.\" Copyright (c) 2008-2013 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_source_create 3
.Os Darwin
.Sh NAME
.Nm dispatch_source_create
.Nd dispatch event sources
.Sh SYNOPSIS
.Fd #include <dispatch/dispatch.h>
.Ft dispatch_source_t
.Fo dispatch_source_create
.Fa "dispatch_source_type_t type"
.Fa "uintptr_t handle"
.Fa "unsigned long mask"
.Fa "dispatch_queue_t queue"
.Fc
.Ft void
.Fo dispatch_source_set_event_handler
.Fa "dispatch_source_t source"
.Fa "void (^block)(void)"
.Fc
.Ft void
.Fo dispatch_source_set_event_handler_f
.Fa "dispatch_source_t source"
.Fa "void (*function)(void *)"
.Fc
.Ft void
.Fo dispatch_source_set_registration_handler
.Fa "dispatch_source_t source"
.Fa "void (^block)(void)"
.Fc
.Ft void
.Fo dispatch_source_set_registration_handler_f
.Fa "dispatch_source_t source"
.Fa "void (*function)(void *)"
.Fc
.Ft void
.Fo dispatch_source_set_cancel_handler
.Fa "dispatch_source_t source"
.Fa "void (^block)(void)"
.Fc
.Ft void
.Fo dispatch_source_set_cancel_handler_f
.Fa "dispatch_source_t source"
.Fa "void (*function)(void *)"
.Fc
.Ft void
.Fo dispatch_source_cancel
.Fa "dispatch_source_t source"
.Fc
.Ft long
.Fo dispatch_source_testcancel
.Fa "dispatch_source_t source"
.Fc
.Ft uintptr_t
.Fo dispatch_source_get_handle
.Fa "dispatch_source_t source"
.Fc
.Ft "unsigned long"
.Fo dispatch_source_get_mask
.Fa "dispatch_source_t source"
.Fc
.Ft "unsigned long"
.Fo dispatch_source_get_data
.Fa "dispatch_source_t source"
.Fc
.Ft void
.Fo dispatch_source_merge_data
.Fa "dispatch_source_t source"
.Fa "unsigned long data"
.Fc
.Ft void
.Fo dispatch_source_set_timer
.Fa "dispatch_source_t source"
.Fa "dispatch_time_t start"
.Fa "uint64_t interval"
.Fa "uint64_t leeway"
.Fc
.Sh 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.
.Pp
When a state change occurs, the dispatch source will submit its event handler
block to its target queue.
.Pp
The
.Fn dispatch_source_create
function creates a new dispatch source object that may be retained and released
with calls to
.Fn dispatch_retain
and
.Fn dispatch_release
respectively. The
.Fa queue
parameter specifies the target queue of the new source object, it will
be retained by the source object. Pass the
.Dv DISPATCH_TARGET_QUEUE_DEFAULT
constant to use the default target queue (the default priority global
concurrent queue).
.Pp
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
.Fn dispatch_resume
before any events will be delivered.
.Pp
Dispatch sources may be one of the following types:
.Bl -bullet -compact -offset indent
.It
DISPATCH_SOURCE_TYPE_DATA_ADD
.It
DISPATCH_SOURCE_TYPE_DATA_OR
.It
DISPATCH_SOURCE_TYPE_MACH_SEND
.It
DISPATCH_SOURCE_TYPE_MACH_RECV
.It
DISPATCH_SOURCE_TYPE_MEMORYPRESSURE
.It
DISPATCH_SOURCE_TYPE_PROC
.It
DISPATCH_SOURCE_TYPE_READ
.It
DISPATCH_SOURCE_TYPE_SIGNAL
.It
DISPATCH_SOURCE_TYPE_TIMER
.It
DISPATCH_SOURCE_TYPE_VNODE
.It
DISPATCH_SOURCE_TYPE_WRITE
.El
.Pp
The
.Fa handle
and
.Fa mask
arguments to
.Fn dispatch_source_create
and the return values of the
.Fn dispatch_source_get_handle ,
.Fn dispatch_source_get_mask ,
and
.Fn dispatch_source_get_data
functions should be interpreted according to the type of the dispatch source.
.Pp
The
.Fn 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.
.Pp
The
.Fn dispatch_source_get_mask
function
returns the set of flags that were specified at source creation time via the
.Fa mask
argument.
.Pp
The
.Fn 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.
.Pp
The
.Fn dispatch_source_merge_data
function is intended for use with the
.Vt DISPATCH_SOURCE_TYPE_DATA_ADD
and
.Vt 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 bitwise OR the data
into the source's data, and trigger the delivery of the source's event handler.
.Pp
.Sh SOURCE EVENT HANDLERS
In order to receive events from the dispatch source, an event handler should be
specified via
.Fn 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 suspended, 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.
