casting long to (void *) to pass

Question

Reading through the pthread tutorial from LLNL I hit the following example code

/******************************************************************************
* FILE: hello.c
* DESCRIPTION:
*   A "hello world" Pthreads program.  Demonstrates thread creation and
*   termination.
* AUTHOR: Blaise Barney
* LAST REVISED: 08/09/11
******************************************************************************/
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#define NUM_THREADS 5

void *PrintHello(void *threadid)
{
   long tid;
   tid = (long)threadid;
   printf("Hello World! It's me, thread #%ld!\n", tid);
   pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
   pthread_t threads[NUM_THREADS];
   int rc;
   long t;
   for(t=0;t<NUM_THREADS;t++){
     printf("In main: creating thread %ld\n", t);
     rc = pthread_create(&threads[t], NULL, PrintHello, (void *)t);
     if (rc){
       printf("ERROR; return code from pthread_create() is %d\n", rc);
       exit(-1);
       }
     }

   /* Last thing that main() should do */
   pthread_exit(NULL);
}

I can see why the long is cast through void * (as if it's not and you pass a pointer to t the numbers printed out by the threads are garbled), my question is should this be considered kosher and always work? Or is this a quick hack to get the simplest possible example of threading working? Is this a standard C thing?

Answer 1

No, it's not strictly kosher in terms of the ISO C standard since there's no guarantee that a pointer is wide enough to hold a long.

A kosher solution would be to pass a pointer to the long, either with one unique long per thread (such as one in an array) or having inter-thread communication (such as condition variables) between the creator thread and created thread so that the latter can make a copy before the former is allowed to change it for the next thread creation.

However, the fact that it's not strictly kosher doesn't mean it won't work in a specific implementation. Provided you can guarantee the the cast between void* and long won't lose any information, it will probably work fine.

From C11 6.3.2.3 Pointers (though it's largely unchanged from C99):

An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.

Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type.