xpc_object(3)

NAME
xpc_connection_create - creation and management of XPC connections

SYNOPSIS
#include <xpc/xpc.h>
xpc_connection_t
xpc_connection_create(const char *name, dispatch_queue_t targetq);
xpc_connection_t
xpc_connection_create_mach_service(const char *name, dispatch_queue_t targetq, uint64_t flags);
xpc_connection_t
xpc_connection_create_from_endpoint(xpc_endpoint_t endpoint);
void
xpc_connection_set_target_queue(xpc_connection_t connection, dispatch_queue_t targetq);
void
xpc_connection_set_event_handler(xpc_connection_t connection, xpc_handler_t handler);
void
xpc_connection_suspend(xpc_connection_t connection);
void
xpc_connection_resume(xpc_connection_t connection);
void
xpc_connection_send_message(xpc_connection_t connection, xpc_object_t message);
void
xpc_connection_send_barrier(xpc_connection_t connection, dispatch_block_t barrier);
void
xpc_connection_send_message_with_reply(xpc_connection_t connection, xpc_object_t message, dispatch_queue_t targetq, xpc_handler_t handler);
xpc_object_t
xpc_connection_send_message_with_reply_sync(xpc_connection_t connection, xpc_object_t message);
void
xpc_connection_cancel(xpc_connection_t connection);
const char *
xpc_connection_get_name(xpc_connection_t connection);
uid_t
xpc_connection_get_euid(xpc_connection_t connection);
gid_t
xpc_connection_get_guid(xpc_connection_t connection);
pid_t
xpc_connection_get_pid(xpc_connection_t connection);
au_asid_t
xpc_connection_get_asid(xpc_connection_t connection);
void
xpc_connection_set_context(xpc_connection_t connection, void *ctx);
void *
xpc_connection_get_context(xpc_connection_t connection);
void
xpc_connection_set_finalizer_f(xpc_connection_t connection, xpc_finalizer_t finalizer);
xpc_endpoint_t
xpc_endpoint_create(xpc_connection_t connection);

DESCRIPTION
Connections are the fundamental primitives for sending and receiving messages. Connections also inform the caller of certain non-message events through errors.

Messages sent to a connection are sent in FIFO order, and message-send operations over a connection are non-blocking. When a message is sent over a connection, it is atomically enqueued on a queue which is managed by the XPC runtime. As it becomes possible to successfully deliver messages to the remote end of the connection, messages will be dequeued from the queue and delivered.

Connections may either be used to communicate with XPC services residing within an application bundle or with a MachService advertised by a launchd job in its launchd.plist(5). XPC connections maintain a one-to-one relationship between the local and remote ends of the connection. Therefore, for every connection created to a service, the remote end will see a distinct peer connection object. This model is semantically similar to the accept(3) model, whereby the server listens on a single file descriptor, and that listening descriptor emits new file descriptors for each connection that occurs.

EVENT HANDLERS
Each connection must have an event handler associated with it. The event handler block takes one argument of type xpc_object_t. The event handler block will deliver different types of objects depending on the nature of the event.

The type of object can be queried using xpc_get_type(3). If the event handler block delivers an object of type XPC_TYPE_DICTIONARY, the event is a message that needs processing. If the event handler delivers an object of type XPC_TYPE_ERROR, an error has occurred on the connection that must be handled.

Regardless of the type of object passed to the event handler, the caller will NOT implicitly gain a reference to the object. Therefore, if the caller wishes to work with the object after the event handler has returned, it should call xpc_retain(3) to keep a reference on the object for itself from within the event handler. It is unsafe to retain the object after the event handler has returned.

The event handler of a connection may be changed while the connection is processing events using the xpc_connection_set_event_handler() API. Calls to this API will not interrupt currently-executing invocations of the connection's event handler. Once the currently-executing event handler returns, the new event handler will take effect. If called from within the event handler itself, the next invocation of the event handler will honor the new one set.

TARGET QUEUES
Each connection has an associated target queue. All connection-related activity will happen on an internal queue which is synchronized with the target queue. Event handler invocations are included in connection-related activity. The target queue may be changed while the connection is processing events using the xpc_connection_set_target_queue() API. Setting of the target queue on a connection is asynchronous, and the caller should not assume that when this API returns, the new target queue is in effect. The actual change will take place at a later time.

