CODEWITHSUNDEEP
assemblygdb
Eric
Eric

Reputation: 24944

Gdb - print array or array element, with element of various size

Assembly code (yasm):

section .data
src db 1,2,3

Each element of src array is 1 byte.

In GDB, how to print the whole array or an element at specified index, e.g print the element with value 2.

Upvotes: 5

Views: 6524

Answers (1)

Eric
Eric

Reputation: 24944

(Ok, I'd like to put an answer by myself, with @Michael Petch's permission, to make it more clear for future searcher)

The code & answer are for x86-64 linux system, variables are defined in assembly the .data section.

Code

tmp.asm (yasm)

; yasm assembly program,
; compile: yasm -f elf64 -g dwarf2 tmp.asm && ld tmp.o
; execute: ./a.out

section .data
a db 1
b dw 2
c dd 4
d dq 0x1234567890abcde

arr_a db 1,2,3,4
arr_b dw 1,2,3,4
arr_c dd 1,2,3,4
arr_d dq 1,2,3,0x1234567890abcde

section .text
global _start

_start:
    mov rax,1

    ; exit
    mov eax,1
    mov ebx,5
    int 0x80

Print array or array element in GDB

In GDB:

  • p command will treat a variable as 4 byte;
  • x command will treat a variable as 8 byte.

So, need different ways to print a number of size 1 / 2 / 4 / 8 bytes.

Examples commands:

  • p for 4 bytes.
    • p/x c, print c as 4 byte, this is default.
  • p with cast, for 1 / 2 byte
    • p/x (char)a, print a as 1 byte
    • p/x (short)b, print b as 2 byte
  • x for 8 bytes
    • x/x &d, print d as 8 byte, this is default.
  • x for 1 / 2 / 4 bytes
    • x/bx &a, print a as 1 byte,
    • x/hx &b, print b as 2 byte,
    • x/wx &c, print c as 4 byte,
  • p for array
    • p/x arr_c@4, print arr_c as array of length 4, contain elements of 4 byte, this is default.
    • p/x (char[4])arr_a, print arr_a as array of length 4, contain elements of 1 byte,
    • p/x (short[4])arr_b, print arr_b as array of length 4, contain elements of 2 byte,
    • p/x (long[4])arr_d, print arr_d as array of length 4, contain elements of 8 byte,
  • p for individual array element
    • p/x ((long[4])arr_d)[1], treat arr_d as array of length 4, contain elements of 8 byte, and print the element with index 1.

Tips:

  • It seems p could print 8 byte array, but can't print individual 8 byte value correctly (which is not part of array). In that case, use x instead.

(This is just tested on my machine, if there is any issue, please feel free to correct it.)

Upvotes: 5

Related Questions