pthread_threadid_np() reports an incorrect thread ID after fork() on Mojave b9

Number:rdar://43843552 Date Originated:2018-08-29
Status:Open Resolved:
Product:macOS + SDK Product Version:10.14db9 18A377a
Classification:Serious Bug Reproducible:Always
Something not on this list

Summary: The thread ID reported by pthread_threadid_np() is not correct after calling fork(). In the forked process, the thread receives a new thread ID from the kernel, but the ID reported by pthread_threadid_np() continues to reflect the original pre-forked thread’s ID.

This is a regression somewhere in the Mojave beta series. I experience it on 10.14db9 18A377a.

Steps to Reproduce: Run the attached test program, which compares the results of pthread_threadid_np(pthread_self()) with the thread_selfid() system call.

mark@garbage zsh% clang thread_id_test.c -o thread_id_test
mark@garbage zsh% ./thread_id_test 

Expected Results:
The exact numbers are irrelevant, but three lines should be printed, and all should report “ok”.

mark@garbage zsh% ./thread_id_test 
prefork: pthread_thread_id 0x48bfe == thread_id 0x48bfe (ok)
parent: pthread_thread_id 0x48bfe == thread_id 0x48bfe (ok)
child: pthread_thread_id 0x48bff == thread_id 0x48bff (ok)
mark@garbage zsh% sw_vers
ProductName:	Mac OS X
ProductVersion:	10.13.6
BuildVersion:	17G65

Actual Results:
On 10.14, the “child” line prints BAD. On the child side of a fork(), pthread_threadid_np(pthread_self()) is incorrect.

litterbox@ten-fourteen zsh% sw_vers; ./thread_id_test
prefork: pthread_thread_id 0x3072 == thread_id 0x3072 (ok)
parent: pthread_thread_id 0x3072 == thread_id 0x3072 (ok)
child: pthread_thread_id 0x3072 != thread_id 0x3073 (BAD)
litterbox@ten-fourteen zsh% sw_vers
ProductName:	Mac OS X
ProductVersion:	10.14
BuildVersion:	18A377a

10.14db9 18A377a, xnu-4903.201.2~4/RELEASE_X86_64, libpthread-330.201.1

This bug is a regression. It is not present in 10.13.6 17G65. It was not present in at least some earlier 10.14 developer betas, although I’m not certain which beta introduced the bug.



// clang thread_id_test.c -o thread_id_test

#include <err.h>
#include <errno.h>
#include <limits.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/syscall.h>
#include <unistd.h>

void Test(const char* tag) {
  uint64_t pthread_thread_id;
  errno = pthread_threadid_np(pthread_self(), &pthread_thread_id);
  if (errno != 0) {
    err(EXIT_FAILURE, "%s: pthread_threadid_np()", tag);

#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wdeprecated-declarations"
  int thread_id = syscall(SYS_thread_selfid);
#pragma clang diagnostic pop
  if (thread_id == -1) {
    err(EXIT_FAILURE, "%s: thread_selfid()", tag);

  if (pthread_thread_id > INT_MAX) {
    printf("%s: pthread_thread_id 0x%llx is too large, thread_id is 0x%x\n",

  _Bool ok = pthread_thread_id == thread_id;
  printf("%s: pthread_thread_id 0x%llx %s thread_id 0x%x (%s)\n",
         ok ? "==" : "!=",
         ok ? "ok" : "BAD");

int main(int argc, char* argv[]) {

  pid_t pid = fork();
  if (pid < 0) {
    err(EXIT_FAILURE, "fork()");

  if (pid == 0) {
  } else {

  return EXIT_SUCCESS;


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