I am new to assembly language programing, and here I am trying to call C standard library function puts from my assembly code, but I am continuously getting segmentaion fault. Please help; Operating system : LINUX 16.04 Assembler : nasm Machine : intel x86 – 64bit
;comiple and build: ; nasm -f elf64 -F stabs helloc.asm ; gcc -o helloC helloC.o [SECTION .data] msg: db "Hello C",0 [SECTION .bss] [SECTION .text] extern puts global main main: push rsp push dword msg call puts add rsp,4 pop rsp ret
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Answer
to explain Comments More, start with x86 calling convention and your code.
x86 Calling Convention
In x86, arguments are located in stack. So basically your function call is x86 way. for example, If you build your code for x86,
[SECTION .data] msg: db "Hello C",0 [SECTION .bss] [SECTION .text] extern puts global main main: push ebp mov ebp, esp and esp, 0xfffffff0 sub esp, 0x10 mov DWORD PTR [esp], msg call puts mov esp, ebp pop ebp ret
It may works fine.
x86-64 Calling Convention
Main difference is two things.
- using 8 bytes to represent address, of course
- use 6 registeres (rdi, rsi, rdx, rcx, r8, r9) for represent first 6 arguments (rest is located in stack)
so first, you should change push dword msg
to mov rdi, msg
, and don’t clean stack after call (because you didn’t push anything to stack)
after change:
[SECTION .data] msg: db "Hello C",0 [SECTION .bss] [SECTION .text] extern puts global main main: push rbp mov rbp, rsp and rsp, 0xfffffffffffffff0 mov rdi, msg call puts mov rsp, rbp pop rbp ret
EDIT: from System V ABI, for call instruction stack be should 16-byte aligned. so push rbp
has effect to alignment, but it is not correct purpose to use. to change that, make stack save logic for both x86 and x86-64.