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Special Forums UNIX Desktop Questions & Answers Knowing the size and location of variables in a C program Post 302833651 by Cambria on Wednesday 17th of July 2013 02:39:12 PM
Old 07-17-2013
Question Knowing the size and location of variables in a C program

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.

For example how does the computer know that the disassembled instructions from main() are <main+##>? Is there a flag between each variable in memory on the stack? Or does the CPU reference the text segment with the variable in memory to know where a variable begins and ends?

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?

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?

When I use the examine command "x" I don't know how to know where my variable begins and ends. Would it be the $ESP register on the stack minus the word size of my variable? $EIP shows how many bytes from main and the previous instruction when you disassemble something but everything on the stack is just numbers.

Any help would be much appreciated!
 

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Devel::GDB::LowLevel(3pm)				User Contributed Perl Documentation				 Devel::GDB::LowLevel(3pm)

NAME
Devel::GDB::LowLevel - Low-level interface for communicating with GDB DESCRIPTION
This module is used internally by Devel::GDB. It handles the low-level I/O of communicating with the GDB process. CONSTRUCTOR
new Spawns a GDB process. Because this class only facilitates communication with GDB (not with the inferior process being debugged), you have to decide what to do with the "STDIN", "STDOUT", and "STDERR" of that process. There are a few options available: * If STDIN is a tty, we can have the inferior process communicate directly with the controlling tty (emulating the default behavior of gdb): $gdb = new Devel::GDB::LowLevel( '-execfile' => $path_to_gdb, '-params' => $extra_gdb_params ); * Or, we can create an "Expect" object to communicate with the inferior process: $gdb = new Devel::GDB::LowLevel( '-create-expect' => 1 ); $expect = $gdb->get_expect_obj(); * Or, we can create our own tty and use that: $gdb = new Devel::GDB::LowLevel( '-use-tty' => '/dev/pts/123' ); METHODS
send Sends a raw line of text to GDB. This should not contain any newlines (they will be stripped). This method only sends a request, and does not wait for a response. get_reader Returns the file handle from which to read GDB responses. get_expect_obj Returns the "Expect" object created in the constructor. Dies if '-create-expect' was not passed to "new". interrupt Send SIGINT to the GDB session, interrupting the inferior process (if any). SEE ALSO
IPC::Open2 AUTHORS
Antal Novak <afn@cpan.org>, Josef Ezra <jezra@cpan.org> COPYRIGHT AND LICENSE
Copyright (C) 2007 by Antal Novak & Josef Ezra This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself, either Perl version 5.8.8 or, at your option, any later version of Perl 5 you may have available. perl v5.8.8 2008-02-03 Devel::GDB::LowLevel(3pm)
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