Show current assembly instruction in GDB

Question

I'm doing some assembly-level debugging in GDB. Is there a way to get GDB to show me the current assembly instruction in the same way that it shows the current source line? The default output after every command looks like this:

0x0001433f      990         Foo::bar(p);

This gives me the address of the current instruction, but I have to keep referring back to the output of disassemble in order to see which instruction I'm currently executing.

Answer 1

You can switch to assembly layout in GDB:

(gdb) layout asm

See here for more information. The current assembly instruction will be shown in assembler window.

   ┌───────────────────────────────────────────────────────────────────────────┐
   │0x7ffff740d756 <__libc_start_main+214>  mov    0x39670b(%rip),%rax        #│
   │0x7ffff740d75d <__libc_start_main+221>  mov    0x8(%rsp),%rsi              │
   │0x7ffff740d762 <__libc_start_main+226>  mov    0x14(%rsp),%edi             │
   │0x7ffff740d766 <__libc_start_main+230>  mov    (%rax),%rdx                 │
   │0x7ffff740d769 <__libc_start_main+233>  callq  *0x18(%rsp)                 │
  >│0x7ffff740d76d <__libc_start_main+237>  mov    %eax,%edi                   │
   │0x7ffff740d76f <__libc_start_main+239>  callq  0x7ffff7427970 <exit>       │
   │0x7ffff740d774 <__libc_start_main+244>  xor    %edx,%edx                   │
   │0x7ffff740d776 <__libc_start_main+246>  jmpq   0x7ffff740d6b9 <__libc_start│
   │0x7ffff740d77b <__libc_start_main+251>  mov    0x39ca2e(%rip),%rax        #│
   │0x7ffff740d782 <__libc_start_main+258>  ror    $0x11,%rax                  │
   │0x7ffff740d786 <__libc_start_main+262>  xor    %fs:0x30,%rax               │
   │0x7ffff740d78f <__libc_start_main+271>  callq  *%rax                       │
   └───────────────────────────────────────────────────────────────────────────┘
multi-thre process 3718 In: __libc_start_main     Line: ??   PC: 0x7ffff740d76d
#3  0x00007ffff7466eb5 in _IO_do_write () from /lib/x86_64-linux-gnu/libc.so.6
#4  0x00007ffff74671ff in _IO_file_overflow ()
   from /lib/x86_64-linux-gnu/libc.so.6
#5  0x0000000000408756 in ?? ()
#6  0x0000000000403980 in ?? ()
#7  0x00007ffff740d76d in __libc_start_main ()
   from /lib/x86_64-linux-gnu/libc.so.6
(gdb)

Answer 2

You can do

display/i $pc

and every time GDB stops, it will display the disassembly of the next instruction.

GDB-7.0 also supports set disassemble-next-line on , which will disassemble the entire next line, and give you more of the disassembly context.

Answer 3

The command

x/i $pc

can be set to run all the time using the usual configuration mechanism.

Answer 4

Setting the following option:

set  disassemble-next-line on
show disassemble-next-line

Will give you results that look like this:

(gdb) stepi
0x000002ce in ResetISR () at startup_gcc.c:245
245 {
   0x000002cc <ResetISR+0>: 80 b5   push    {r7, lr}
=> 0x000002ce <ResetISR+2>: 82 b0   sub sp, #8
   0x000002d0 <ResetISR+4>: 00 af   add r7, sp, #0
(gdb) stepi
0x000002d0  245 {
   0x000002cc <ResetISR+0>: 80 b5   push    {r7, lr}
   0x000002ce <ResetISR+2>: 82 b0   sub sp, #8
=> 0x000002d0 <ResetISR+4>: 00 af   add r7, sp, #0

Answer 5

If you want the next few instructions to display automatically while stepping through the program you can use the display command as follows -

display /3i $pc

The above will display 3 instructions whenever a breakpoint is hit or when you single step the program.

More details at the blog entry here .

Answer 6

GDB Dashboard

https://github.com/cyrus-and/gdb-dashboard

This GDB configuration uses the official GDB Python API to show us whatever we want whenever GDB stops after for example next , much like TUI.

However I have found that this implementation is a more robust and configurable alternative to the built-in GDB TUI mode as explained at: gdb split view with code

For example, we can configure GDB Dashboard to show disassembly, source, registers and stack with:

dashboard -layout source assembly registers stack

Here is what it looks like if you enable all available views instead:

Related questions:

• Switching to assembly in gdb

Answer 7

From within gdb press Ctrl x 2 and the screen will split into 3 parts.

First part will show you the normal code in high level language.

Second will show you the assembly equivalent and corresponding instruction Pointer .

Third will present you the normal gdb prompt to enter commands.

Answer 8

有一个简单的解决方案是使用stepi ，它stepi向前移动 1 个 asm 指令并显示周围的 asm 代码。