Please check the code

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define N 16

void *thread(void *vargp);

int main()
{
pthread_t tid;
int i,*ptr;

for (i = 0; i < N; i++)
{
ptr=malloc(sizeof(int));
*ptr=i;
pthread_create(&tid, NULL, thread, ptr);
}
sleep(3); /*wait all peer threads to exit*/
return 0;
}

/* thread routine */
void *thread(void *vargp)
{
int myid = *((int *)vargp);
pthread_detach(pthread_self());
free(vargp);
printf("Hello from thread %d, thread id is %ld\n", myid,pthread_self());
return NULL;
}

when you execute it .
it print the thread id number like this:

Hello from thread 0, thread id is -1208124528
Hello from thread 1, thread id is -1218614384
Hello from thread 2, thread id is -1229104240
Hello from thread 3, thread id is -1239594096
Hello from thread 4, thread id is -1250083952
Hello from thread 5, thread id is -1260573808
Hello from thread 6, thread id is -1271063664
Hello from thread 7, thread id is -1218614384
Hello from thread 8, thread id is -1208124528
Hello from thread 9, thread id is -1281553520
Hello from thread 10, thread id is -1292043376
Hello from thread 11, thread id is -1302533232
Hello from thread 12, thread id is -1302533232
Hello from thread 13, thread id is -1292043376
Hello from thread 14, thread id is -1281553520
Hello from thread 15, thread id is -1208124528

you can see that, lots of thread id are the same.

if i use the joinable function, it won't have this problem. What's the problem is the pthread_detach func? Am I write this program wrong.

That's purely random chance. It's not "wrong."

Thanks for answer. I got it.

I think as this is detached thread once the thread exits its id can be reused for some other thread. Hence u see the thread id being same.

That is exactly what's happening; i.e. reuse because the thread/process no longer exists. Now, if the thread type was 'joinable' it would persist in zombie mode until joined, and the id's could/would not be reused! Although pthread_create(pthread_detach()) and pthread_join() may not be compatible.