CODEWITHSUNDEEP
cgcc
Quantico
Quantico

Reputation: 2446

How to find compiled variable/function address from debug symbols

I found the following post (How to generate gcc debug symbol outside the build target?) on how to split a the compiled file and the debugging symbols.

However, I cannot find any useful information in the debugging file.

For example,

My helloWorld code is: 

#include<stdio.h>

int main(void) {

    int a;
    a = 5;
    printf("The memory address of a is: %p\n", (void*) &a);
    return 0;
}

I ran gcc -g -o hello hello.c

objcopy --only-keep-debug hello hello.debug
gdb -s main.debug -e main

In gdb, anything I tried won't give me any information on a, I cannot find its address, I cannot find the main function address

For example :

(gdb) info variables
All defined variables:

Non-debugging symbols:
0x0000000000400618  _IO_stdin_used
0x0000000000400710  __FRAME_END__
0x0000000000600e3c  __init_array_end
0x0000000000600e3c  __init_array_start
0x0000000000600e40  __CTOR_LIST__
0x0000000000600e48  __CTOR_END__
0x0000000000600e50  __DTOR_LIST__
0x0000000000600e58  __DTOR_END__
0x0000000000600e60  __JCR_END__
0x0000000000600e60  __JCR_LIST__
0x0000000000600e68  _DYNAMIC
0x0000000000601000  _GLOBAL_OFFSET_TABLE_
0x0000000000601028  __data_start
0x0000000000601028  data_start
0x0000000000601030  __dso_handle
0x0000000000601038  __bss_start
0x0000000000601038  _edata
0x0000000000601038  completed.6603
0x0000000000601040  dtor_idx.6605
0x0000000000601048  _end

Am I doing something wrong? Am I understanding the debug file incorrectly? Is there even a way to find out an address of compiled variable/function from a saved debugging information?

Upvotes: 4

Views: 9950

Answers (1)

jforberg
jforberg

Reputation: 6762

int a is a stack variable so it does not have a fixed address unless you are in a call to that specific function. Furthermore, each call to that function will allocate its own variable.

When we say "debugging symbols" we usually mean functions and global variables. A local variable is not a "symbol" in this context. In fact, if you compile with optimisations enabled int a would almost certainly be optimised to a register variable so it would not have an address at all, unless you forced it to be written to memory by doing some_function(&a) or similar.

You can find the address of main just by writing print main in GDB. This is because functions are implicitly converted to pointers in C when they appear in value context, and GDB's print uses C semantics.

Upvotes: 3

Related Questions