get-free-port.cpp

// C++Builder code
struct sockaddr_in addr;
SOCKET s;
int i;

memset(&addr, NULL, sizeof(struct sockaddr));
addr.sin_family = AF_INET;
addr.sin_addr.s_addr = INADDR_ANY;
addr.sin_port = htons(0);

s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);

i = bind(s, (sockaddr *)&addr, sizeof(addr));
if (i == 0) {
	i = sizeof(struct sockaddr);
	getsockname(s, (sockaddr *)&addr, &i);
	ShowMessage(AnsiString(ntohs(addr.sin_port)));
}
closesocket(s);