How to find a free local port using Swift? - swift

Question

For a local server I need to specify a port, which must not be in use. There's a really neat solution in Python to get a free port. However, such a socket library is not available in Swift. So I tried Using BSD Sockets in Swift, but that actually wants a port to be specified upfront and I cannot get the bind command to work. Here's the code I tried:
let socketFD = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if socketFD == -1 {
print("Error creating BSD Socket")
return
}
var hints = addrinfo(
ai_flags: AI_PASSIVE, // Assign the address of the local host to the socket structures
ai_family: AF_UNSPEC, // Either IPv4 or IPv6
ai_socktype: SOCK_STREAM, // TCP
ai_protocol: 0,
ai_addrlen: 0,
ai_canonname: nil,
ai_addr: nil,
ai_next: nil)
var servinfo: UnsafeMutablePointer<addrinfo>? = nil
let addrInfoResult = getaddrinfo(
nil, // Any interface
"8000", // The port on which will be listenend
&hints, // Protocol configuration as per above
&servinfo);
if addrInfoResult != 0 {
print("Error getting address info: \(errno)")
return
}
let bindResult = Darwin.bind(socketFD, servinfo!.pointee.ai_addr, socklen_t(servinfo!.pointee.ai_addrlen));
if bindResult == -1 {
print("Error binding socket to Address: \(errno)")
return
}
let listenResult = Darwin.listen(socketFD, 1);
if listenResult == -1 {
print("Error setting our socket to listen")
return
}
let port = Darwin.getsockname(socketFD, nil, nil);
The bind call always returns -1 and since I want to get a free port it makes no sense to specify one in getaddrinfo. What's the correct way here?

Answer 1

Here's a working solution:
func findFreePort() -> UInt16 {
var port: UInt16 = 8000;
let socketFD = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if socketFD == -1 {
//print("Error creating socket: \(errno)")
return port;
}
var hints = addrinfo(
ai_flags: AI_PASSIVE,
ai_family: AF_INET,
ai_socktype: SOCK_STREAM,
ai_protocol: 0,
ai_addrlen: 0,
ai_canonname: nil,
ai_addr: nil,
ai_next: nil
);
var addressInfo: UnsafeMutablePointer<addrinfo>? = nil;
var result = getaddrinfo(nil, "0", &hints, &addressInfo);
if result != 0 {
//print("Error getting address info: \(errno)")
close(socketFD);
return port;
}
result = Darwin.bind(socketFD, addressInfo!.pointee.ai_addr, socklen_t(addressInfo!.pointee.ai_addrlen));
if result == -1 {
//print("Error binding socket to an address: \(errno)")
close(socketFD);
return port;
}
result = Darwin.listen(socketFD, 1);
if result == -1 {
//print("Error setting socket to listen: \(errno)")
close(socketFD);
return port;
}
var addr_in = sockaddr_in();
addr_in.sin_len = UInt8(MemoryLayout.size(ofValue: addr_in));
addr_in.sin_family = sa_family_t(AF_INET);
var len = socklen_t(addr_in.sin_len);
result = withUnsafeMutablePointer(to: &addr_in, {
$0.withMemoryRebound(to: sockaddr.self, capacity: 1) {
return Darwin.getsockname(socketFD, $0, &len);
}
});
if result == 0 {
port = addr_in.sin_port;
}
Darwin.shutdown(socketFD, SHUT_RDWR);
close(socketFD);
return port;
}