소켓프로그래밍 accpet함수 마지막인자(3번째인자)의 주소값으로 넘겨주는 이유가 궁금합니다.

int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

The argument addr is a pointer to a sockaddr structure.  This structure is filled in with the address of the peer socket, as known to the communications layer.  
The exact format of the address returned addr is determined by the socket’s address family (see socket(2) and the respective protocol man pages).  
When addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL.
 
The addrlen argument is a value-result argument: the caller must initialize it to contain the size (in bytes) of the structure pointed to by addr; 
on return it will contain the actual size of the peer address.
 
The returned address is truncated if the buffer provided is too small; 
in this case, addrlen will return a value greater than was supplied to the call.