xpc_main(3) BSD Library Functions Manual xpc_main(3)
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 con-
nection 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 deter-
mine 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.
Activity is tracked with a transaction count maintained by the XPC run-
time. 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 transac-
tion 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 con-
sidered idle.
Services may extend the default behavior using xpc_transaction_begin()
and xpc_transaction_end(), which increment and decrement the transaction
count respectivley. This may be necessary for services that send periodic
messages to their clients, not in direct reply to a received message.
If the service has a non-zero transaction count at a time when the system
deems it necessary to terminate the service, all peer connections in the
service will 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 termi-
nate the process.
Important: xpc_transaction_begin() and xpc_transaction_end() are not safe
to call before xpc_main(). In general, an XPC service's main() should
have no other code in it other than a call to xpc_main() followed by a
call to exit(2) as illustrated above.
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 ser-
vice, the act of sending that message will boost the destination ser-
vice'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 mes-
sage 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 key-
chain 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) func-
tion to manage the service's main event loop. This behavior may be over-
ridden with the RunLoopType key in the service's Info.plist.
See xpcservice.plist(5) for more information about these keys.
SEE ALSO
xpc(3), xpc_connection_create(3)
Darwin 1 July, 2011 Darwin
Mac OS X 10.9.1 - Generated Fri Jan 10 05:42:06 CST 2014