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dispatch_source_creat... BSD Library Functions Manual dispatch_source_creat...
NAME
dispatch_source_create -- dispatch event sources
SYNOPSIS
#include <dispatch/dispatch.h>
dispatch_source_t
dispatch_source_create(dispatch_source_type_t type, uintptr_t handle,
unsigned long mask, dispatch_queue_t queue);
void
dispatch_source_set_event_handler(dispatch_source_t source,
void (^block)(void));
void
dispatch_source_set_event_handler_f(dispatch_source_t source,
void (*function)(void *));
void
dispatch_source_set_registration_handler(dispatch_source_t source,
void (^block)(void));
void
dispatch_source_set_registration_handler_f(dispatch_source_t source,
void (*function)(void *));
void
dispatch_source_set_cancel_handler(dispatch_source_t source,
void (^block)(void));
void
dispatch_source_set_cancel_handler_f(dispatch_source_t source,
void (*function)(void *));
void
dispatch_source_cancel(dispatch_source_t source);
long
dispatch_source_testcancel(dispatch_source_t source);
uintptr_t
dispatch_source_get_handle(dispatch_source_t source);
unsigned long
dispatch_source_get_mask(dispatch_source_t source);
unsigned long
dispatch_source_get_data(dispatch_source_t source);
void
dispatch_source_merge_data(dispatch_source_t source, unsigned long data);
void
dispatch_source_set_timer(dispatch_source_t source,
dispatch_time_t start, uint64_t interval, uint64_t leeway);
DESCRIPTION
Dispatch event sources may be used to monitor a variety of system objects
and events including file descriptors, mach ports, processes, virtual
filesystem nodes, signal delivery and timers.
When a state change occurs, the dispatch source will submit its event
handler block to its target queue.
The dispatch_source_create() function creates a new dispatch source
object that may be retained and released with calls to dispatch_retain()
and dispatch_release() respectively. The queue parameter specifies the
target queue of the new source object, it will be retained by the source
object. Pass the DISPATCH_TARGET_QUEUE_DEFAULT constant to use the
default target queue (the default priority global concurrent queue).
Newly created sources are created in a suspended state. After the source
has been configured by setting an event handler, cancellation handler,
registration handler, context, etc., the source must be activated by a
call to dispatch_resume() before any events will be delivered.
Dispatch sources may be one of the following types:
o DISPATCH_SOURCE_TYPE_DATA_ADD
o DISPATCH_SOURCE_TYPE_DATA_OR
o DISPATCH_SOURCE_TYPE_DATA_REPLACE
o DISPATCH_SOURCE_TYPE_MACH_SEND
o DISPATCH_SOURCE_TYPE_MACH_RECV
o DISPATCH_SOURCE_TYPE_MEMORYPRESSURE
o DISPATCH_SOURCE_TYPE_PROC
o DISPATCH_SOURCE_TYPE_READ
o DISPATCH_SOURCE_TYPE_SIGNAL
o DISPATCH_SOURCE_TYPE_TIMER
o DISPATCH_SOURCE_TYPE_VNODE
o DISPATCH_SOURCE_TYPE_WRITE
The handle and mask arguments to dispatch_source_create() and the return
values of the dispatch_source_get_handle(), dispatch_source_get_mask(),
and dispatch_source_get_data() functions should be interpreted according
to the type of the dispatch source.
The dispatch_source_get_handle() function returns the underlying handle
to the dispatch source (i.e. file descriptor, mach port, process identi-
fer, etc.). The result of this function may be cast directly to the
underlying type.
The dispatch_source_get_mask() function returns the set of flags that
were specified at source creation time via the mask argument.
The dispatch_source_get_data() function returns the currently pending
data for the dispatch source. This function should only be called from
within the source's event handler. The result of calling this function
from any other context is undefined.
The dispatch_source_merge_data() function is intended for use with the
DISPATCH_SOURCE_TYPE_DATA_ADD, DISPATCH_SOURCE_TYPE_DATA_OR and
DISPATCH_SOURCE_TYPE_DATA_REPLACE source types. The result of using this
function with any other source type is undefined. Data merging is per-
formed according to the source type:
o DISPATCH_SOURCE_TYPE_DATA_ADD data is atomically added to
the source's data
o DISPATCH_SOURCE_TYPE_DATA_OR data is atomically bitwise
ORed into the source's data
o DISPATCH_SOURCE_TYPE_DATA_REPLACE data atomically replaces the
source's data.
If the source data value resulting from the merge operation is 0, the
source handler will not be invoked. This can happen if:
o the atomic addition wraps for sources of type
DISPATCH_SOURCE_TYPE_DATA_ADD,
o 0 is merged for sources of type
DISPATCH_SOURCE_TYPE_DATA_REPLACE.
SOURCE EVENT HANDLERS
In order to receive events from the dispatch source, an event handler
should be specified via dispatch_source_set_event_handler(). The event
handler block is submitted to the source's target queue when the state of
the underlying system handle changes, or when an event occurs. If a
source is resumed with no event handler block set, events will be quietly
ignored. If the event handler block is changed while the source is sus-
pended, or from a block running on a serial queue that is the source's
target queue, then the next event handler invocation will use the new
block.
