Re: How to debug a segmentation fault

On Thu, Nov 09, 2006 at 06:06:11AM -0500, Douglas F. Calvert wrote:
> Hello,
> I am testing out nmap4.20a11 with linux 2.6.19-rc5. I am getting a lot
> of segmentation faults when I scan with:
>
> nmap -sV -O -n -oA basename -PE --version_all --allports
> --randomize_hosts -n -v -v --max_retries 3 --log-errors -d5
> 1.1.14-16.1-254 > nmap.out 2>&1

*snip*
 
> How would I go about figuring out why nmap is dying?

Typically you would compile with nmap debug enabled (the "-g" option
in gcc), then start gdb with the nmap binary.

shell$ gdb nmap

(gdb) set args -sV -O -n -oA basename -PE --version_all --allports (etc)

(gdb) run

When the program segfaults, type "up" to move up the stack frame
until you can see the line of the source code that is causing the
segfault (usually overwriting memory, or NULL pointer or some such).

I like running gdb under Emacs, so I can see the source code
easily, if you're not experienced with Emacs then that probably
isn't a good idea.

HTH, Brett

-- 
Brett Hutley
mailto:brett_at_hutley.net
http://hutley.net/brett/
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