为什么引用的int被强制转换为void指针？

Question

void Link::keepalive(bool enable){
    int opt = enable? 1 : 0;
    ::setsockopt(sock, SOL_SOCKET, SO_KEEPALIVE, (void *)&opt, sizeof(opt));
}

I keep seeing code like the following in some of the more low-level c functions especially when it pertains to sockets and networking operations.

I'm coming from a python background, and I've certain looked around for good explanations as to what (void *)&opt is doing, but have yet to find a good explaination.

I get that a reference is merely a pointer with restrictions.

Answer 1

The signature for setsockopt

int setsockopt(int socket, int level, int option_name,
   const void *option_value, socklen_t option_len);  

Its second last argument should be of type void * . To make &opt of type void * it is catsted to that type.

Also note that &opt is the address of opt object. Its not reference.

Answer 2

It is indeed quite a mess which exists in the first place because setsockopt() is supporting tons of different options (SO_KEEPALIVE is just one of them).

• To enable passing any parameter (including future extensions), they've said that setsockopt() will accept a pointer to the parameter, where parameter type is any pointer (and is implicitly defined by option_name parameter, in this case SO_KEEPALIVE).

  • this ensures that setsockopt() prototype remains the same for all the future values of SO_* options

• in C, "any pointer" is written as "void*"

• &opt is "pointer to opt" (or "address of opt")

• as "opt" has type int, "&opt" has type int*, so to convert it to void*, there is an "explicit cast" (void*)&opt in your code

  • however, in most cases it is not necessary, as cast to void* is implicit, so simple "&opt" should do

• this kind of trickery, while common in C APIs, is generally discouraged in C++, so don't take it as an example of "how to write C++ programs"

Answer 3

Take a look at MSDN's documentation for the setsockopt function, in particular the Remarks section:

There are two types of socket options: Boolean options that enable or disable a feature or behavior, and options that require an integer value or structure. To enable a Boolean option, the optval parameter points to a nonzero integer. To disable the option optval points to an integer equal to zero. The optlen parameter should be equal to sizeof(int) for Boolean options. For other options, optval points to an integer or structure that contains the desired value for the option, and optlen is the length of the integer or structure.

It is a common practice in C APIs to pass function arguments that can be of different type/length as the most general pointer type (void *) pointing on their address in memory.

&opt returns pointer/address to opt of type (int *) in your case, so it needs to be cast to (void *) . Also when writing for C++, prefer using explicit type conversions .