Dispatch sources may be suspended or resumed independently of their tar-
get queues using dispatch_suspend() and dispatch_resume() on the dispatch
source directly. The data describing events which occur while a source is
suspended are coalesced and delivered once the source is resumed.
The handler block need not be reentrant safe, as it is not resubmitted to
the target queue until any prior invocation for that dispatch source has
completed. When the handler is set, the dispatch source will perform a
Block_copy() on the handler block.
To unset the event handler, call dispatch_source_set_event_handler_f()
and pass NULL as function. This unsets the event handler regardless of
whether the handler was a function pointer or a block. Registration and
cancellation handlers (see below) may be unset in the same way, but as
noted below, a cancellation handler may be required.
REGISTRATION
When dispatch_resume() is called on a suspended or newly created source,
there may be a brief delay before the source is ready to receive events
from the underlying system handle. During this delay, the event handler
will not be invoked, and events will be missed.
Once the dispatch source is registered with the underlying system and is
ready to process all events its optional registration handler will be
submitted to its target queue. This registration handler may be specified
via dispatch_source_set_registration_handler().
The event handler will not be called until the registration handler fin-
ishes. If the source is canceled (see below) before it is registered,
its registration handler will not be called.
CANCELLATION
The dispatch_source_cancel() function asynchronously cancels the dispatch
source, preventing any further invocation of its event handler block.
Cancellation does not interrupt a currently executing handler block (non-
preemptive). If a source is canceled before the first time it is resumed,
its event handler will never be called. (In this case, note that the
source must be resumed before it can be released.)
The dispatch_source_testcancel() function may be used to determine
whether the specified source has been canceled. A non-zero value will be
returned if the source is canceled.
When a dispatch source is canceled its optional cancellation handler will
be submitted to its target queue. The cancellation handler may be speci-
fied via dispatch_source_set_cancel_handler(). This cancellation handler
is invoked only once, and only as a direct consequence of calling
dispatch_source_cancel().
Important: a cancellation handler is required for file descriptor and
mach port based sources in order to safely close the descriptor or
destroy the port. Closing the descriptor or port before the cancellation
handler has run may result in a race condition: if a new descriptor is
allocated with the same value as the recently closed descriptor while the
source's event handler is still running, the event handler may read/write
data to the wrong descriptor.
DISPATCH SOURCE TYPES
The following section contains a summary of supported dispatch event
types and the interpretation of their parameters and returned data.
DISPATCH_SOURCE_TYPE_DATA_ADD, DISPATCH_SOURCE_TYPE_DATA_OR,
DISPATCH_SOURCE_TYPE_DATA_REPLACE
Sources of this type allow applications to manually trigger the source's
event handler via a call to dispatch_source_merge_data(). The data will
be merged with the source's pending data via an atomic add or atomic bit-
wise OR, or direct replacement (based on the source's type), and the
event handler block will be submitted to the source's target queue. The
data is application defined. These sources have no handle or mask and
zero should be used.
DISPATCH_SOURCE_TYPE_MACH_SEND
Sources of this type monitor a mach port with a send right for state
changes. The handle is the mach port (mach_port_t) to monitor and the
mask may be:
o DISPATCH_MACH_SEND_DEAD
The port's corresponding receive right has
been destroyed
The data returned by dispatch_source_get_data() is a bitmask that indi-
cates which of the events in the mask were observed. Note that because
this source type will request notifications on the provided port, it
should not be mixed with the use of mach_port_request_notification() on
the same port.
DISPATCH_SOURCE_TYPE_MACH_RECV
Sources of this type monitor a mach port with a receive right for state
changes. The handle is the mach port (mach_port_t) to monitor and the
mask is unused and should be zero. The event handler block will be sub-
mitted to the target queue when a message on the mach port is waiting to
be received.
DISPATCH_SOURCE_TYPE_MEMORYPRESSURE
Sources of this type monitor the system memory pressure condition for
state changes. The handle is unused and should be zero. The mask may be
one or more of the following:
o DISPATCH_MEMORYPRESSURE_NORMAL The system memory pressure con-
dition has returned to normal.
o DISPATCH_MEMORYPRESSURE_WARN The system memory pressure con-
dition has changed to warning.
o DISPATCH_MEMORYPRESSURE_CRITICAL The system memory pressure con-
dition has changed to critical.
The data returned by dispatch_source_get_data() indicates which of the
events in the mask were observed.
Elevated memory pressure is a system-wide condition that applications
registered for this source should react to by changing their future mem-
ory use behavior, e.g. by reducing cache sizes of newly initiated opera-
tions until memory pressure returns back to normal.
However, applications should NOT traverse and discard existing caches for
past operations when the system memory pressure enters an elevated state,
as that is likely to trigger VM operations that will further aggravate
system memory pressure.
