Provided by: libdispatch-dev_0~svn197-3.3ubuntu2_amd64 
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_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);
void
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.
Newly created sources are created in a suspended state. After the source has been configured
by setting an event handler, cancellation 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:
• DISPATCH_SOURCE_TYPE_DATA_ADD
• DISPATCH_SOURCE_TYPE_DATA_OR
• DISPATCH_SOURCE_TYPE_MACH_SEND
• DISPATCH_SOURCE_TYPE_MACH_RECV
• DISPATCH_SOURCE_TYPE_PROC
• DISPATCH_SOURCE_TYPE_READ
• DISPATCH_SOURCE_TYPE_SIGNAL
• DISPATCH_SOURCE_TYPE_TIMER
• DISPATCH_SOURCE_TYPE_VNODE
• 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 identifer, 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 and DISPATCH_SOURCE_TYPE_DATA_OR source types. The result of
using this function with any other source type is undefined. Calling this function will
atomically add or logical OR the data into the source's data, and trigger the delivery of
the source's event handler.
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.
Dispatch sources may be suspended or resumed independently of their target 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.
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).
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 specified 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
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 logic OR (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:
• DISPATCH_MACH_SEND_DEAD
The port's corresponding receive right has been destroyed
The data returned by dispatch_source_get_data() indicates which of the events in the mask
were observed.
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 submitted to the target queue when a message on the mach port is
waiting to be received.
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:
• DISPATCH_PROC_EXIT The process has exited and is available to wait(2).
• DISPATCH_PROC_FORK The process has created one or more child processes.
• DISPATCH_PROC_EXEC The process has become another executable image via a call to
execve(2) or posix_spawn(2).
• DISPATCH_PROC_REAP The process status has been collected by its parent process
via wait(2).
• DISPATCH_PROC_SIGNAL A signal was delivered to the process.
The data returned by dispatch_source_get_data() indicates 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 on an
interval specified by dispatch_source_set_timer(). The handle and mask arguments are unused
and should be zero.
A best effort attempt is made to submit the event handler block to the target queue at the
specified time; however, actual invocation may occur at a later time.
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 function dispatch_source_set_timer() takes as an argument the start time of the timer
(initial fire time) represented as a dispatch_time_t. The timer dispatch source will use
the same clock as the function used to create this value. (See dispatch_time(3) for more
information.) The interval, in nanoseconds, specifies the period at which the timer should
repeat. All timers will repeat indefinitely until dispatch_source_cancel() is called. The
leeway, in nanoseconds, is a hint to the system that it may defer the timer in order to
align with other system activity for improved system performance or reduced power
consumption. (For example, an application might perform a periodic task every 5 minutes with
a leeway of up to 30 seconds.) Note that some latency is to be expected for all timers even
when a value of zero is used.
Note: Under the C language, untyped numbers default to the int type. This can lead to
truncation bugs when arithmetic operations with other numbers are expected to generate a
uint64_t sized result. When in doubt, use ull as a suffix. For example:
3ull * NSEC_PER_SEC
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:
• DISPATCH_VNODE_DELETE The referenced node was removed from the filesystem namespace
via unlink(2).
• DISPATCH_VNODE_WRITE A write to the referenced file occurred
• DISPATCH_VNODE_EXTEND The referenced file was extended
• DISPATCH_VNODE_ATTRIB The metadata attributes of the referenced node have changed
• DISPATCH_VNODE_LINK The link count on the referenced node has changed
• DISPATCH_VNODE_RENAME The referenced node was renamed
• DISPATCH_VNODE_REVOKE Access to the referenced node was revoked via revoke(2) or
the underlying fileystem was unmounted.
The data returned by dispatch_source_get_data() indicates 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)