demonstrate the difference between user space PID, TID, and kernel space light-weight process (LWP) id

Makefile

all:
  gcc pid_tid.c -lpthread
  ./a.out

pid_tid_lwpid.c

#include <pthread.h>
#include <stdio.h>
#include <sys/types.h>
#include <syscall.h>
#include <unistd.h>
void *pr_pid_tid(void *arg) {
  pid_t my_pid = getpid();
  printf("User space PID is %d\n", my_pid);
  pthread_t my_tid = pthread_self();
  printf("User space TID is %ld\n", my_tid);
  int my_sid = syscall(SYS_gettid);
  printf("Kernel space LWP ID is %d\n", my_sid);

  return NULL;
}
int main() {
  pthread_t new_thread_tid = 0;

  printf("Child thread:\n");
  pthread_create(&new_thread_tid, NULL, pr_pid_tid, NULL);
  pthread_join(new_thread_tid, NULL);

  printf("New thread id is %ld\n", new_thread_tid);
  printf("Parent thread:\n");
  pr_pid_tid(NULL);
  return 0;
}