int getsockname(int sockfd, struct sockaddr *addr, socklen_t *addrlen);
getsockname() returns the current address to which the socket sockfd is bound,in the buffer pointed to by addr. The addrlen argument should be initialized to indicate the amount of space (in bytes) pointed to by addr. On return it contains the actual size of the socket 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.
通过手册说明，可以看到addrlen参数所指的对象必须初始化，另外，如果初始提供的值太小，getsockname()函数在返回时，新写入addrlen指向的对象的值将会大于所调用时提供的值。