When creating threads, some are getting a thread ID 0 - c

Question

I have a server application that creates new thread for every incoming request.
However, every once in a while, it will create a thread with thread ID = 0 (used pthread_equal to check this). I have a structure that contains the thread ID that I pass to the function specified in pthread_create, and am checking this there.
Why would a thread get created with ID = 0?
Is there anything I can do if this happens? I cannot use this thread and want to exit it immediately.
=====================================================================
typedef struct
{
pthread_t tid;
other_struct_t Other;
} data_ptr_t;
void * worker(void * arg)
{
data_ptr_t local_data;
data_ptr_t * incoming_data = (data_ptr_t *) arg;
if (NULL == incoming_data || NULL == incoming_data->Other)
{
printf("invalid input\n");
}
else if (pthread_equal(incoming_data->tid, 0))
{
printf("invalid thread id\n");
}
else
{
// add to global thread pool
// do other stuff here
// remove from global thread pool
}
}
int main()
{
// server socket stuff
while (1)
{
// if incoming connection is valid
data_ptr_t data;
int error = pthread_create(&(data.tid), NULL, (void * (*) (void *)) worker, (void *) &data);
if (0 != errror)
{
printf("could not create thread (%d)\n", error);
}
else
{
pthread_detach(data.tid);
printf("thread dispatched\n");
}
}
}
Note: If the number of threads I'm creating is under 50 or so, it works fine. Upwards of 70, most threads go through just fine, the rest end up printing the "invalid thread id".
Note: This is on Linux.

Answer 1

You can't do this:
while (1)
{
// if incoming connection is valid
data_ptr_t data;
int error = pthread_create(&(data.tid),
NULL, (void * (*) (void *)) worker, (void *) &data);
your data_ptr_t is a local variable on the stack. On the next iteration of the while loop, that variable is destroyed/gone/not-valid.
The while loop might start another iteration long before the new worker thread starts running and makes use of the data you pass to it. Instead, dynamically allocate the data you pass to the worker thread so you can be sure it's still valid.