is nice() used to change the thread priority or the process priority?

Question

The man page for nice says "nice() adds inc to the nice value for the calling process. So, can we use it to change the nice value for a thread created by pthread_create?

EDIT: It seems that we can set the nice value per thread.

I wrote an application, setting different nice values for different threads, and observed that the "nicer" thread has been scheduled with lower priority. Checking the output, I found that the string "high priority ................" gets outputted more frequently.

void * thread_function1(void *arg)
{
  const pid_t tid = syscall(SYS_gettid);

  int ret = setpriority(PRIO_PROCESS, tid, -10);
  printf("tid of high priority thread %d , %d\n", tid ,getpriority(PRIO_PROCESS, tid));
  while(1)
  {
    printf("high priority ................\n");
  }
}


void * thread_function(void *arg)
{
  const pid_t tid = syscall(SYS_gettid);
  int ret = setpriority(PRIO_PROCESS, tid, 10);
  printf("tid of low priority thread %d , %d \n", tid ,getpriority(PRIO_PROCESS, tid));
  while(1)
  {
    printf("lower priority\n");
  }
}


int main()
{
  pthread_t id1;
  pthread_t id2;

  pid_t pid = getpid();
  pid_t tid = syscall(SYS_gettid);

  printf("main thread : pid = %d , tid = %d \n" , pid, tid);
  pthread_create(&id1, NULL, thread_function1,  NULL);
  pthread_create(&id2, NULL,thread_function,   NULL);

  pthread_join(id1, NULL);
  pthread_join(id2, NULL);
}

Answer 1

The pthreads man page says:

POSIX.1 also requires that threads share a range of other attributes (i.e., these attributes are process-wide rather than per-thread):

[...]

• nice value (setpriority(2))

So, theoretically, the "niceness" value is global to the process and shared by all threads, and you should not be able to set a specific niceness for one or more individual threads.

However, the very same man page also says:

LinuxThreads

The notable features of this implementation are the following:

[...]

• Threads do not share a common nice value.

NPTL

[...]

NPTL still has a few non-conformances with POSIX.1:

• Threads do not share a common nice value.

So it turns out that both threading implementations on Linux (LinuxThreads and NPTL) actually violate POSIX.1, and you can set a specific niceness for one or more individual threads by passing a tid to setpriority() on these systems.