warning: cast from pointer to integer of different size

Question

I'm working on socket programming.. my code executes the way I want it to, I'm able to use it. BUT it gives me a warning on compilation.

I compile using

gcc server1.c -o server1 -lpthread

And I get the warning

warning: cast from pointer to integer of different size [-Wpointer-to-int-cast]

This error comes for the following code

int newsockfd;
newsockfd = (int)newsockfdd; //this line

And I'm using newsockfdd (which is int) in the following chunk of code

if (pthread_create(&threadID[i++], NULL, serverThread, (void *)(intptr_t)newsockfdd) != 0)
    {
        perror("Thread create error");
    }

As you can probably tell, the code is not written too well (I am working on making it better). I know that this warning comes because of something to do with the size of int. But I really dunno how to fix it. Before I put in (intptr_t) in the pthread_create statement, it was showing a warning on that line, but that time the warning was

warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

It seems like there should be a simple fix to this? But I can't find it. I'm using Ubuntu 64-bit. Is that a reason for the warning?

Hmm, given the warning you got from (void*)newsockfdd before you interposed the cast to intptr_t, it seems unlikely that the cast from pointer to integer of different size warning is actually from the indicated line. — Daniel Fischer
– Daniel Fischer, Commented Nov 25, 2012 at 4:08
Unless... Is that line in serverThread and newsockfdd is the argument that serverThread receives? Then it would be clear. — Daniel Fischer
– Daniel Fischer, Commented Nov 25, 2012 at 4:10
@DanielFischer Yes.. I pass it as an argument to serverThread. Like so: void *serverThread(void *newsockfdd). So what is wrong? — user1825770
– user1825770, Commented Nov 25, 2012 at 6:58
Well, the function you're running as a pthread takes a void* as argument. So your int is first cast to void* when it is passed, then in serverThread, the void* is cast back to int. It's bad style, bad probably will work. But, if possible, pass a pointer to newsockfdd and in serverThread, cast that pointer to int* and dereference it. — Daniel Fischer
– Daniel Fischer, Commented Nov 25, 2012 at 7:41

Daniel Fischer · Accepted Answer · 2012-11-25 14:07:25Z

6

As has been established in the comments, the situation is (modulo renaming to avoid confusing occurrences of newsockfdd as passed argument or received parameter)

void *serverThread(void *arg) {
    // ...
    int newsockfd = (int)arg;
    // ...
}

and in main (or a function called from there)

// ...
int newsockfdd = whatever;
// ...
if (pthread_create(&threadID[i++], NULL, serverThread, (void *)(intptr_t)newsockfdd) != 0)
// ..

So when the int newsockfdd is passed as an argument to serverThread, it is cast to a void*. Originally, that cast was direct, but the intermediate cast to intptr_t was inserted to remove the warning about the cast to pointer from integer of different size.

And in serverThread, the received void* is cast to int, resulting in the warning about the cast from pointer to integer of different size.

That warning could probably also be removed by inserting an intermediate cast to intptr_t.

But, while the standard allows casting integers to pointers and vice versa, the results are implementation-defined and there's no guarantee that int -> void* -> int round-trips (although, a footnote in the standard says

The mapping functions for converting a pointer to an integer or an integer to a pointer are intended to be consistent with the addressing structure of the execution environment.

so probably it will round-trip and work as intended in this case - but it would likely not work [only for values of small enough absolute value] if the size of a void* is smaller than that of the integer type [consider long long -> void* -> long long on 32-bit systems]).

The proper fix is to avoid the casting between integers and pointers,

void *serverThread(void *arg) {
    // ... check that arg isn't NULL
    int newsockfd = *(int *)arg;
    // ...
}

in severThread, cast the received pointer to a pointer of appropriate type, and read the pointed-to value, and in main

if// ...
int newsockfdd = whatever;
// ...
if (pthread_create(&threadID[i++], NULL, serverThread, &newsockfdd) != 0)

pass the address of newsockfdd.

One caveat: if serverThread is called from multiple places, the calls in all these places need to be fixed.

answered Nov 25, 2012 at 14:07

Daniel Fischer

184k19 gold badges319 silver badges436 bronze badges

Sign up to request clarification or add additional context in comments.

3 Comments

davec Over a year ago

In main you suggest passing the address of a local variable as the parameter. This is a bug, if main exits before serverThread() executes, and the latter tries to dereference the address (which is no longer a valid address, since newsockfdd will be freed when main exits) your program will crash. Better to either intermediate cast, or pass the address of a static variable instead of a local (local) one.

Daniel Fischer Over a year ago

Hmm, I had the impression that if main exits, it takes all pthread_created threads with it (but it should join the spawned threads anyway).

davec Over a year ago

I don't think that's guaranteed but I could be wrong. In any case though it's better safe than sorry; consider if the thread creation is done in a helper function called by main. Then the problem occurs regardless of whether main waits for children to exit. Just as a rule of thumb, it's best not to pass local addresses to other threads...

Collectives™ on Stack Overflow

warning: cast from pointer to integer of different size

1 Answer 1

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related