15.3.2 UDP

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► | 15 | Operating System

15.1 | Paths
15.2 | Filesystem
15.3 | Networking
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15.7 | Environment Variables
15.8 | Environment and Runtime Information
15.9 | Command-Line Parsing
15.10 | Additional Operating System Functions

▼ | 15.3 | Networking

15.3.1 | TCP
15.3.2 | UDP

On this page:

udp-open-socket
udp-bind!
udp-connect!
udp-send-to
udp-send
udp-send-to*
udp-send*
udp-send-to/ enable-break
udp-send/ enable-break
udp-receive!
udp-receive!*
udp-receive!/ enable-break
udp-set-receive-buffer-size!
udp-close
udp?
udp-bound?
udp-connected?
udp-send-ready-evt
udp-receive-ready-evt
udp-send-to-evt
udp-send-evt
udp-receive!-evt
udp-addresses
udp-set-ttl!
udp-ttl
udp-multicast-join-group!
udp-multicast-leave-group!
udp-multicast-interface
udp-multicast-set-interface!
udp-multicast-set-loopback!
udp-multicast-loopback?
udp-multicast-set-ttl!
udp-multicast-ttl

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#### 15.3.2 UDP🔗ℹ

(require racket/udp) | package: base

The bindings documented in this section are provided by the racket/udp and racket libraries, but not racket/base.

For information about UDP in general, see TCP/IP Illustrated, Volume 1 by W. Richard Stevens.

> > procedure
> > (udp-open-socket | [ | family-hostname |  |  |  |
> > |  | family-port-no]) |  | → |  | udp?
> family-hostname : (or/c string? #f) = #f
> family-port-no : (or/c port-number? #f) = #f

Creates and returns a UDP socket to send and receive datagrams (broadcasting is allowed). Initially, the socket is not bound or connected to any address or port.

If family-hostname or family-port-no is not #f, then the socket’s protocol family is determined from these arguments. The socket is not bound to the hostname or port number. For example, the arguments might be the hostname and port to which messages will be sent through the socket, which ensures that the socket’s protocol family is consistent with the destination. Alternately, the arguments might be the same as for a future call to udp-bind!, which ensures that the socket’s protocol family is consistent with the binding. If neither family-hostname nor family-port-no is non-#f, then the socket’s protocol family is IPv4.

On variants of Unix and MacOS that support FD_CLOEXEC, a socket is given that flag so that it is not shared with a subprocess created by subprocess.

Changed in version 8.11.1.6 of package base: Changed to use FD_CLOEXEC where supported by the operating system.

> > procedure
> > (udp-bind! |  | udp-socket |  |  |  |
> > |  | hostname-string |  |  |  |
> > |  | port-no |  |  |  |
> > | [ | reuse?]) |  | → |  | void?
> udp-socket : udp?
> hostname-string : (or/c string? #f)
> port-no : listen-port-number?
> reuse? : any/c = #f

Binds an unbound udp-socket to the local port number port-no. If port-no is 0 the udp-socket is bound to an ephemeral port, which can be determined by calling udp-addresses.

If hostname-string is #f, then the socket accepts connections to all of the listening machine’s IP addresses at port-no. Otherwise, the socket accepts connections only at the IP address associated with the given name. For example, providing "127.0.0.1" as hostname-string typically creates a listener that accepts only connections to "127.0.0.1" from the local machine.

A socket cannot receive datagrams until it is bound to a local address and port. If a socket is not bound before it is used with a sending procedure udp-send, udp-send-to, etc., the sending procedure binds the socket to a random local port. Similarly, if an event from udp-send-evt or udp-send-to-evt is chosen for a synchronization (see Events), the socket is bound; if the event is not chosen, the socket may or may not become bound.

The binding of a bound socket cannot be changed, with one exception: on some systems, if the socket is bound automatically when sending, if the socket is disconnected via udp-connect!, and if the socket is later used again in a send, then the later send may change the socket’s automatic binding.

If udp-socket is already bound or closed, the exn:fail:network exception is raised.

