用户输入和输出在我的汇编代码中不起作用

Question

The following program compiles without errors, but when run it doesn't prompt for any input and nothing prints. What's the problem, and how can I fix it?

I use these commands to assemble and link:

/usr/local/bin/nasm -f macho32 $1
ld -macosx_version_min 10.9.0 -lSystem -o run $filename.o -e _start -lc

My code is:

section .data
    ;New line string
    NEWLINE: db 0xa, 0xd
    LENGTH: equ $-NEWLINE

section .bss    
INPT: resd 1

section .text   
global _start
_start:


;Read character
mov eax, 0x3
mov ebx, 0x1
mov ecx, INPT
mov edx, 0x1
int 80h

;print character
mov eax, 0x4
mov ebx, 0x1
mov ecx, INPT
mov edx, 0x1
int 80h

;Print new line after the output 
mov eax, 0x4
mov ebx, 0x1
mov ecx, NEWLINE
mov edx, LENGTH
int 0x80

;Terminate
mov eax, 0x1
xor ebx, ebx
int 0x80

Answer 1

There are signs in your code that you may have been using a Linux tutorial when producing code for OS/X(BSD). Linux and OS/X have differing SYSCALL calling conventions. In OS/X 32-bit programs int 0x80 requires parameters (except the syscall in EAX ) to be passed on a stack.

The important things to be aware of with 32-bit SYSCALL s via int 0x80 on OS/X are:

• arguments passed on the stack, pushed right-to-left
• you must allocate an additional 4 bytes (a DWORD ) on the stack after you push all the arguments
• syscall number in the eax register
• call by interrupt 0x80

After pushing arguments on the stack in reverse order for int 0x80 you must allocate an additional 4 bytes (a DWORD ) on the stack. The value in that memory location on the stack doesn't matter. This requirement is an artifact from an old UNIX convention .

A list of the SYSCALL numbers and their parameters can be found in the APPLE header files . You'll need these SYSCALL s:

 1 AUE_EXIT ALL { void exit(int rval); } 3 AUE_NULL ALL { user_ssize_t read(int fd, user_addr_t cbuf, user_size_t nbyte); } 4 AUE_NULL ALL { user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte); }

I have commented some example code that would be similar in functionality to what you may have been attempting to achieve:

section .data
    ;New line string
    NEWLINE: db 0xa, 0xd
    LENGTH: equ $-NEWLINE

section .bss
    INPT: resd 1

global _start

section .text
_start:
    and     esp, -16      ; Make sure stack is 16 byte aligned at program start
                          ;     not necessary in this example since we don't call 
                          ;     external functions that conform to the OS/X 32-bit ABI

    push    dword 1       ; Read 1 character
    push    dword INPT    ; Input buffer
    push    dword 0       ; Standard input = FD 0
    mov     eax, 3        ; syscall sys_read
    sub     esp, 4        ; Extra 4 bytes on stack needed by int 0x80
    int     0x80
    add     esp, 16       ; Restore stack

    push    dword 1       ; Print 1 character
    push    dword INPT    ; Output buffer = buffer we read characters into
    push    dword 1       ; Standard output = FD 1
    mov     eax, 4        ; syscall sys_write
    sub     esp, 4        ; Extra 4 bytes on stack needed by int 0x80
    int     0x80
    add     esp, 16       ; Restore stack

    push    dword LENGTH  ; Number of characters to write
    push    dword NEWLINE ; Write the data in the NEWLINE string
    push    dword 1       ; Standard output = FD 1
    mov     eax, 4        ; syscall sys_write
    sub     esp, 4        ; Extra 4 bytes on stack needed by int 0x80
    int     0x80
    add     esp, 16       ; Restore stack

    push    dword 0       ; Return value from program = 0
    mov     eax, 1        ; syscall sys_exit
    sub     esp, 4        ; Extra 4 bytes on stack needed by int 0x80
    int     0x80

The and esp, -16 is only necessary if you need to align the stack to a 16-byte boundary as a baseline for future stack operations. If you intend to call external functions that conform to the OS/X 32-bit ABI the stack is expected to be 16-byte aligned immediately preceding a function CALL . This alignment is not necessary for system calls via int 0x80 .

You should be able to assemble and link it with:

nasm -f macho32 test.asm -o test.o
ld -macosx_version_min 10.9.0 -o test test.o -e _start -lSystem

And run it with:

./test