Why thread_id creates not in order? - multithreading

Question

I tried to create 10 threads, and output each tread index. My code is shown as below, I am wondering why they are repeating instead of arranging in order?
#include <stdio.h>
#include <pthread.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include "util.h"
#include <errno.h>
#include <unistd.h>
#include <signal.h>
#include <time.h>
pthread_mutex_t request_buf_lock = PTHREAD_MUTEX_INITIALIZER;
void * worker(void *arg)
{
int thread_id = *(int*)arg;
// int requests_handled = 0;
//requests_handled = requests_handled + 1;
printf("%d\n",thread_id);
}
int main(int argc, char** argv)
{
pthread_t dispatchers[100];
pthread_t workers[100];
int i;
int * thread_id = malloc(sizeof(int));
for (i = 0; i < 10; i++) {
*thread_id = i;
pthread_create(&workers[i], NULL, worker, (void*)thread_id);
}
for (i = 0; i < 10; i++) {
pthread_join(workers[i], NULL);
}
return 0;
}
And the output result is:
4
5
5
6
6
6
7
8
9
9
But I expected it as:
0
1
2
3
4
5
6
7
8
9
Anyone has any idea or advice?

Answer 1

All 10 threads execute in parallel, and they all share a single int object, the one created by the call to malloc.
By the time your first thread executes its printf call, the value of *thread_id has been set to 4. Your second and third threads execute their printf calls when *thread_id has been set to 5. And so on.
If you allocate a separate int object for each thread (either by moving the malloc call inside the loop or just by declaring an array of ints), you'll get a unique thread id in each thread. But they're still likely to be printed in arbitrary order, since there's no synchronization among the threads.