If the reuse? argument is true, then udp-bind! will set the SO_REUSEADDR socket option before binding, permitting the sharing of access to a UDP port between many processes on a single machine when using UDP multicast.

> > procedure
> > (udp-connect! |  | udp-socket |  |  |  |
> > |  | hostname-string |  |  |  |
> > |  | port-no) |  | → |  | void?
> udp-socket : udp?
> hostname-string : (or/c string? #f)
> port-no : (or/c port-number? #f)

Connects the socket to the indicated remote address and port if hostname-string is a string and port-no is an exact integer.

If hostname-string is #f, then port-no also must be #f, and the port is disconnected (if connected). If one of hostname-string or port-no is #f and the other is not, the exn:fail:contract exception is raised.

A connected socket can be used with udp-send (not udp-send-to), and it accepts datagrams only from the connected address and port. A socket need not be connected to receive datagrams. A socket can be connected, re-connected, and disconnected any number of times.

If udp-socket is closed, the exn:fail:network exception is raised.

> > procedure
> > (udp-send-to |  | udp-socket |  |  |  |
> > |  | hostname |  |  |  |
> > |  | port-no |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | void?
> udp-socket : udp?
> hostname : string?
> port-no : port-number?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Sends (subbytes bytes start-pos end-pos) as a datagram from the unconnected udp-socket to the socket at the remote machine hostname-address on the port port-no. The udp-socket need not be bound or connected; if it is not bound, udp-send-to binds it to a random local port. If the socket’s outgoing datagram queue is too full to support the send, udp-send-to blocks until the datagram can be queued.

If start-pos is greater than the length of bstr, or if end-pos is less than start-pos or greater than the length of bstr, the exn:fail:contract exception is raised.

If udp-socket is closed or connected, the exn:fail:network exception is raised.

> procedure (udp-send udp-socket bstr [start-pos end-pos]) → void?
> udp-socket : udp?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-send-to, except that udp-socket must be connected, and the datagram goes to the connection target. If udp-socket is closed or unconnected, the exn:fail:network exception is raised.

> > procedure
> > (udp-send-to* |  | udp-socket |  |  |  |
> > |  | hostname |  |  |  |
> > |  | port-no |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | boolean?
> udp-socket : udp?
> hostname : string?
> port-no : port-number?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-send-to, but never blocks; if the socket’s outgoing queue is too full to support the send, #f is returned, otherwise the datagram is queued and the result is #t.

> procedure (udp-send* udp-socket bstr [start-pos end-pos]) → boolean?
> udp-socket : udp?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-send, except that (like udp-send-to) it never blocks and returns #f or #t.

> > procedure
> > (udp-send-to/enable-break |  | udp-socket |  |  |  |
> > |  | hostname |  |  |  |
> > |  | port-no |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | void?
> udp-socket : udp?
> hostname : string?
> port-no : port-number?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-send-to, but breaking is enabled (see Breaks) while trying to send the datagram. If breaking is disabled when udp-send-to/enable-break is called, then either the datagram is sent or the exn:break exception is raised, but not both.

> > procedure
> > (udp-send/enable-break |  | udp-socket |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | void?
> udp-socket : udp?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-send, except that breaks are enabled like udp-send-to/enable-break.

> > procedure
> > (udp-receive! |  | udp-socket |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  |
> > exact-nonnegative-integer?
> > string?
> > port-number?
> udp-socket : udp?
> bstr : (and/c bytes? (not immutable?))
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Accepts up to end-pos-start-pos bytes of udp-socket’s next incoming datagram into bstr, writing the datagram bytes starting at position start-pos within bstr. The udp-socket must be bound to a local address and port (but need not be connected). If no incoming datagram is immediately available, udp-receive! blocks until one is available.

Three values are returned: the number of received bytes (between 0 and end-pos-start-pos, a hostname string indicating the source address of the datagram, and an integer indicating the source port of the datagram. If the received datagram is longer than end-pos-start-pos bytes, the remainder is discarded.

If start-pos is greater than the length of bstr, or if end-pos is less than start-pos or greater than the length of bstr, the exn:fail:contract exception is raised.

