如何在使用pedantic标志进行编译时，在没有得到任何警告的情况下，在C ++中从（void *）重新键入int？

Question

I have mutlithread client-server application that communicates via sockets . I create new thread with this construction:

 pthread_t thread;
 pthread_create(&thread, NULL, c->sendMessage, (void *) fd);

where fd is a ID of the connection and c->sendMessage is a function, that is called after a new thread is created and that handles this thread. Within this function I need to send some message via send(int sockfd, const void *buf, size_t len, int flags);

so I get sockfd this way:

void * Client::sendMessage(void *threadid) {
   int sockfd = (int) threadid;
   // some more code here and in the end I send the data via send(int sockfd, const void *buf, size_t len, int flags)
}

I compile with -pedantic flag and most of the compilers (mine including) don't throw any warning or error during the compilation. But some throw an error during compilation saying that this retyping from void * to int is unsafe and can cause loose of precision . I understand, that this is not a good solution and it should be done cleaner. But I can't find out how. Could anybody advice me any clean practice, how to retype ponter to int and avoid any warning during compilation?

Answer 1

What is problem to send pointer, instead of integer? Convert int to void* , or void* to int , it's not standard conformant solution.

pthread_create(&thread, NULL, c->sendMessage, new int(fd));

int* sockfd_ptr = (int*)threadid;
// some usage
delete sockfd_ptr;

Any pointer is convertible to void* , so it should works well. Don't forget to delete threadid in some place of your program. May be it will better to create some class, where store reference/pointer.

Also, I can't understand, how you send member function into C function, it's not correct too.

Answer 2

It's an extremely bad idea to cast from int to void* and then back to int because there are absolutely no guarantees in the C++ standard that those two types are the same size.

If you really must cast between an integer type and a pointer (which is not a very good idea, but sometimes there is no other choice), use intptr_t or uintptr_t instead of int since they are guaranteed to be the same size as a pointer. This way you won't get any warning.

As a side note, legacy C casts like you're doing are frowned upon in C++ because you can't be sure what they are doing under the hood (it could be a static_cast or a reinterpret_cast , who knows?). Prefer using proper C++ casts, namely reinterpret_cast in your case.

Answer 3

您可以添加诸如static_assert(sizeof(fd) <= sizeof(void*), "problem") ，并依赖于您的实现文档，reinterpret_cast将在这种情况下往返。