Can you find the memory for a local character array in a function using objdump?

Question

If I define a local character array within a function and then use objdump to grab the assembly code for that particular function, can I find the memory for that array within the assembly code?

This is a question I have for a homework assignment.

Answer 1

Sure, as long as your array has a non-zero initializer, you should be able to find it. Here's an example I made for ARM:

char function(int i)
{
    char arr[] = { 1, 2, 3, 4, 5, 6, 7, 8 };
    return arr[i];
}

Build it:

$ clang -O2 -Wall -c -o example.o example.c

Disassemble the output:

$ objdump -d example.o

example.o:     file format elf32-littlearm


Disassembly of section .text:

00000000 <function>:
   0:   e59f1004    ldr r1, [pc, #4]    ; c <function+0xc>
   4:   e7d10000    ldrb    r0, [r1, r0]
   8:   e12fff1e    bx  lr
   c:   00000000    .word   0x00000000

Hmm - notice that .word 0x0000000 at offset 0xc? That's going to be fixed up by the linker to point to the array. Let's go check out the relocation table:

$ objdump -r example.o 

example.o:     file format elf32-littlearm

RELOCATION RECORDS FOR [.text]:
OFFSET   TYPE              VALUE 
00000008 R_ARM_V4BX        *ABS*
0000000c R_ARM_ABS32       .rodata.cst8

Aha! The word at 0xc is going to get fixed up with an absolute pointer to the .rodata.cst8 section - that sounds like what we want. Let's take a peek:

$ objdump -s -j .rodata.cst8  example.o 

example.o:     file format elf32-littlearm

Contents of section .rodata.cst8:
 0000 01020304 05060708                    ........        

And there you have the contents of the array!