> > procedure
> > (udp-receive!* |  | udp-socket |
> > |  | bstr |
> > | [ | start-pos |
> > |  | end-pos]) |
> | → |  |
> (or/c exact-nonnegative-integer? #f)
> (or/c string? #f)
> (or/c port-number? #f)
> udp-socket : udp?
> bstr : (and/c bytes? (not immutable?))
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-receive!, except that it never blocks. If no datagram is available, the three result values are all #f.

> > procedure
> > (udp-receive!/enable-break |  | udp-socket |
> > |  | bstr |
> > | [ | start-pos |
> > |  | end-pos]) |
> | → |  |
> exact-nonnegative-integer?
> string?
> port-number?
> udp-socket : udp?
> bstr : (and/c bytes? (not immutable?))
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like udp-receive!, but breaking is enabled (see Breaks) while trying to receive the datagram. If breaking is disabled when udp-receive!/enable-break is called, then either a datagram is received or the exn:break exception is raised, but not both.

> > procedure
> > (udp-set-receive-buffer-size! |  | udp-socket |  |  |  |
> > |  | size) |  | → |  | void?
> udp-socket : udp?
> size : exact-positive-integer?

Set the receive buffer size (SO_RCVBUF) for udp-socket. Using a larger buffer can minimize packet loss that can occur due to slow polling of a connection, including during a major garbage collection.

If size is greater than the maximum allowed by the system, the exn:fail:network exception is raised.

Added in version 7.1.0.11 of package base.

> procedure (udp-close udp-socket) → void?
> udp-socket : udp?

Closes udp-socket, discarding unreceived datagrams. If the socket is already closed, the exn:fail:network exception is raised.

> procedure (udp? v) → boolean?
> v : any/c

Returns #t if v is a socket created by udp-open-socket, #f otherwise.

> procedure (udp-bound? udp-socket) → boolean?
> udp-socket : udp?

Returns #t if udp-socket is bound to a local address and port, #f otherwise.

> procedure (udp-connected? udp-socket) → boolean?
> udp-socket : udp?

Returns #t if udp-socket is connected to a remote address and port, #f otherwise.

> procedure (udp-send-ready-evt udp-socket) → evt?
> udp-socket : udp?

Returns a synchronizable event (see Events) that is in a blocking state when udp-send-to on udp-socket would block. The synchronization result is the event itself.

> procedure (udp-receive-ready-evt udp-socket) → evt?
> udp-socket : udp?

Returns a synchronizable event (see Events) that is in a blocking state when udp-receive! on udp-socket would block. The synchronization result is the event itself.

> > procedure
> > (udp-send-to-evt |  | udp-socket |  |  |  |
> > |  | hostname |  |  |  |
> > |  | port-no |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | evt?
> udp-socket : udp?
> hostname : string?
> port-no : port-number?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Returns a synchronizable event. The event is in a blocking state when udp-send-to on udp-socket would block. Otherwise, if the event is chosen in a synchronization, data is sent as for (udp-send-to udp-socket hostname-address port-no bstr start-pos end-pos), and the synchronization result is #<void>. (No bytes are sent if the event is not chosen.)

> > procedure
> > (udp-send-evt |  | udp-socket |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | evt?
> udp-socket : udp?
> bstr : bytes?
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Returns a synchronizable event. The event is ready for synchronization when udp-send on udp-socket would not block. Otherwise, if the event is chosen in a synchronization, data is sent as for (udp-send-to udp-socket bstr start-pos end-pos), and the synchronization result is #<void>. (No bytes are sent if the event is not chosen.) If udp-socket is closed or unconnected, the exn:fail:network exception is raised during a synchronization attempt.

> > procedure
> > (udp-receive!-evt |  | udp-socket |  |  |  |
> > |  | bstr |  |  |  |
> > | [ | start-pos |  |  |  |
> > |  | end-pos]) |  | → |  | evt?
> udp-socket : udp?
> bstr : (and/c bytes? (not immutable?))
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Returns a synchronizable event. The event is ready for synchronization when udp-receive on udp-socket would not block. Otherwise, if the event is chosen in a synchronization, data is received into bstr as for (udp-receive! udp-socket bytes start-pos end-pos), and the synchronization result is a list of three values, corresponding to the three results from udp-receive!. (No bytes are received and the bstr content is not modified if the event is not chosen.)

