[saagarjha]

macOS, in some sense a BSD (at least nominally), would like you to not make system calls yourself as well. Actually, not linking against libc has a number of hilarious consequences, one of which is that you bypass the platform sandbox because apparently the engineers thought it couldn't be possible to write a program without it :P

[khrbtxyz]

Is there an example somewhere on how to link without libc and make my own syscalls? I tried this a while ago (can't remember which version of macOS it was), fiddling with Csu, nasm etc. but couldn't quite figure it out.

[jmillikin]

https://john-millikin.com/unix-syscalls#darwin is a small, "hello world" example.

[saagarjha]

> Note that I have left out the instructions to statically link binaries because they are documented as unsupported

That's a bit annoying, especially since you're already using raw syscall numbers anyways. Here's how to make it static:

  .intel_syntax noprefix
  
  #include <sys/syscall.h>
  
  #define UNIX_SYSCALL 0x2000000
  
  .globl start
  start:
      mov rax, UNIX_SYSCALL | SYS_write
      mov rdi, 1
      lea rsi, text[rip]
      lea rdx, length
      syscall
      mov rax, UNIX_SYSCALL | SYS_exit
      xor rdi, rdi
      syscall
  
  text:
  .asciz "Hello, world!\n"
  .equ length, . - text

Compile that with clang -static -nostdlib.

[jmillikin]

You don't have to change the source or compile with `clang` -- switching the LD command to:

  ld -arch x86_64 -o hello hello.o -macosx_version_min 10.8 -static -e _main

is sufficient if you're determined to violate the OS vendor's compatibility requirements.

[saagarjha]

That works too, but I'm lazy :P

[saagarjha]

-static -nostdlib, and make sure you have an entry point set.