launch.h API for getting launchd job by label

launch_key_getjob.c

/**
 ./launch_key_getjob_test com.apple.Spotlight

  LimitLoadToSessionType: Aqua
  MachServices: (cba) 0x7fbfbb504700
  Label: com.apple.Spotlight
  OnDemand: (cba) 0x7fff9464d490
  LastExitStatus: 0
  PID: 562
  Program: /System/Library/CoreServices/Spotlight.app/Contents/MacOS/Spotlight
  ProgramArguments: (cba) 0x7fbfbb504b70
  PerJobMachServices: (cba) 0x7fbfbb5049b0
 */

#include <stdio.h>
#include <launch.h>
#include <errno.h>

void print_job(launch_data_t j, const char *key, void *context)
{
  printf("%s: ", key);
  
  if (launch_data_get_type(j) == LAUNCH_DATA_STRING) {
    printf("%s\n", launch_data_get_string(j));
  } 
  else if (launch_data_get_type(j) == LAUNCH_DATA_INTEGER) {
    printf("%lli\n", launch_data_get_integer(j));
  }
  else {
    printf("(cba) %p\n", j);
  }
}

int main(int argc, char** argv) {
  if (argc < 2) {
    printf("Usage: %s [label]\n", argv[0]);
    return 1;
  }

  launch_data_t msg, resp = NULL;
  msg = launch_data_alloc(LAUNCH_DATA_DICTIONARY);

  launch_data_dict_insert(msg, launch_data_new_string(argv[1]), LAUNCH_KEY_GETJOB);
  resp = launch_msg(msg);
  launch_data_free(msg);

  if (launch_data_get_type(resp) == LAUNCH_DATA_DICTIONARY)
    launch_data_dict_iterate(resp, print_job, NULL);
  else
    printf("%s not found\n", argv[1]);

  launch_data_free(resp);
  return 0;
}

it seems that 'launch_msg' can not get jobs of root?