C pointer principle - AT&T assembly

The standard assembler of Linux platform is GAS, which is the background assembly tool relied on by GCC, and is usually included in binutils package,
--gstabs tells the assembler to add a symbol table to the generated object code. First, we complete the assembly:
as -gstabs -o hello.o hello.s
The target code generated by assembler must be processed by linker to generate executable code. Linux uses ld as the standard linker, which is also included in binutils package. We then link:
 ld -o hello hello.o
With the symbol table, we can debug.
Run it first to see the effect:

GDB Do as LINUX An important debugging tool for programmers, which is also applicable to programs written in assembly, we use GDB Perform some simple debugging operations on the above code
//First open the hello program:
 gdb hello
GNU gdb (GDB) 7.1-ubuntu
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-linux-gnu".
For bug reporting instructions, please see:
Reading symbols from /home/deepfuture-lx/private/mytest/hello...done.
//Then, we can use the list command to list the source code
(gdb) list
1 .section .data#Initialized variables
2 output:
3   .ascii "hello,world\n"
4   #The string to print,. Data is the variable of the initialization value. output is the label, indicating the start position of the string, ascii is the data type
5 .section .bss#Uninitialized variable, buffer filled by 0
6   .lcomm num,20
7   #lcomm is the local memory area, that is, it can't be accessed outside the local assembly. . comm is the general memory area.
8 .section .text#Assembly language instruction code
9   .globl _start#Start entry
10   _start:
//Use the break command to set breakpoints
(gdb) break 17
Breakpoint 1 at 0x4000c6: file hello.s, line 17.
//Run to breakpoint
(gdb) run
Starting program: /home/deepfuture-lx/private/mytest/hello 
Breakpoint 1, _start () at hello.s:17
//Continue with next statement
17   movl $0,%eax
(gdb) next
18   movl $num,%edi
//Display the values of all registers
(gdb) info registers
rax            0x0 0
rbx            0x1 1
rcx            0x60011c 6291740
rdx            0xc 12
rsi            0x0 0
rdi            0x0 0
rbp            0x0 0x0
rsp            0x7fffffffe2d0 0x7fffffffe2d0
r8             0x0 0
r9             0x0 0
r10            0x0 0
r11            0x0 0
r12            0x0 0
r13            0x0 0
r14            0x0 0
r15            0x0 0
rip            0x4000cb 0x4000cb <_start+27>
eflags         0x202 [ IF ]
cs             0x33 51
ss             0x2b 43
ds             0x0 0
es             0x0 0
fs             0x0 0
---Type <return> to continue, or q <return> to quit---
gs             0x0 0
(gdb) next

19   movl $65,1(%edi)#ascii of A

//Output the value of edi register in hexadecimal format. /x is hexadecimal, / d is hexadecimal, / t is binary
(gdb) print/x $rdi
$3 = 0x600128
//Continue operation
(gdb) next
20   movl $66,2(%edi)#ascii of B
//Display the value of a memory location, x / N Y z, where n is the number of fields, y is the format (c is character, d is decimal, x is hexadecimal), z is the field length (b is byte, n is 16 bit word, w is 32-bit word)
(gdb) next
21   movl $67,3(%edi)#ascii of C
(gdb) x/3cb &num
0x600128 <num>: 0 '\000' 65 'A' 66 'B'
(gdb) next
22   movl $68,4(%edi)#ascii of D
(gdb) next
23   movl $10,5(%edi)#ascii of \n
(gdb) next
25   movl $4,%eax#The system function called is write
(gdb) x/4cb &num
0x600128 <num>: 0 '\000' 65 'A' 66 'B' 67 'C'
//Quit gdb

Keywords: C++ ascii Linux Linker Ubuntu

Added by blogger3 on Fri, 06 Dec 2019 15:30:17 +0200