P thread - cast to pointer from integer of different size - c

Question

So I am trying to figure out why I am getting this error in regards to p thread.
I get the below error when I'm trying to run the code. I know it has to be something with p thread but not sure exactly what it is.
prime.c: In function ‘main’:
prime.c:33: warning: cast to pointer from integer of different size
prime.c: In function ‘isPrime’:
prime.c:50: warning: cast from pointer to integer of different size
any idea of what I'm doing wrong? I am including my code below. Thanks in advance for help.
#include<stdio.h>
#include<stdlib.h>
#include <pthread.h>
int *ptr;
int count;
int n;
void *isPrime(void *argp);
int main(int argc, char *argv[]) {
int i;
int num = 2;
int p;
pthread_t tid;
count = 0;
if (argc == 2 || argc == 3) {
printf("Argument : %s\n", argv[1]);
n = atoi(argv[1]);
if (argc == 3) {
p = atoi(argv[2]);
}
else {
p = 1;
}
ptr = (int *) malloc(n*sizeof(int));
while (count <= n) {
for (i = 1; i <= p; i++)
pthread_create(&tid, NULL, isPrime, (void *)num++);
}
for ( i = 0;i < n; i++) {
printf("%d ", *(ptr+i));
}
}
else {
printf("Invalid number of arguments\n");
}
}
void *isPrime(void *vargp) {
int i,flag = 0;
int num = (int)vargp;
for (i = 2; i <= num/2; i++) {
if (num % i == 0) {
flag = 1;
break;
}
}
if (!flag && count <= n) {
*(ptr + count) = num;
count++;
}
}

Answer 1

Here's the obvious way to avoid the implementation-defined behavior inherent in casting between integers and pointers:
while (count <= n) {
for (i = 1; i <= p; i++) {
int * iptr = malloc(sizeof *iptr);
if ( !iptr ) {
perror("Couldn't allocate memory."); /* Or other failure code */
exit(EXIT_FAILURE);
}
*iptr = num++;
pthread_create(&tid, NULL, isPrime, iptr);
}
}
and then in your thread function:
void *isPrime(void *vargp) {
int * iptr = vargp;
int num = *iptr;
free(iptr);
/* Rest of function */
return NULL;
}
As #WhozCraig points out, this is only one of the thread-related problems in your code.

Answer 2

Edit
As WhozCraig and user3386109 point out, the changes I suggest below introduce a race condition. I haven't encountered this exact use case (passing different values to different threads through a single variable), so I can't offer any useful advice; simply take the explanation for why the warnings occur as what they are.
Original
pthread_create(&tid, NULL, isPrime, (void *)num++);
The expression (void *)num++ attempts to convert the type of the expression num++ to void *; since integers and pointers may have different sizes and representations, this is potentially a bad idea.
Ideally, you should do something more like the following:
{
pthread_create( &tid, NULL, isPrime, (void *) &num );
num++;
}
Unfortunately, (void *) &num++ won't work; the result of the ++ operator isn't an lvalue, so you can't apply the & operator to it.
(void *) &num is converting from one pointer type to void *, which is safer than converting from an integer type to a pointer type.
Then, in the isprime function, you would need to make the corresponding change:
int num = *((int *)vargp);
That is, convert vargp back to a pointer to int and dereference it.