Threads receiving the same ID in C - c

Question

Here is a block of code that creates a number of threads provided by the user, each thread then generates a random number and calculates its squareroot. I cannot figure out why the threads are getting the same ID, line 64 is the culprit as it is where the threads are being created. I suspect that there is something happening in the loop that is causing the threads to all be generated at the same time.
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#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <pthread.h>
#include <math.h>
void *squareroot(void *num1)
{
int *id = (int *)num1;
int incoming = rand()/100;
float *outgoing = (float *)malloc(sizeof(float));
printf("%d \n", *id);
printf("%d\n", incoming);
*outgoing = 5.000;
//Calculate the square root of the number passed to the function
*outgoing = sqrt(incoming);
return outgoing;
}
int main(int argc, char* argv[])//testing funcion
{
srand(time(NULL));
int i, j;
int *temp = (int *)malloc(sizeof(int));
if (argc != 2)
{
printf ("ERROR: Enter a number\n");
return 1;
}
int loop = atoi(argv[1]); //grabbing the integer supplied by user
pthread_t thread_id[loop];
void *exit_status;
float *thread_result;
for(i = 0; i < loop; i++)
{
pthread_create(&thread_id[i], NULL, squareroot, &i);
}
for(j = 0; j < loop; j++)
{
pthread_join(thread_id[j], &exit_status);
thread_result = (float *)exit_status;
printf("%f\n", *thread_result);
}
}

Answer 1

I think what is happening is that your loop finishes creating all the threads (or at least some of them) before any of the threads actually run and extract their unique id.
Because you're passing a pointer to i, when each thread finally gets around to checking its parameter, i is already finished... Or at least partway through. The danger is that multiple threads might see the same value for i. It's even worse that you never copy the value out of that pointer - you always dereference it. That means it might change in the middle of your thread's execution.
What you should do instead is pretend it is a pointer:
pthread_create(&thread_id[i], NULL, squareroot, (void*)i);
And in your thread function:
int id = (int)num1;
This works because the pointer is passed by value. Whatever value you provide is the value that goes into the thread function. Previously it didn't work because you passed a pointer to a value that could change in another thread.
PS: Don't forget to free the result from each thread in your loop at the end. At the moment you're not cleaning up memory that you allocated.