By default, all connections target the DISPATCH_TARGET_QUEUE_DEFAULT(3) queue. This queue will be used if NULL is given as the targetq argument to xpc_connection_set_target_queue(), xpc_connection_create() or xpc_connection_create_mach_service()

Important: The result of calling dispatch_get_current_queue(3) from within a connection's event handler is undefined and should not be considered reliable for attempting to avoid deadlocks.

LAUNCH-ON-DEMAND
When the caller obtains a connection to a named service, the fact that it has a connection does not imply anything about whether the remote end is alive and running. Connections are virtual, and if the remote end is not yet running, the act of sending a message will cause it to launch on-demand.

If the caller has a connection to a named service, then the remote process closing the connection or crashing will deliver the XPC_ERROR_CONNECTION_INTERRUPTED error to the event handler. This error is recoverable, and after receiving it, the connection is still usable. If the caller had previously sent state over the connection, this error indicates that that state should be updated, if needed, and resent.

NOTE: Services work best when they are as stateless as possible. Even if you write perfectly bug-free code, the libraries and frameworks your service links against may have bugs that could crash the service. So a service must be able to recover from such abnormal exits.

One strategy for implementing a robust and recoverable service is to have each client of the service maintain state for the service. If the service crashes, then each client will detect that condition and resend the needed state to the service so that it can resume any interrupted operations.

PEERING
The local and remote ends of a connection have a one-to-one association. So when a new connection to a service is created and has a message sent over it, the service will receive a new connection in the event handler it specified to xpc_main(3). If the service is a MachService advertised by launchd(8), then the listener connection for the named service will receive the new connection in its event handler.

Even if the same process creates multiple connections to the same service, each connection will be distinct. The peer connection received by the service will deliver XPC_ERROR_CONNECTION_INVALID to its event handler when the connection has been closed. These peer connections cannot be re-created by the XPC runtime, and therefore they will never deliver the XPC_ERROR_CONNECTION_INTERRUPTED error to their event handlers.

SUSPEND AND RESUME
All connections are created in a suspended state. Therefore, they will not begin processing messages or events until an initial call to xpc_connection_resume(). Before making this first call to resume the connection, the caller must set an event handler using xpc_connection_set_event_handler(). Note that the initial resume does not need to immediately follow setting the event handler. The caller is free to delay the initial resume as long as it chooses.

Once a connection has been resumed, it may be suspended to halt the processing of incoming events and outgoing messages. This behavior is useful to rate-limit or throttle over-active clients who are sending too many messages or to allow certain synchronization behaviors with the internal state engine.

Each connection maintains a suspend count, so xpc_connection_suspend() may be called multiple times on the same connection. The connection will resume processing events when an equal number of calls to xpc_connection_resume() have been performed on the connection, resetting the suspend count to zero.

Important: All calls to xpc_connection_suspend() must be balanced by a call to xpc_connection_resume() before the final reference on a connection is released. It is not valid to release the last reference on a suspended connection.

Important: It is invalid to underflow the suspend count by calling xpc_connection_resume() more times than xpc_connection_suspend() has been called.

CONTEXT
Connections may have associated context that can be set and retrieved using the xpc_connection_set_context() and xpc_connection_get_context() APIs, respectively. When setting context on a connection, an optional finalizer may be specified using xpc_connection_set_finalizer_f(). The function given as the finalizer argument will be invoked just before the connection's memory is being deallocated. For simple context structures allocated through malloc(3), this provides a convenient shortcut. For example:

    struct my_context_s *ctx = malloc(sizeof(*ctx));
    xpc_connection_set_context(connection, ctx);
    xpc_connection_set_finalizer_f(connection, free);

Important: The connection object itself should not be referenced or modified in any way within the context of the finalizer.

MESSAGE SENDING
Messages are sent to the remote end of a connection with the xpc_connection_send_message() API. This API will enqueue the message in a FIFO queue which will be drained asynchronously by the XPC runtime. The caller should not assume that, when this API returns, the message has been delivered to the remote end. If the caller needs to know when the message has been processed by the runtime, it should call the xpc_connection_send_barrier() API directly after calling xpc_connection_send_message(). The supplied barrier block will be invoked by the connection when the runtime has finished processing the message.