.Pp
Dispatch sources may be suspended or resumed independently of their target
queues using
.Fn dispatch_suspend
and
.Fn 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.
.Pp
The
.Fa handler
block
need not be reentrant safe, as it is not resubmitted to the target
.Fa queue
until any prior invocation for that dispatch source has completed.
When the handler is set, the dispatch source will perform a
.Fn Block_copy
on the
.Fa handler
block.
.Pp
To unset the event handler, call
.Fn dispatch_source_set_event_handler_f
and pass NULL as
.Fa 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.
.Sh REGISTRATION
When 
.Fn 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.
.Pp
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
.Fn dispatch_source_set_registration_handler .
.Pp
The event handler will not be called until the registration handler finishes.
If the source is canceled (see below) before it is registered,
its registration handler will not be called.
.Pp
.Sh CANCELLATION
The
.Fn 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.)
.Pp
The
.Fn 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.
.Pp
When a dispatch source is canceled its optional cancellation handler will be
submitted to its target queue. The cancellation handler may be specified via
.Fn dispatch_source_set_cancel_handler .
This cancellation handler is invoked only once, and only as a direct consequence
of calling
.Fn dispatch_source_cancel .
.Pp
.Em 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.
.Pp
.Sh DISPATCH SOURCE TYPES
The following section contains a summary of supported dispatch event types and
the interpretation of their parameters and returned data.
.Pp
.Vt DISPATCH_SOURCE_TYPE_DATA_ADD ,
.Vt DISPATCH_SOURCE_TYPE_DATA_OR
.Pp
Sources of this type allow applications to manually trigger the source's event
handler via a call to
.Fn 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
.Fa data
is application defined. These sources have no
.Fa handle
or
.Fa mask
and zero should be used.
.Pp
.Vt DISPATCH_SOURCE_TYPE_MACH_SEND
.Pp
Sources of this type monitor a mach port with a send right for state changes.
The
.Fa handle
is the mach port (mach_port_t) to monitor and the
.Fa mask
may be:
.Bl -tag -width "XXDISPATCH_PROC_SIGNAL" -compact -offset indent
.It \(bu DISPATCH_MACH_SEND_DEAD
The port's corresponding receive right has been destroyed
.El
.Pp
The data returned by
.Fn dispatch_source_get_data
indicates which of the events in the
.Fa mask
were observed.
.Pp
.Vt DISPATCH_SOURCE_TYPE_MACH_RECV
.Pp
Sources of this type monitor a mach port with a receive right for state changes.
The
.Fa handle
is the mach port (mach_port_t) to monitor and the
.Fa 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.
.Pp
.Vt DISPATCH_SOURCE_TYPE_MEMORYPRESSURE
.Pp
Sources of this type monitor the system memory pressure condition for state changes.
The
.Fa handle
is unused and should be zero. The
.Fa mask
may be one or more of the following:
.Bl -tag -width "XXDISPATCH_MEMORYPRESSURE_CRITICAL" -compact -offset indent
.It \(bu DISPATCH_MEMORYPRESSURE_NORMAL
The system memory pressure condition has returned to normal.
.It \(bu DISPATCH_MEMORYPRESSURE_WARN
The system memory pressure condition has changed to warning.
.It \(bu DISPATCH_MEMORYPRESSURE_CRITICAL
The system memory pressure condition has changed to critical.
.El
.Pp
The data returned by
.Fn dispatch_source_get_data
indicates which of the events in the
.Fa mask
were observed.
.Pp
Elevated memory pressure is a system-wide condition that applications
registered for this source should react to by changing their future memory use
behavior, e.g. by reducing cache sizes of newly initiated operations until
memory pressure returns back to normal.
.Pp
However, applications should
.Em 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.
.Pp
.Vt DISPATCH_SOURCE_TYPE_PROC
.Pp
Sources of this type monitor processes for state changes.
The
.Fa handle
is the process identifier (pid_t) of the process to monitor and the
.Fa mask
may be one or more of the following:
.Bl -tag -width "XXDISPATCH_PROC_SIGNAL" -compact -offset indent
.It \(bu DISPATCH_PROC_EXIT
The process has exited and is available to
.Xr wait 2 .
.It \(bu DISPATCH_PROC_FORK
The process has created one or more child processes.
.It \(bu DISPATCH_PROC_EXEC
The process has become another executable image via a call to
.Xr execve 2
or
.Xr posix_spawn 2 .
.It \(bu DISPATCH_PROC_SIGNAL
A signal was delivered to the process.
.El
.Pp
The data returned by
.Fn dispatch_source_get_data
indicates which of the events in the
.Fa mask
were observed.
.Pp
.Vt DISPATCH_SOURCE_TYPE_READ
.Pp
Sources of this type monitor file descriptors for pending data.
