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Special Forums UNIX Desktop Questions & Answers Knowing the size and location of variables in a C program Post 302833697 by Corona688 on Wednesday 17th of July 2013 04:30:15 PM
Old 07-17-2013
Quote:
Originally Posted by Cambria
So I need some help with this. Pardon me if I'm posting in the wrong forum, after some googling for my answer and finding nothing I found this forum. It seemed appropriate for what I was seeking. I just didnt find a forum that concerned the use of GDB. I'm learning to use the C language and GDB. What I don't understand is how the computer knows how big each piece of a program is in memory, and how I could find my variable's in memory using GDB.
To get nice debugging information like that, you have to build the executable with debugging information(i.e. -ggdb). This embeds lots of offsets and labels inside the program file for gdb's convenience.

This is also why gdb has trouble when it steps into code outside your program, like libc... Libraries are probably not built with debugging information, so details about their insides will be very limited.

Quote:
I mean if all memory is numbered how can anyone including the CPU know where a word or giant or w/e starts and ends?
To put it bluntly -- it doesn't. They all become hardcoded segment offsets, in the end. Without debugging information, you're left with detective work.

Quote:
If I wanted to find my variable in memory after setting a break point in it and accessing the $esp register how would I know where my variables began and ended?
If your executable wasn't built with debugging info, that'd mean detective work.

Last edited by Corona688; 07-17-2013 at 05:38 PM..
 

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SHELL-QUOTE(1p) 					User Contributed Perl Documentation					   SHELL-QUOTE(1p)

NAME
shell-quote - quote arguments for safe use, unmodified in a shell command SYNOPSIS
shell-quote [switch]... arg... DESCRIPTION
shell-quote lets you pass arbitrary strings through the shell so that they won't be changed by the shell. This lets you process commands or files with embedded white space or shell globbing characters safely. Here are a few examples. EXAMPLES
ssh preserving args When running a remote command with ssh, ssh doesn't preserve the separate arguments it receives. It just joins them with spaces and passes them to "$SHELL -c". This doesn't work as intended: ssh host touch 'hi there' # fails It creates 2 files, hi and there. Instead, do this: cmd=`shell-quote touch 'hi there'` ssh host "$cmd" This gives you just 1 file, hi there. process find output It's not ordinarily possible to process an arbitrary list of files output by find with a shell script. Anything you put in $IFS to split up the output could legitimately be in a file's name. Here's how you can do it using shell-quote: eval set -- `find -type f -print0 | xargs -0 shell-quote --` debug shell scripts shell-quote is better than echo for debugging shell scripts. debug() { [ -z "$debug" ] || shell-quote "debug:" "$@" } With echo you can't tell the difference between "debug 'foo bar'" and "debug foo bar", but with shell-quote you can. save a command for later shell-quote can be used to build up a shell command to run later. Say you want the user to be able to give you switches for a command you're going to run. If you don't want the switches to be re-evaluated by the shell (which is usually a good idea, else there are things the user can't pass through), you can do something like this: user_switches= while [ $# != 0 ] do case x$1 in x--pass-through) [ $# -gt 1 ] || die "need an argument for $1" user_switches="$user_switches "`shell-quote -- "$2"` shift;; # process other switches esac shift done # later eval "shell-quote some-command $user_switches my args" OPTIONS
--debug Turn debugging on. --help Show the usage message and die. --version Show the version number and exit. AVAILABILITY
The code is licensed under the GNU GPL. Check http://www.argon.org/~roderick/ or CPAN for updated versions. AUTHOR
Roderick Schertler <roderick@argon.org> perl v5.8.4 2005-05-03 SHELL-QUOTE(1p)
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