从NASM调用C函数_printf会导致分段错误

Question

I've been trying to learn 64-bit assembly on both Mac-OS and Windows using NASM.

My code is

extern _printf

section .data
    msg db "Hello World!", 10, 0

section .text
    global _main

_main:
    mov rax, 0
    mov rdi, msg
    call _printf

    mov rax, 0x2000001
    mov rdi, 0
    syscall

and I compile it with

nasm -f macho64 -o main.o main.asm 
gcc -o main main.o

While trying to call _printf , I got the error

Segmentation fault: 11

When I remove the call to _printf , my code runs fine. Why does the call to _printf cause a segmentation fault?

I have found the ABI Calling Convention here , but had no luck successfully calling a C-function.

I would expect Hello World! to be printedd, however I get 'Segmentation Fault: 11' instead.

Answer 1

~~You need to setup a stack frame before calling _printf

TL;DR: The System V AMD64 ABI requires the stack-pointer to be 16-byte-aligned. By the time of calling _printf , the stack-pointer is misaligned by 8 bytes.

Debugging the binary with LLDB gives:

frame #0: 0x00007fff527d430a libdyld.dylib`stack_not_16_byte_aligned_error

MacOS uses the System V AMD64 ABI and therefore relies on 16-byte alignment for the stack-pointer ( see this question ), in short this means the stack-pointer ( rsp ) should always be divisible by 16 when calling a function.

By the time of calling _printf , the stack-pointer ( rsp ) is misaligned by 8 bytes. How did this come?

I found the answer on this page , calling the _main function pushes the return address (8 bytes) on the stack and therefore misaligns it.

My initial idea - the setup of the stack frame - pushed another address on the stack and therefore rsp was divisible by 16 again.

However an easier solution would be just sub rsp, 8 as suggested by Margaret Bloom

Change your code to:

extern _printf

section .data
    msg: db "Hello World!", 10, 0

section .text
    global _main

_main:
    ;; Fix the stack alignment
    sub rsp, 8
    mov rax, 0
    mov rdi, msg
    call _printf

    mov rax, 0x2000001
    mov rdi, 0
    syscall

Tested on macOS 10.13.6