('L'), CCHR('X'), cXcL, NULL
+                       CCHR('J'), CCHR('X'), cXcJ, NULL
                },
                {
                        '(', ')', cXlp, NULL
Index: mg.1
===================================================================
RCS file: /cvs/src/usr.bin/mg/mg.1,v
retrieving revision 1.120
diff -u -p -r1.120 mg.1
--- mg.1        23 Feb 2021 18:45:33 -0000      1.120
+++ mg.1        19 Mar 2021 06:02:05 -0000
@@ -198,6 +198,8 @@ list-buffers
 save-buffers-kill-emacs
 .It C-x C-f
 find-file
+.It C-x C-j
+dired-jump
 .It C-x C-g
 keyboard-quit
 .It C-x C-l
@@ -523,6 +525,8 @@ Display the name of the function current
 View the differences between buffer and its associated file.
 .It digit-argument
 Process a numerical argument for keyboard-invoked functions.
+.It dired-jump
+Open a dired buffer containing the current buffer's directory location.
 .It downcase-region
 Set all characters in the region to lower case.
 .It downcase-word

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