编辑10/2/16 - 它不再是segfaulting但小写字符没有转换 - 这是我在小写标签中更改的内容：
lowercase:
cmp ax, 'a' ; is it a?
jb slashprint ; jump out if it's below a
cmp ax, 'z' ; is it z?
ja slashprint ; jump out if it's above z
mov eax, [esi] ; move the char to eax to index
mov ebx, mappings ; point to the mappings array
sub eax, 97 ; convert the lower case letter to decimal
mov ax, [ebx + eax] ; indexed addressing done here
jmp done
我正在开发一个程序，允许我从用户那里获取一个字符串，检测转义序列并正确输出转换后的消息。我已经知道我需要清理子程序handle_ESC中的一些逻辑，但我试图一次测试和调试一种情况。我的问题是，每次从子程序返回并将我修改的字符添加到存储在edi中的新字符串（在标签L1_cont的第一行）时，我得到一个段错误。到目前为止，我一直专注于让它接受小写的转义序列。编辑：改变了一些事情。现在，当我输入2个斜杠时，我并没有分裂，但它也没有做我想做的事情。它不会打印任何东西。在查看同一行的小写字母时仍然是segfaulting。随着我所做的改变，它永远不应该达到那个指示。
%define STDIN 0
%define STDOUT 1
%define SYSCALL_EXIT 1
%define SYSCALL_READ 3
%define SYSCALL_WRITE 4
%define BUFLEN 256
SECTION .data ; initialized data section
msg1: db "Enter string: " ; user prompt
len1: equ $-msg1 ; length of first message
msg2: db "Original: " ; original string label
len2: equ $-msg2 ; length of second message
msg3: db "Convert: " ; converted string label
len3: equ $-msg3
msg4: db 10, "Read error", 10 ; error message
len4: equ $-msg4
msg5: db 10, "Error: unknown escape sequence", 10 ; unknown escape error (not sure this works)
len5: equ $-msg5
msg6: db 10, "Error: octal value overflow!", 10 ; overflow error
len6: equ $-msg6
; Declare the array using mappings.
mappings: db 7, 8, -1, -1, -1, 12, -1, -1, -1, -1, -1, -1, -1, 10, -1, -1, -1, 13, -1, 9, -1, 11, -1, -1, -1, -1
SECTION .bss ; uninitialized data section
buf: resb BUFLEN ; buffer for read
newstr: resb BUFLEN ; converted string
rlen: resb 4 ; length
SECTION .text ; Code section.
global _start ; let loader see entry point
_start: nop ; Entry point.
start: ; address for gdb
; prompt user for input
;
mov eax, SYSCALL_WRITE ; write function
mov ebx, STDOUT ; Arg1: file descriptor
mov ecx, msg1 ; Arg2: addr of message
mov edx, len1 ; Arg3: length of message
int 080h ; ask kernel to write
; read user input
;
mov eax, SYSCALL_READ ; read function
mov ebx, STDIN ; Arg 1: file descriptor
mov ecx, buf ; Arg 2: address of buffer
mov edx, BUFLEN ; Arg 3: buffer length
mov [ecx + edx], byte 0 ; null terminate the read buffer
sub edx, 1 ; make sure the buffer length is 1 less than BUFLEN
int 080h
; error check
;
mov [rlen], eax ; save length of string read
cmp eax, 0 ; check if any chars read
jg read_OK ; >0 chars read = OK
mov eax, SYSCALL_WRITE ; ow print error mesg
mov ebx, STDOUT
mov ecx, msg4
mov edx, len4
int 080h
jmp exit ; skip over rest
read_OK:
; Loop for iterating over string
L1_init:
mov esi, buf ; point to start of buffer
mov edi, newstr ; point to start of new string
mov bx, [mappings] ; point to the mappings array
L1_top:
mov al, [esi] ; get a character
inc esi ; point to the next character
cmp al, '\' ; is current char a backslash?
je escapectrl ; if so, run the subroutine
jmp L1_cont ; if not, jump to L1_cont and add the char
escapectrl:
call handle_ESC ; run subroutine
inc edi
cmp [esi], byte 0
je L1_end
jmp L1_top
L1_cont:
mov [edi], al ; store char in new string
inc edi ; update dest pointer
cmp [esi], byte 0 ; if not null, keep looping
je L1_end ; end loop if null
jmp L1_top ; loop if there are more chars
L1_end:
; print out user input for feedback
;
mov eax, SYSCALL_WRITE ; write message
mov ebx, STDOUT
mov ecx, msg2
mov edx, len2
int 080h
mov eax, SYSCALL_WRITE ; write user input
mov ebx, STDOUT
mov ecx, buf
mov edx, [rlen]
int 080h
; print out converted string
;
mov eax, SYSCALL_WRITE ; write message
mov ebx, STDOUT
mov ecx, msg3
mov edx, len3
int 080h
mov eax, SYSCALL_WRITE ; write out string
mov ebx, STDOUT
mov ecx, newstr
mov edx, [rlen]
int 080h
; final exit
;
exit: mov eax, SYSCALL_EXIT ; exit function
mov ebx, 0 ; exit code, 0=normal
int 080h ; ask kernel to take over
; Subroutine begins here.
handle_ESC:
mov di, [esi] ; take a copy of the current char
inc esi ; point to the next char
cmp di, '7' ; is it above 7?
ja lowercase ; if so, skip octal check
cmp di, '0' ; is it below zero?
jb lowercase ; if so, skip octal check
octalinit:
mov bl, 0 ; holds the total for the digit
mov cl, 3 ; count for how many times the loop should run if it's octal
mov dh, 8 ; holds the octal multiple
octalloop:
mov ah, [di] ; store the new char in ah for operations
sub ah, 48 ; convert char to integer
mul dl ; multiply the octal value by 8
add ax, dx ; update the total value
mov di, [esi] ; store the next character in di for consideration
inc esi ; point to the character after that
cmp di, '0' ; is it below 0?
jb lowercase ; if below, jump out
cmp di, '7' ; is it above 7?
ja lowercase ; if above, jump out
dec cl ; count down
jnz octalloop ; loop if needed
dec esi ; undo the prefetch if no longer looping
jo octaloverflow ; if the overflow flag was set, handle the overflow
mov al, bl ; stores the finalized value in al
jmp done ; exit
octaloverflow:
mov [edi], byte 32 ; makes the second char blank due to error
jmp done ; not sure if this part is gonna work right
lowercase:
cmp di, 'a' ; is it a?
jb slashprint ; jump out if it's below a
cmp di, 'z' ; is it z?
ja slashprint ; jump out if it's above z
sub di, 97 ; convert the lower case letter to decimal
mov cl, [bx + di] ; indexed addressing done here
cmp cl, -1 ; is it -1?
mov [edi], cl ; store the converted char in the string
inc esi ; point to the spot after
inc edi ; point to the next entry in the new string
mov [edi], byte 32 ; add a blank space in the new string where the lowercase letter was
jmp done
slashprint:
cmp di, '\' ; is it another slash?
jne errormsg ; jump and print an error message
mov [edi], byte 92 ; store a slash in the new string
inc edi ; move to the next position of the new string
mov [edi], byte 32 ; make the second char blank since both are slashes
jmp done ; return
errormsg:
inc esi ; skip over the character after the slash
done:
ret
; end Subroutine.
答案 0 :(得分：0)
此代码中存在许多错误，但我会突出显示可能导致崩溃的错误：