在pthread_create（）中，将int类型转换为void *是什么意思？

Question

What does such code mean:

int main() {
    typedef int UDTSOCKET;
    UDTSOCKET recver;
    pthread_create(&rsvthread, NULL, recvAndSend, (void *)(unsigned long)recver);
    return 0;
}

void* recvAndSend(void* usocket)
{
    UDTSOCKET recver = (UDTSOCKET)(unsigned long)usocket;
}

Such code is right? What does (void *)(unsigned long)recver) mean, how can unsigned int convert to void* , and how can void* convert to UDTSOCKET in

UDTSOCKET recver = (UDTSOCKET)(unsigned long)usocket;

I think it should be

pthread_create(&rsvthread, NULL, recvAndSend, (void *)((unsigned long*)&recver));

and

void* recvAndSend(void* usocket)
{
    UDTSOCKET recver = (UDTSOCKET)(unsigned long)(*usocket);
}

Someone can explain it?

Answer 1

The first approach does pass by value, the second does pass by reference.

Note : The way how the data is passed to the thread function has an essential impact on how the variable can be used after the call to pthread_create() had returned.

When passing by value the variable's value is copied to the stack during the call to pthread_create() , which has the effect, that the original variable could be used again immediately after the calls return.

If passed by reference the value pointed to by reference isn't copied immediately, but at a later point in time, typically after pthread_create() had returned. This dues to the asynchronous nature of the way the thread's function (as passed to pthread_create() ) is started, so the variable which's address had been passed in could not be reused immediately. If it shall be reused, access to it needs to by synchronised.

---

The first approach makes use of the fact that on some platforms the size of an integer is less or equal the size of a pointer.

This might work or not.

If going this way, make sure to use an integer type which is guaranteed to be of the same size as pointer. That is intptr_t for signed and uintptr_t for unsigned integers.

To pass in the data:

#include <stdint.h> /* for intptr_t */

...

pthread_create(&rsvthread, NULL, recvAndSend, (void *)((intptr_t) recver));

To pull out the data:

void * recvAndSend(void * pvsocket)
{
  UDTSOCKET recver = (UDTSOCKET) ((intptr_t) pvsocket);
  ...

---

The second approach is the portable way to go, assuming we are doing C, do the following

to pass in the data:

pthread_create(&rsvthread, NULL, recvAndSend, &recver);

to pull out the data:

void * recvAndSend(void * pvsocket)
{
  UDTSOCKET recver = *((UDTSOCKET *) pvsocket); /* First cast to pointer to UDTSOCKET, then dereference this pointer to read out what it ispointing to. */
  ...