section .text
global _start
_start:
nop
main:
mov eax, 1
mov ebx, 2
xor eax, eax
ret
I compile with these commands:
nasm -f elf main.asm
ld -melf_i386 -o main main.o
When I run the code, Linux throw a segmentation fault error
(I am using Linux Mint Nadia 64 bits). Why this error is produced?
Because ret is NOT the proper way to exit a program in Linux, Windows, or Mac!!!!
_start is not a function, there is no return address on the stack because there is no user-space caller to return to. Execution in user-space started here (in a static executable), at the process entry point. (Or with dynamic linking, it jumped here after the dynamic linker finished, but same result).
On Linux / OS X, the stack pointer is pointing at argc on entry to _start (see the i386 or x86-64 System V ABI doc for more details on the process startup environment); the kernel puts command line args into user-space stack memory before starting user-space. (So if you do try to ret, EIP/RIP = argc = a small integer, not a valid address. If your debugger shows a fault at address 0x00000001 or something, that's why.)
For Windows it is ExitProcess and Linux is is system call -
int 80H using sys_exit, for x86 or using syscall using 60 for 64Bit or a call to exit from the C Library if you are linking to it.
32 bit Linux
mov eax, sys_exit ; sys_exit = 1
xor ebx, ebx
int 80H
64 bit Linux
mov rax, 60
xor rdi, rdi
syscall
Windows
push 0
call ExitProcess
Or Windows/Linux linking against the C Library
call exit
exit (unlike a raw exit system call or libc _exit) will flush stdio buffers first. If you used printf from _start, use exit to make sure all output is printed before you exit, even if stdout is redirected to a file (making stdout full-buffered, not line-buffered).
It's generally recommended that if you use libc functions, you write a main function and link with gcc so it's called by the normal CRT start functions which you can ret to.
Defining main as something that _start falls through into doesn't make it special, it's just confusing to use a main label if it's not like a C main function called by a _start that's prepared to exit after main returns.
