Help using dbx

[Mark Rosenthal]

I posted the following article some weeks ago.  I fear it may have dropped
into the bit-bucket, so I am reposting it.

It appears that most vendors have given up on 'sdb' in favor of 'dbx' or
some derivative of 'dbx'.  I find that although 'dbx' is generally more
powerful than 'sdb' it lacks some capabilities that I used to depend on quite
heavily.  If anyone is familiar with how to do the following with 'dbx',
please let me know.  You will earn my undying gratitude.

Note that I have not used sdb in over a year.  The examples I give below are
from my own faulty memory and may not be 100% accurate.

By the way, I am running 'dbx' on a Sun workstation.

1.  I'd like to be able to specify the format in which a variable is to be
    displayed.  If I have a variable (char *cp), I could do the following
    with 'sdb':

	me:	cp/x		or:	cp/s		or:	cp/
	sdb:	0x7fff3ec		string			string

    With 'dbx', the dialogue is something like:

	me:	print cp
	dbx:	0x7fff3ec
	me:	0x7fff3ec/s
	dbx:	string

    Having to type back into dbx what it just typed at me is a pain.

2.  The man page for 'dbx' claims that the command to display a memory location
    is:

	[address] / [count] [mode]

    where everything but the "/" is optional, and that "if no address is
    specified, the address following the one displayed most recently is used".
    Unfortunately, there is also a "/" command which searches the source file
    for a string.  Typing "/" reports:

	"No saved search string"

    rather than showing me the next location.  This makes it quite a chore to
    examine an array.  Arrays of structures are even worse.

3.  With 'sdb', I could display lines of code relative to the current line.
    'dbx' appears to limit me to specifying absolute line numbers.  This means
    that anytime I hit a breakpoint and want to see some context, I have to
    read the current line number from the screen, do some quick arithmetic,
    and type "list from_line,to_line".  The concentration required to do that
    is a distraction from the problem I am trying to solve, namely finding and
    fixing bugs in a program.  A symbolic way of referencing dbx's line counter
    would solve the problem, but I can find no mention of any such capability
    in the documentation.  I'd like to be able to do something like:

	alias w list .-5,.+5

    and then just type "w" to see context around the current line.

4.  A common construct in C is a linked list of structures.

	struct foo
	{
		struct foo *next;
		int	data;
		char	*moredata;
		/* Etc., etc., etc. */
	};

	struct foo *foobar;

    Let's say my code malloc()s instances of foo, and links them in to the list
    beginning at foobar.  I am at a breakpoint, and I want to examine the list.

    I don't remember the exact sequence I used with sdb, but it went something
    like:

	me:	foobar/		show the value of foobar
	sdb?:	0x7ff2ea
	me:	.->/		show what the last displayed value points at
				    (i.e. foobar->next)
	sdb?:	0x7e3720
	me:	+/d		show the next location in decimal
				    (i.e. foobar->data)
	sdb?:	3
	me:	+/s		show the next location as a string pointer
				    (i.e. foobar->moredata)
	sdb?:	string data
	me:	-		back up one location (to foobar->data)
	sdb?:	3
	me:	-		back up one location (to foobar->next)
	sdb?:	0x7e3720
	me:	.->/		show what the last displayed value points at
				    (i.e. foobar->next->next)
	sdb?:	0x748238
	me:	+/d		show the next location in decimal
				    (i.e. foobar->next->data)
	sdb?:	2
	me:	+/s		show the next location as a string pointer
				    (i.e. foobar->next->moredata)
	sdb?:	more chars
	me:	-		back up one location (to foobar->next->data)
	sdb?:	2
	me:	-		back up one location (to foobar->next->next)
	sdb?:	0x748238
	me:	.->/		show what the last displayed value points at
				    (i.e. foobar->next->next->next)
	sdb?:	0		end of list
	me:	+/d		show the next location in decimal
				    (i.e. foobar->next->next->data)
	sdb?:	1
	me:	+/s		show the next location as a string pointer
				    (i.e. foobar->next->next->moredata)
	sdb?:	still more chars

    To do the same thing with dbx, the best I've been able to come up with
    so far is:

	me:	print foobar
	dbx:	0x7ff2ea
	me:	print foobar->data
	dbx:	3
	me:	print foobar->moredata
	dbx:	string data
	me:	print foobar->next
	dbx:	0x7e3720
	me:	print foobar->next->data
	dbx:	2
	me:	print foobar->next->moredata
	dbx:	more chars
	me:	print foobar->next->next
	dbx:	0x748238
	me:	print foobar->next->next->data
	dbx:	1
	me:	print foobar->next->next->moredata
	dbx:	still more chars
	me:	print foobar->next->next->next
	dbx:	0		end of list

    Note that the sequence of commands I type to sdb is relatively simple,
    and stays the same no matter how far through the list I've gone:

	me:	foobar/
	me:	.->/
	me:	+/d
	me:	+/s
	me:	-
	me:	-
	me:	.->/
	me:	+/d
	me:	+/s
	me:	-
	me:	-
	me:	.->/
	me:	+/d
	me:	+/s

    whereas with dbx, I have to retype the entire sequence from the starting
    point every time:

	me:	print foobar
	me:	print foobar->data
	me:	print foobar->moredata
	me:	print foobar->next
	me:	print foobar->next->data
	me:	print foobar->next->moredata
	me:	print foobar->next->next
	me:	print foobar->next->next->data
	me:	print foobar->next->next->moredata
	me:	print foobar->next->next->next

    This makes a list of length 4 a pain to deal with, and a list of length 16
    prohibitive.  Linked lists with hundreds of elements are not uncommon.
    Do the authors of 'dbx' really intend that I should have to type:

print foobar->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->next->next->next->next->next->next->next->next->next->next->next->next->
next->data

    to get to the 128'th (or did I only type next 127 times?) item in a list?
    There MUST be a better way.  Can somebody PLEASE enlighten me.

Sorry.  I got a bit carried away there.  I'll be alright in a second.

-- 

	Mark of the Valley of Roses
	...!{decvax,linus,ima,ihnp4}!bbncca!aoa!mbr
	...!{wjh12,mit-vax}!biomed!aoa!mbr