binpref.py

import ctypes
import os
import sys

libSystem = ctypes.CDLL('libSystem.dylib')

# `typedef  void *posix_spawnattr_t;` on darwin
attrp = ctypes.c_void_p()
e = libSystem.posix_spawnattr_init(ctypes.byref(attrp))
print('spawnattr init ret:', e)

count = ctypes.c_size_t(1)
ocount = ctypes.c_size_t()

CPU_ARCH_ABI64    = 0x01000000 # 64 bit ABI
CPU_ARCH_ABI64_32 = 0x02000000 # ABI for 64-bit hardware with 32-bit types; LP32
CPU_TYPE_ARM      = 12
CPU_TYPE_ARM64    = CPU_TYPE_ARM | CPU_ARCH_ABI64

pref = (ctypes.c_int * 1)(CPU_TYPE_ARM64)
e = libSystem.posix_spawnattr_setbinpref_np(ctypes.byref(attrp), count, pref, ctypes.byref(ocount))
print('binpref ret:', e, 'ocount:', ocount)

spawn = libSystem.posix_spawnp
spawn.restype = ctypes.c_int
spawn.argtypes = (
        ctypes.POINTER(ctypes.c_int),
        ctypes.c_char_p, ctypes.c_void_p, ctypes.c_void_p,
        ctypes.POINTER(ctypes.c_char_p),
        ctypes.POINTER(ctypes.c_char_p)
)

def charpp(a): return (ctypes.c_char_p * len(a))(*a)

argv = list(map(ctypes.c_char_p, b'git -c core.deltaBaseCacheLimit=2g clone --no-checkout --progress https://chromium.googlesource.com/chromium/src.git _gclient_src_mmdd8v1w'.split())) + [None]
envp = [ctypes.c_char_p(('%s=%s' % (k, os.environ[k])).encode('utf-8')) for k in os.environ] + [None]

pid = ctypes.c_int()
e = spawn(ctypes.byref(pid), ctypes.c_char_p(b"git"), None, ctypes.byref(attrp), charpp(argv), charpp(envp))

if e != 0:
  print('spawn failed')
  sys.exit(1)

e = libSystem.posix_spawnattr_destroy(ctypes.byref(attrp))
print('spawnattr destroy ret:', e)

os.waitpid(pid.value, 0)

Hm, posix_spawnattr_setbinpref_np() seems to not work when using ctypes in an intel python running under rosetta. Not that surprising, come to think of it.

Using arch(1) is way less code and works, something like this:

p = subprocess.check_call('arch -arch arm64 git -c core.deltaBaseCacheLimit=2g clone --no-checkout --progress https://chromium.googlesource.com/chromium/src.git _gclient_src_mmdd8v1w'.split())

posix_spawnattr_setbinpref_np works of course fine from C:

% clang -arch x86_64 -arch arm64 -o mysleep sleep.c
clang: error: no such file or directory: 'sleep.c'
clang: error: no input files
thakis@Nicos-Mac gittry % cat mysleep.c
#include <unistd.h>

int main() {
  sleep(30);
}
% clang -arch x86_64 -arch arm64 -o mysleep mysleep.c
% cat launch.c
#include <spawn.h>
#include <sys/wait.h>
#include <unistd.h>

#include <stdio.h>

extern char** environ;

int main() {
  printf("%x\n", CPU_TYPE_ARM64);

  posix_spawnattr_t attr;
  posix_spawnattr_init(&attr);

  cpu_type_t pref[] = { CPU_TYPE_ARM64 };
  size_t ocount;
  posix_spawnattr_setbinpref_np(&attr, 1, pref, &ocount);
  printf("count %zu\n", ocount);

  char* argv[] = { "mysleep", NULL };
  pid_t pid;
  posix_spawn(&pid, "mysleep", NULL, &attr, argv, environ);

  posix_spawnattr_destroy(&attr);

  int status;
  waitpid(pid, &status, 0);
}
% clang launch.c -o launch -arch x86_64
% ./launch

With this, mysleep shows up as "Apple" in Activity Monitor, but without the spawattr, launch launches the Intel slice instead.