  Execution Ordering

Send barriers are NOT immediately enqueued on the connection's target queue and therefore has no guaranteed execution order with respect to other blocks scheduled on that queue. The following code illustrates this anti-pattern:

    xpc_connection_set_target_queue(connection, queue);
    static bool aboolean = false;
    xpc_connection_send_barrier(connection, ^{
    	aboolean = true;
    });
    dispatch_async(queue, ^{
    	// Assertion will fail.
    	assert(aboolean == true);
    });

To achieve the desired effect of deferring the second block's execution until after the barrier has completed, the caller can use a dispatch group (dispatch_group_create(3)) as follows:

    xpc_connection_set_target_queue(connection, queue);
    static bool aboolean = false;
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group)
    xpc_connection_send_barrier(connection, ^{
    	aboolean = true;
    	dispatch_group_leave(group);
    });
    dispatch_group_notify(group, queue, ^{
    	assert(aboolean == true);
    });

Alternatively, the caller can also dispatch_async(3) the second block from within the barrier block.

Important: The caller should not assume that the remote end of the connection has received the message when a barrier is invoked. Even though the message has been delivered to the remote end, the remote end may not have yet been scheduled for execution or may have suspended its end of the connection. The only way for the sender to know whether the remote end has received the message is to specify in its message protocol that the remote end must send a message back to the sender acknowledging receipt of the message.

REPLY MESSAGES
By default, all messages sent to a connection will result in an invocation of the remote end's connection's event handler with that message as the argument. If the caller wishes to tie the invocation of a particular block to a reply to a particular message, however, it may use the xpc_connection_send_message_with_reply() API. Like xpc_connection_send_message(), this API will return immediately and, when the remote end sends a reply back, the supplied handler block will be submitted to the supplied targetq instead of causing the connection's event handler to be invoked. The reply handler block may deliver an error to the caller, which indicates that the remote end will never send a reply.

xpc_dictionary_create_reply(3) and sending it to its peer connection as it normally would. The caller must, in turn, specify in the message itself whether it expects a reply to be delivered.

CLIENT SIDE

    xpc_connection_send_message_with_reply(connection, message, replyq, ^(xpc_object_t reply) {
    	if (xpc_get_type(reply) == XPC_TYPE_DICTIONARY) {
    		// Process reply message that is specific to the message sent.
    	} else {
    		// There was an error, indicating that the caller will never receive
    		// a reply to this message. Tear down any associated data structures.
    	}
    });

SERVICE SIDE

    void
    handle_message(xpc_object_t message)
    {
    	if (xpc_dictionary_get_bool(message, "ExpectsReply")) {
    		// Sender has set the protocol-defined "ExpectsReply" key, and therefore
    		// it expects the reply to be delivered specially.
    		xpc_object_t reply = xpc_dictionary_create_reply(message);
    		// Populate 'reply' as a normal dictionary.
    		// This is the connection from which the message originated.
    		xpc_connection_t remote = xpc_dictionary_get_remote_connection(message);
    		xpc_connection_send_message(remote, reply);
    		xpc_release(reply);
    	} else {
    		// The sender does not expect any kind of special reply.
    	}
    }

Important: The invocations of reply handlers are independent of the connection's normal incoming message stream. Therefore, reply messages are delivered to the recipient independently of the connection's normal FIFO semantics.

SYNCHRONOUS OPERATION
If the caller needs to block execution until a reply to a message is received, it should use the xpc_connection_send_message_with_reply_sync() API. This result of this API will be the reply sent by the server. Like the handler given to xpc_connection_send_message_with_reply(), this API may return errors indicating that the remote end of the connection will never deliver a reply.

