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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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CORE(5) 							File Formats Manual							   CORE(5)

NAME
core - format of memory image file SYNOPSIS
#include <sys/param.h> DESCRIPTION
The UNIX System writes out a memory image of a terminated process when any of various errors occur. See sigvec(2) for the list of reasons; the most common are memory violations, illegal instructions, bus errors, and user-generated quit signals. The memory image is called `core' and is written in the process's working directory (provided it can be; normal access controls apply). The core file consists of the u. area, whose size (in 64 byte `clicks') is defined by the USIZE manifest in the <sys/param.h> file. The u. area starts with a user structure as given in <sys/user.h>. The rest of the u. area consists of the kernel stack for the terminated process which includes (among other things) the processor registers at the time of the fault; see the system listings for the format of this area. The remainder of the core file consists first of the data pages and then the stack pages of the process image. The amount of data space image in the core file is given (in clicks) by the variable u_dsize in the u. area. If the text segment was not write-only and and shared it is included as the first etext bytes of the data image where etext is taken from the symbol table of the object file which generated the memory image. The amount of stack image in the core file is given (in clicks) by the variable u_ssize in the u. area. In general the debugger adb(1) is sufficient to deal with core images. SEE ALSO
adb(1), sigvec(2), stack(5) 3rd Berkeley Distribution January 26, 1987 CORE(5)
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