How to get this simple assembly to run? - macos

Question

This is my first ever attempt at programming with assembly. I'm using a 64 bit Mac OS. I'm also using NASM. I've done a lot of looking around for a solution, but I can't find anything that works for my machine.
Can anyone help me solve this problem? Here is the code and error, thanks!
hello.asm
global start
section .text
start:
mov rax, 1
mov rdi, 1
mov rsi, message
mov rdx, 13
syscall
mov eax, 60
xor rdi, rdi
syscall
message:
db "Hello, World", 10
my attempt at executing:
nasm -f macho64 hello.asm -o hello.o
ld -arch i386 -o hello hello.o
./hello
the error
ld: warning: -macosx_version_min not specified, assuming 10.10
ld: warning: ignoring file hello.o, file was built for unsupported file format ( 0xCF 0xFA 0xED 0xFE 0x07 0x00 0x00 0x01 0x03 0x00 0x00 0x00 0x01 0x00 0x00 0x00 ) which is not the architecture being linked (i386): hello.o
Undefined symbols for architecture i386:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture i386

Answer 1

The reason for your linker error is that you created a 64-bit macho object using NASM, but then targeted i386 for the executable. What you likely were after was a 64-bit executable, which could be done by removing -arch like this:
ld -o hello hello.o
As for your segfault when running your program, it seems that you likely followed a tutorial that may have been designed for Linux. OS/X isn't base upon Linux, it derived from BSD so the Syscalls are different. We could tell you were using Linux Syscalls because syscall 1 is sys_write and sys_exit is rax = 60. This unfortunately isn't the same for OS/X. In 64-bit OS/X code sys_exit is rax=0x20000001 and sys_write is rax=0x20000004 .
Your code would have to be changed to:
global start
section .data
message: db "Hello, World", 10
section .text
start:
mov rax, 0x20000004
mov rdi, 1
mov rsi, message
mov rdx, 13
syscall
mov rax, 0x20000001
xor rdi, rdi
syscall
You'll also observe I explicitly declared a .data section and placed your variable in it. In some environments it may cause problems if data variables are placed in the code.
If creating 32-bit code on OS/X (you aren't in this case) the Syscalls have 0x20000000 subtracted from each. So in 32-bit OS/X code sys_exit is eax=0x1 and sys_write is eax=0x4 .
A reference for all the Syscalls (and their parameters) on OS/X can be found in this Apple information. Just add 0x20000000 to each number in the first column of the chart for 64-bit assembler code.
You probably want to find a 64-bit OS/X tutorial about Syscalls. This is a simple one