Important: This API is primarily intended for allowing existing synchronous API to be re- implemented in terms of XPC. But in cases where the you are designing a new API that calls out to a service to retrieve a value, we strongly encourage you to have the API return the value asynchronously using a queue/block pair rather than blocking the caller until the service returns the requested value:

    void
    retrieve_uint64(dispatch_queue_t q, void (^handler)(uint64_t value))
    {
    	xpc_object_t message = xpc_dictionary_create(NULL, NULL, 0);
    	xpc_dictionary_set_string(message, "RetrieveValue", "uint64");
    	// 'connection' is a previously-created singleton.
    	xpc_connection_send_message_with_reply(connection, message, q, ^(xpc_object_t reply) {
    		if (xpc_get_type(reply) == XPC_TYPE_DICTIONARY) {
    			uint64_t value = xpc_dictionary_get_uint64(reply, "Value");
    			// 'reply' is captured by this block and copied to the heap. It will
    			// be released when this block is disposed of.
    			handler(value);
    		} else {
    			// Invoke 'reply' with a value indicating that there was an error.
    		}
    		xpc_release(message);
    	});
    }

However, such a scheme may introduce unwanted complexity in the API. The trade- off for making the example implementation above synchronous involves factors such as where the data for the response comes from and how likely it is that the API will be called on the main thread.

If the response will be constructed with data that exists in-memory in the server, it is usually safe to make the API synchronous. But if constructing the response requires I/O, and it is likely to be called from the main thread (or a thread which synchronizes with the main thread), we highly encourage that you take the asynchronous route to avoid the risk of blocking the UI.

CREDENTIALS
Identifying information about the sending processs can be obtained from a connection. Available credential information includes the sending process identifer (PID), effective user identifier (EUID), effective group identifier (EGID) and audit session identifier (ASID). These values can be obtained with the functions xpc_connection_get_pid(), xpc_connection_get_euid(), xpc_connection_get_egid() and xpc_connection_get_asid() respectively.

Credentials for a connection may not be immediately available. For example, when creating a new connection with xpc_connection_create(), XPC will not know the credentials of the remote end of the connection until it has actually exchanged messages with it. Until this credential information is filled in, these methods will return sensible values to indicate absence of crucial information. xpc_connection_get_pid() will return 0, xpc_connection_get_euid() and xpc_connection_get_egid() will return -1 and xpc_connection_get_asid() will return AU_ASSIGN_ASID (see setaudit_addr(2)).

For peer connections received through a listener's event handler or through the handler given to xpc_main(3), credentials will be immediately available.

Connection credentials have similar semantics to file descriptor credentials. That is, the credentials that the connection was created with are "baked in" to it and do not change as a result of calls to setuid(3) and friends. Use of these APIs is heavily discouraged in IPC protocols due to the inherently racy nature of credential checking.

Important: PIDs on OS X roll over when they reach a relatively small value, and a given PID cannot be assumed to be unique for a given boot session. For services bundled with an application, this is not a practical concern because the application is the only process capable of looking up its services. But MachServices advertised through launchd have a much higher visibility, so extra care should be taken when checking credentials to mitigate fork(2) bomb-style attacks.

CANCELLATION
A connection may be canceled when it is no longer needed. Once canceled, a connection will receive the XPC_ERROR_CONNECTION_INVALID error in its event handler, and no further events will be delivered. Cancellation does not affect the reference count of the connection, so if you hold references to the connection, they must still be released in order for all of the connection's associated resources to be freed.

Note that, if a connection receives XPC_ERROR_CONNECTION_INVALID in its event handler due to other circumstances, it is already in a canceled state, and therefore a call to xpc_connection_cancel() is unnecessary (but harmless) in this case.

Canceling a connection on one side has effects on the other side of a connection. For example, if you cancel a connection received through a listener connection's event handler, the remote peer connection will receive XPC_ERROR_CONNECTION_INTERRUPTED in its event handler. Even though the connection was canceled, the remote end is still able to send messages to the connection.

If, on the other hand, the creator of a named connection cancels the connection, the peer connection given to the remote end through a listener connection will receive XPC_ERROR_CONNECTION_INVALID in its event handler.

Important: As discussed previously, some connections (such as named connections created through xpc_connection_create()) will not receive XPC_ERROR_CONNECTION_INVALID in the normal course of their operation. But if another part of your code can end up calling xpc_connection_cancel(), then the connection's event handler must handle this error.

BUNDLED SERVICES
Applications may include XPC service bundles in their own bundle. When the application is run, the XPC runtime automatically recognizes each bundled service and makes it accessible to the application through the xpc_connection_create() API. To connect to a bundled service, the caller must pass the CFBundleIdentifier specified in the service's Info.plist as the name argument. The service itself will call xpc_main(3) to initialize its runtime, and the provided event handler function will be invoked with any incoming connections.

Services bundled with an application are only accessible to that application. An external process cannot connect to those services.

