c - 在 gdb 中查看 ASCII 格式的寄存器内容

Question

假设，我现在在这个位置，想看ascii格式的cl,ch,cx,ecx等内容。

怎么办？

下面是layout asm命令中显示的子程序。

RemCharCodeFromAToB:
    ; standard entry sequence
    push    ebp             ; save the previous value of ebp for the benefi$
    mov     ebp, esp        ; copy esp -> ebp so that ebp can be used as a $   

    ; accessing arguments  
                            ; [ebp + 0] = old ebp stack frame
                            ; [ebp + 4] = return address
    mov     edx, [ebp + 8]  ; string address

    while_loop_rcc:
        mov cl, [edx]       ; obtain the address of the 1st character of the string
        cmp cl, 0           ; check the null value  

        je  while_loop_exit_rcc     ; exit if the null-character is reached

        mov al, cl ; save cl
        mov cl, [ebp + 16]      ; end-char
        push cx                 ; push end-char
        mov cl, [ebp + 12]      ; start-char
        push cx                 ; push start-char
        push ax;                ; push ch
        call IsBetweenAandB
        add esp, 12

        cmp eax, 0          ; if(ch is not between 'a' and 'e')

        je inner_loop_exit_rcc

        mov eax, edx    ; copy the current address

        inner_loop_rcc:
            mov cl, [eax+1]
            cmp cl, 0
            je  inner_loop_exit_rcc

            mov [eax], cl

            inc eax
            jmp inner_loop_rcc
        inner_loop_exit_rcc:

        inc edx             ; increment the address
        jmp while_loop_rcc  ; start the loop again
    while_loop_exit_rcc:

    ; standard exit sequence
    mov     esp, ebp        ; restore esp with ebp
    pop     ebp             ; remove ebp from stack
    ret                     ; return the value of temporary variable

• 完整的汇编代码 is here .
• C++ 代码 is here .
• 生成文件 is here .

Answer 1

这将定义一个 asciiprint 宏，它将其参数寄存器的值打印为字符序列，从最低有效字节到最高有效字节。

(gdb) define asciiprint
 set $len = sizeof($arg0)
 set $val = (unsigned long long)($arg0)
 while $len-- > 0
   set $char = $val & 0xff
   if $char >= 0x20 && $char <= 0x7e
     printf "%c", $char
   else
     printf "\\x%02x", $char
   end
   set $val >>= 8
 end
 printf "\n"
end

(gdb) set $rcx=0x6b63616a70616c66
(gdb) asciiprint $rcx
flapjack
(gdb) asciiprint $ecx
flap
(gdb) asciiprint $cx
fl
(gdb) asciiprint $cl
f
(gdb) asciiprint $ch
l