NAME
xpc_main —
XPC service runtime
SYNOPSIS
#include
<xpc/xpc.h>
void
xpc_main(xpc_connection_handler_t
handler);
void
xpc_transaction_begin(void);
void
xpc_transaction_end(void);
DESCRIPTION
The
xpc_main()
function is called by an XPC service to initialize the runtime and start
listening for incoming connections.
HANDLER
The handler provided to
xpc_main() will be invoked when a new connection has
been established with the service. For each new connection, an
xpc_connection_t will be passed as the parameter to
the handler. Each connection corresponds to a call to
xpc_connection_create(3) made by a client of the service.
The service is responsible for setting an event handler on the new connection and resuming it in the same fashion as new connections returned by xpc_connection_create(3).
Important:
The new connection passed to
handler()
must be retained using
xpc_retain(3) if it will be stored in data structures that persist
beyond the scope of that function.
static void
new_connection_handler(xpc_connection_t peer)
{
xpc_connection_set_event_handler(peer, ^(xpc_object_t event) {
// Handle messages and errors.
});
xpc_connection_resume(peer);
}
int
main(void)
{
xpc_main(new_connection_handler);
exit(EXIT_FAILURE);
}
launchd jobs which advertise MachServices may
not call
xpc_main().
RUNTIME MANAGEMENT
The XPC runtime automatically keeps track of message activity to
determine whether a service is busy or idle. If the service remains idle
after a period of inactivity (defined by the system),
xpc_main() will exit the process. This behavior is
automatically enabled for XPC services, but
launchd(8) jobs wishing to opt into the same behavior may do so by
adding the EnablePressuredExit key to their
launchd.plist(5).
Activity is tracked with a transaction count maintained by the XPC runtime. A service is deemed idle when its transaction count is zero. The transaction count is incremented immediately before the receipt and delivery of a message to a peer connection's event handler. The transaction count is correspondingly decremented when the event handler returns.
The transaction count is also incremented when a reply message is created with xpc_dictionary_create_reply(3), and decremented when the reply is sent. As a result, a service with outstanding reply messages is not considered idle.
Services may extend the default
behavior using
xpc_transaction_begin()
and
xpc_transaction_end(),
which increment and decrement the transaction count respectively. This may
be necessary for services that send periodic messages to their clients, not
in direct reply to a received message.
Beginning a transaction and never ending it is considered an anti-pattern and is strongly discouraged.
If the service has a non-zero transaction count at a time when the system deems it necessary to terminate the service, peer connections in the service may receive the XPC_ERROR_TERMINATION_IMMINENT event. This event indicates that the service should unwind all outstanding work as quickly as possible and not begin any new work, as the system will terminate the process if it does not exit in a timely fashion. After this event is received, no further messages will be delivered to the peers, and the end of the service's last outstanding transaction will automatically terminate the process.
The XPC runtime will also automatically manage the service's priority based on where a message came from. If an app sends a message to the service, the act of sending that message will boost the destination service's priority and resource limits so that it can more quickly fill the request. If, however, a service gets a message from a background process, the service stays at a lower priority so as not to interfere with work initiated as a direct result of user interaction.
The lifetime of these boosts is tied to the lifetime of the message or reply object, just like transactions. So while the service maintains a reference to a message which boosted it, the boost will remain. If a reply message is created using xpc_dictionary_create_reply(3), the boost transfers to the reply object and will remain with the process until until the reply has been sent or deallocated.
Note that boosts happen as a result of a message-send operation. So even if the service isn't running when a boosting message is sent, it will be launched on-demand at the elevated priority necessary to receive the message in a timely fashion.
launchd jobs which use XPC for their IPC may opt into priority boosting by specifying their ProcessType as Adaptive. This will apply priority boosting behavior only to the MachServices that are in the launchd.plist. See launchd.plist(5) for more details.
DEFAULT ENVIRONMENT
The execution environment for XPC services bundled with applications is tightly controlled. By default, services are executed in a new security audit session and therefore do not have access to the current user's keychain or the ability to draw UI. This behavior may be overridden with the JoinExistingSession key in the service's Info.plist.
By default, the
xpc_main()
function will call the
dispatch_main(3) function to manage the service's main event loop.
This behavior may be overridden with the RunLoopType
key in the service's Info.plist.
See xpcservice.plist(5) for more information about these keys.