MACH SERVICES
If a caller wishes to connect to a MachService advertised in a launchd.plist(5), it should pass the MachService name to which is wishes to connect to xpc_connection_create_mach_service(). If the destination service is advertised in the root Mach bootstrap (i.e. the launchd.plist(5) lives in /Library/LaunchDaemons), the caller may ensure that the service that it connects to is privileged and not being spoofed through a man-in-the-middle attack by OR'ing the XPC_CONNECTION_MACH_SERVICE_PRIVILEGED flag into the flags argument. This flag will cause XPC_ERROR_CONNECTION_INVALID to be given to the event handler if the service name was not found in the root Mach bootstrap. If the launchd.plist(5) lives in /Library/LaunchAgents or ~/Library/LaunchAgents, then this flag should not be passed.

The launchd job using XPC is required to create a listener connection manually by calling xpc_connection_create_mach_service() with the XPC_CONNECTION_MACH_SERVICE_LISTENER flag OR'ed into the flags argument. The XPC_CONNECTION_MACH_SERVICE_PRIVILEGED flag has no effect on these connections. If the service name for the connection is not present in your launchd.plist's MachServices dictionary, your listener connection's event handler will receive the XPC_ERROR_CONNECTION_INVALID error, as XPC disallows ad-hoc service name registrations. However, assuming your configuration is correct, the listener connection will only ever deliver new peer connections to its event handler. The connections received by the event handler must have an event hander set on them and resumed along with an optional target queue, just like the peer connections delivered to the handler given to xpc_main(3). Note connections received through listener connection's event handler do not inherit the target queue of the listener.

    int
    main(void)
    {
    	xpc_connection_t listener = xpc_connection_create_mach_service("com.apple.myservice", NULL, XPC_CONNECTION_MACH_SERVICE_LISTENER);
    	xpc_connection_set_event_handler(listener, ^(xpc_object_t peer) {
    		// It is safe to cast 'peer' to xpc_connection_t assuming
    		// we have a correct configuration in our launchd.plist.
    		xpc_connection_set_event_handler(peer, ^(xpc_object_t event) {
    			// Handle event, whether it is a message or an error.
    		});
    		xpc_connection_resume(peer);
    	});
    	xpc_connection_resume(listener);
    	dispatch_main();
    	exit(EXIT_FAILURE);
    }

Important: New service names may NOT be dynamically registered using xpc_connection_create_mach_service(). Only launchd jobs may listen on certain service names, and any service name that the job wishes to listen on must be declared in its launchd.plist(5). XPC may make allowances for dynamic name registration in debug scenarios, but these allowances absolutely will NOT be made in the production scenario.

UNLOADING
An XPC connection to a MachService advertised by a launchd job will receive the XPC_ERROR_CONNECTION_INVALID error if the job is unloaded. There will be no indication of when the job has been loaded again. Using job loading and unloading as a normal part of your job's operation is highly discouraged.

ANONYMOUS CONNECTIONS
If a caller wishes to create a listener connection that is not bound to a particular service name, it may create an anonymous listener connection by calling xpc_connection_create() and passing NULL as the name. This connection may be given to xpc_endpoint_create(3), and the result may be embedded in a message. The recipient of that message will then be able to create a connection from that endpoint using xpc_connection_create_from_endpoint().

The resulting connection will behave like a connection to a named service created using xpc_connection_create(). The fundamental difference is that an anonymous connection is not backed a name that can be looked up. Therefore, if an connection created from an endpoint is closed, there is no guarantee that it can be re-established. So anonymous connections' event handlers must always handle both the XPC_ERROR_CONNECTION_INTERRUPTED and XPC_ERROR_CONNECTION_INVALID errors.

The endpoint type may be thought of as a boxed connection, in the same way that the uint64 type is a boxed uint64_t. Like other types, the collection APIs provide primitive setters and getters for connections, so instead of first boxing a connection in an endpoint, the xpc_dictionary_set_connection(3), xpc_dictionary_create_connection(3), xpc_array_set_connection(3), and xpc_array_create_connection(3) APIs may be used.

SEE ALSO
xpc(3), xpc_main(3), xpc_object(3), xpc_dictionary_create(3), xpc_objects(3), setaudit_addr(2), dispatch_group_create(3)