The
.Fa handle
is the file descriptor (int) to monitor and the
.Fa mask
is unused and should be zero.
.Pp
The data returned by
.Fn 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
.Em minimum
read buffer size. There are no guarantees that a complete read of this size
will be performed.
.Pp
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
.Xr fcntl 2
for additional information about setting the
.Vt O_NONBLOCK
flag on a file descriptor.
.Pp
.Vt DISPATCH_SOURCE_TYPE_SIGNAL
.Pp
Sources of this type monitor signals delivered to the current process. The
.Fa handle
is the signal number to monitor (int) and the
.Fa mask
is unused and should be zero.
.Pp
The data returned by
.Fn dispatch_source_get_data
is the number of signals received since the last invocation of the event handler
block.
.Pp
Unlike signal handlers specified via
.Fn 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
.Xr sigaction 2 .
Furthermore, multiple observers of a given signal are supported; thus allowing
applications and libraries to cooperate safely. However, a dispatch source
.Em 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
.Fn main :
.Bd -literal -offset ident
signal(SIGTERM, SIG_IGN);
.Ed
.Pp
.Vt DISPATCH_SOURCE_TYPE_TIMER
.Pp
Sources of this type periodically submit the event handler block to the target
queue. The
.Fa handle
argument is unused and should be zero.
.Pp
The data returned by
.Fn dispatch_source_get_data
is the number of times the timer has fired since the last invocation of the
event handler block.
.Pp
The timer parameters are configured with the
.Fn 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
.Fa start ,
and every
.Fa interval
nanoseconds thereafter until the timer source is canceled.
.Pp
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 allowable delay may
be configured with the
.Fa leeway
argument, the lower limit is under the control of the system.
.Pp
For the initial timer fire at
.Fa start ,
the upper limit to the allowable delay is set to
.Fa leeway
nanoseconds. For the subsequent timer fires at
.Fa start
.Li "+ N *"
.Fa interval ,
the upper limit is
.Li MIN(
.Fa leeway ,
.Fa interval
.Li "/ 2 )" .
.Pp
The lower limit to the allowable delay may vary with process state such as
visibility of application UI. If the specified timer source was created with a
.Fa mask
of
.Vt DISPATCH_TIMER_STRICT ,
the system will make a best effort to strictly observe the provided
.Fa 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.
.Pp
The
.Fa start
argument also determines which clock will be used for the timer: If
.Fa start
is
.Vt DISPATCH_TIME_NOW
or was created with
.Xr dispatch_time 3 ,
the timer is based on
.Fn mach_absolute_time .
If
.Fa start
was created with
.Xr dispatch_walltime 3 ,
the timer is based on
.Xr gettimeofday 3 .
.Pp
.Em Note :
Under the C language, untyped numbers default to the
.Vt int
type. This can lead to truncation bugs when arithmetic operations with other
numbers are expected to generate a
.Vt uint64_t
sized result. When in doubt, use
.Vt ull
as a suffix. For example:
.Bd -literal -offset indent
3ull * NSEC_PER_SEC
.Ed
.Pp
.Vt DISPATCH_SOURCE_TYPE_VNODE
.Pp
Sources of this type monitor the virtual filesystem nodes for state changes.
The
.Fa handle
is a file descriptor (int) referencing the node to monitor, and
the
.Fa mask
may be one or more of the following:
.Bl -tag -width "XXDISPATCH_VNODE_ATTRIB" -compact -offset indent
.It \(bu DISPATCH_VNODE_DELETE
The referenced node was removed from the filesystem namespace via
.Xr unlink 2 .
.It \(bu DISPATCH_VNODE_WRITE
A write to the referenced file occurred
.It \(bu DISPATCH_VNODE_EXTEND
The referenced file was extended
.It \(bu DISPATCH_VNODE_ATTRIB
The metadata attributes of the referenced node have changed
.It \(bu DISPATCH_VNODE_LINK
The link count on the referenced node has changed
.It \(bu DISPATCH_VNODE_RENAME
The referenced node was renamed
.It \(bu DISPATCH_VNODE_REVOKE
Access to the referenced node was revoked via
.Xr revoke 2
or the underlying fileystem was unmounted.
.El
.Pp
The data returned by
.Fn dispatch_source_get_data
indicates which of the events in the
.Fa mask
were observed.
.Pp
.Vt DISPATCH_SOURCE_TYPE_WRITE
.Pp
Sources of this type monitor file descriptors for available write buffer space.
The
.Fa handle
is the file descriptor (int) to monitor and the
.Fa mask
is unused and should be zero.
.Pp
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
.Xr fcntl 2
for additional information about setting the
.Vt O_NONBLOCK
flag on a file descriptor.
.Pp
.Sh SEE ALSO
.Xr dispatch 3 ,
.Xr dispatch_object 3 ,
.Xr dispatch_queue_create 3