> procedure (udp-addresses udp-port [port-numbers?])
> | → |  |
> (or/c (values string? string?)
> (values string? listen-port-number?
> string? listen-port-number?))
> udp-port : udp?
> port-numbers? : any/c = #f

Returns two strings when port-numbers? is #f (the default). The first string is the Internet address for the local machine a viewed by the given UDP socket’s connection. (For most machines, the answer corresponds to the current machine’s only Internet address, but when a machine serves multiple addresses, the result is connection-specific.) The second string is the Internet address for the other end of the connection.

If port-numbers? is true, then four results are returned: a string for the local machine’s address, an exact integer between 1 and 65535 for the local machine’s port number or 0 if the socket is unbound, a string for the remote machine’s address, and an exact integer between 1 and 65535 for the remote machine’s port number or 0 if the socket is unconnected.

If the given port has been closed, the exn:fail:network exception is raised.

> procedure (udp-set-ttl! udp-socket ttl) → void?
> udp-socket : udp?
> ttl : byte?
> procedure (udp-ttl udp-socket) → byte?
> udp-socket : udp?

> > > Time-to-live settings correspond to the IP_TTL setting of the socket.

Sets or retrieves the current time-to-live setting of udp-socket.

Added in version 7.5.0.5 of package base.

> > procedure
> > (udp-multicast-join-group! |  | udp-socket |  |  |  |
> > |  | multicast-addr |  |  |  |
> > |  | hostname) |  | → |  | void?
> udp-socket : udp?
> multicast-addr : string?
> hostname : (or/c string? #f)
> > procedure
> > (udp-multicast-leave-group! |  | udp-socket |  |  |  |
> > |  | multicast-addr |  |  |  |
> > |  | hostname) |  | → |  | void?
> udp-socket : udp?
> multicast-addr : string?
> hostname : (or/c string? #f)

Adds or removes udp-socket to a named multicast group.

The multicast-addr argument must be a valid IPv4 multicast IP address; for example, "224.0.0.251" is the appropriate address for the mDNS protocol. The hostname argument selects the interface that the socket uses to receive (not send) multicast datagrams; if hostname is #f or "0.0.0.0", the kernel selects an interface automatically.

Leaving a group requires the same multicast-addr and hostname arguments that were used to join the group.

> procedure (udp-multicast-interface udp-socket) → string?
> udp-socket : udp?
> > procedure
> > (udp-multicast-set-interface! |  | udp-socket |  |  |  |
> > |  | hostname) |  | → |  | void?
> udp-socket : udp?
> hostname : (or/c string? #f)

Retrieves or sets the interface that udp-socket uses to send (not receive) multicast datagrams. If the result or hostname is either #f or "0.0.0.0", the kernel automatically selects an interface when a multicast datagram is sent.

> > procedure
> > (udp-multicast-set-loopback! |  | udp-socket |  |  |  |
> > |  | loopback?) |  | → |  | void?
> udp-socket : udp?
> loopback? : any/c
> procedure (udp-multicast-loopback? udp-socket) → boolean?
> udp-socket : udp?

> > > Loopback settings correspond to the IP_MULTICAST_LOOP setting of the socket.

Sets or checks whether udp-socket receives its own multicast datagrams: a #t result or a true value for loopback? indicates that self-receipt is enabled, and #f indicates that self-receipt is disabled.

> procedure (udp-multicast-set-ttl! udp-socket ttl) → void?
> udp-socket : udp?
> ttl : byte?
> procedure (udp-multicast-ttl udp-socket) → byte?
> udp-socket : udp?

> > > Time-to-live settings correspond to the IP_MULTICAST_TTL setting of the socket.

Sets or retrieves the current time-to-live setting of udp-socket.

The time-to-live setting should almost always be 1, and it is important that this number is as low as possible. In fact, these functions seldom should be used at all. See the documentation for your platform’s IP stack.

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