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Full Discussion: Knowing the size and location of variables in a C program
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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LEARN ABOUT DEBIAN

msp430-run

run(1)								     GNU Tools								    run(1)

NAME

       run--Simulator front-end

SYNOPSIS

       run    [-v] [-p freq] [-m memory] [--sysroot filepath] program

DESCRIPTION

       Use `run program' to execute a binary by interpreting machine instructions on your host computer.

       run is the same emulator used by GDB's `target sim' command.  You can run it directly by executing run if you just want to see your program
       execute, and do not need any debugger functionality.  You can also use run to generate profiling information for analysis with gprof.

OPTIONS

       -v     Verbose output.  Display the name of the program to run before execution; after execution, display the number of	instructions  exe-
	      cuted, the number of machine cycles emulated, the number of pipeline stalls, the real time taken, the emulated execution time taken,
	      and a summary of how much profiling information was generated.

       -p freq
	      Generate profile information (for use with gprof).  freq is the profiling frequency.  Write the  profiling  information  to  a  file
	      called gmon.out.

       -m memory
	      Set  the	memory	size  for the emulated machine to two to the power memory.  The default value is 19, emulating a board with 524288
	      bytes of memory.

       --sysroot filepath
	      Prepend filepath to all simulator system calls that pass absolute file paths.  Change  working  directory  to  filepath  at  program
	      start.  Not all simulators support this option; those that don't, will ignore it.

SEE ALSO

       `gprof' entry in info; `gdb' entry in info; Using GDB: A Guide to the GNU Source-Level Debugger, Richard M. Stallman and Roland H. Pesch.

COPYING

       Copyright (c) 1993, 2000 Free Software Foundation, Inc.

       This  document is distributed under the terms of the GNU Free Documentation License, version 1.1.  That license is described in the sources
       for this manual page, but it is not displayed here in order to make this manual more consise.  Copies of this license can also be  obtained
       from: http://www.gnu.org/copyleft/.

GNU Tools							     13oct1993								    run(1)
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