DISPATCH_SOURCE_TYPE_PROC
Sources of this type monitor processes for state changes. The handle is
the process identifier (pid_t) of the process to monitor and the mask may
be one or more of the following:
o DISPATCH_PROC_EXIT The process has exited and is available to
wait(2).
o DISPATCH_PROC_FORK The process has created one or more child
processes.
o DISPATCH_PROC_EXEC The process has become another executable
image via a call to execve(2) or
posix_spawn(2).
o DISPATCH_PROC_SIGNAL A signal was delivered to the process.
The data returned by dispatch_source_get_data() is a bitmask that indi-
cates which of the events in the mask were observed.
DISPATCH_SOURCE_TYPE_READ
Sources of this type monitor file descriptors for pending data. The
handle is the file descriptor (int) to monitor and the mask is unused and
should be zero.
The data returned by dispatch_source_get_data() is an estimated number of
bytes available to be read from the descriptor. This estimate should be
treated as a suggested minimum read buffer size. There are no guarantees
that a complete read of this size will be performed.
Users of this source type are strongly encouraged to perform non-blocking
I/O and handle any truncated reads or error conditions that may occur.
See fcntl(2) for additional information about setting the O_NONBLOCK flag
on a file descriptor.
DISPATCH_SOURCE_TYPE_SIGNAL
Sources of this type monitor signals delivered to the current process.
The handle is the signal number to monitor (int) and the mask is unused
and should be zero.
The data returned by dispatch_source_get_data() is the number of signals
received since the last invocation of the event handler block.
Unlike signal handlers specified via sigaction(), the execution of the
event handler block does not interrupt the current thread of execution;
therefore the handler block is not limited to the use of signal safe
interfaces defined in sigaction(2). Furthermore, multiple observers of a
given signal are supported; thus allowing applications and libraries to
cooperate safely. However, a dispatch source does not install a signal
handler or otherwise alter the behavior of signal delivery. Therefore,
applications must ignore or at least catch any signal that terminates a
process by default. For example, near the top of main():
signal(SIGTERM, SIG_IGN);
DISPATCH_SOURCE_TYPE_TIMER
Sources of this type periodically submit the event handler block to the
target queue. The handle argument is unused and should be zero.
The data returned by dispatch_source_get_data() is the number of times
the timer has fired since the last invocation of the event handler block.
The timer parameters are configured with the dispatch_source_set_timer()
function. Once this function returns, any pending source data accumulated
for the previous timer parameters has been cleared; the next fire of the
timer will occur at start, and every interval nanoseconds thereafter
until the timer source is canceled.
Any fire of the timer may be delayed by the system in order to improve
power consumption and system performance. The upper limit to the allow-
able delay may be configured with the leeway argument, the lower limit is
under the control of the system.
For the initial timer fire at start, the upper limit to the allowable
delay is set to leeway nanoseconds. For the subsequent timer fires at
start + N * interval, the upper limit is MIN( leeway, interval / 2 ).
The lower limit to the allowable delay may vary with process state such
as visibility of application UI. If the specified timer source was cre-
ated with a mask of DISPATCH_TIMER_STRICT, the system will make a best
effort to strictly observe the provided leeway value even if it is
smaller than the current lower limit. Note that a minimal amount of delay
is to be expected even if this flag is specified.
The start argument also determines which clock will be used for the
timer: If start is DISPATCH_TIME_NOW or was created with
dispatch_time(3), the timer is based on mach_absolute_time(). If start
was created with dispatch_walltime(3), the timer is based on
gettimeofday(3).
DISPATCH_SOURCE_TYPE_VNODE
Sources of this type monitor the virtual filesystem nodes for state
changes. The handle is a file descriptor (int) referencing the node to
monitor, and the mask may be one or more of the following:
o DISPATCH_VNODE_DELETE The referenced node was removed from the
filesystem namespace via unlink(2).
o DISPATCH_VNODE_WRITE A write to the referenced file occurred.
o DISPATCH_VNODE_EXTEND The referenced file was extended.
o DISPATCH_VNODE_ATTRIB The metadata attributes of the referenced
node have changed.
o DISPATCH_VNODE_LINK The link count on the referenced node has
changed.
o DISPATCH_VNODE_RENAME The referenced node was renamed.
o DISPATCH_VNODE_REVOKE Access to the referenced node was revoked
via revoke(2) or the underlying fileystem
was unmounted.
o DISPATCH_VNODE_FUNLOCK
The referenced file was unlocked by
flock(2) or close(2).
The data returned by dispatch_source_get_data() is a bitmask that indi-
cates which of the events in the mask were observed.
DISPATCH_SOURCE_TYPE_WRITE
Sources of this type monitor file descriptors for available write buffer
space. The handle is the file descriptor (int) to monitor and the mask
is unused and should be zero.
Users of this source type are strongly encouraged to perform non-blocking
I/O and handle any truncated reads or error conditions that may occur.
See fcntl(2) for additional information about setting the O_NONBLOCK flag
on a file descriptor.
SEE ALSO
dispatch(3), dispatch_object(3), dispatch_queue_create(3)
Darwin May 1, 2009 Darwin