Async
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NAME
Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke,
Tcl_AsyncDelete - handle asynchronous events
SYNOPSIS
#include <tcl.h>
extern int tcl_AsyncReady;
Tcl_AsyncHandler
Tcl_AsyncCreate(proc, clientData)
Tcl_AsyncMark(async)
int
Tcl_AsyncInvoke(interp, code)
Tcl_AsyncDelete(async)
ARGUMENTS
Tcl_AsyncProc *proc (in) Procedure to
invoke to handle
an asynchronous
event.
ClientData clientData (in) One-word value to
pass to proc.
Tcl_AsyncHandler async (in) Token for
asynchronous event
handler.
Tcl_Interp *interp (in) Tcl interpreter in
which command was
being evaluated
when handler was
invoked, or NULL
if handler was
invoked when there
was no interpreter
active.
int code (in) Completion code
from command that
just completed in
interp, or 0 if
interp is NULL.
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DESCRIPTION
These procedures provide a safe mechanism for dealing with
asynchronous events such as signals. If an event such as a
signal occurs while a Tcl script is being evaluated then it
isn't safe to take any substantive action to process the
event. For example, it isn't safe to evaluate a Tcl script
since the intepreter may already be in the middle of
evaluating a script; it may not even be safe to allocate
memory, since a memory allocation could have been in
progress when the event occurred. The only safe approach is
to set a flag indicating that the event occurred, then
handle the event later when the world has returned to a
clean state, such as after the current Tcl command
completes.
Tcl_AsyncCreate creates an asynchronous handler and returns
a token for it. The asynchronous handler must be created
before any occurrences of the asynchronous event that it is
intended to handle (it is not safe to create a handler at
the time of an event). When an asynchronous event occurs
the code that detects the event (such as a signal handler)
should call Tcl_AsyncMark with the token for the handler.
Tcl_AsyncMark will mark the handler as ready to execute, but
it will not invoke the handler immediately. Tcl will call
the proc associated with the handler later, when the world
is in a safe state, and proc can then carry out the actions
associated with the asynchronous event. Proc should have
arguments and result that match the type Tcl_AsyncProc:
typedef int Tcl_AsyncProc(
ClientData clientData,
Tcl_Interp *interp,
int code);
The clientData will be the same as the clientData argument
passed to Tcl_AsyncCreate when the handler was created. If
proc is invoked just after a command has completed execution
in an interpreter, then interp will identify the interpreter
in which the command was evaluated and code will be the
completion code returned by that command. The command's
result will be present in interp->result. When proc
returns, whatever it leaves in interp->result will be
returned as the result of the command and the integer value
returned by proc will be used as the new completion code for
the command.
It is also possible for proc to be invoked when no
interpreter is active. This can happen, for example, if an
asynchronous event occurs while the application is waiting
for interactive input or an X event. In this case interp
will be NULL and code will be 0, and the return value from
proc will be ignored.
The procedure Tcl_AsyncInvoke is called to invoke all of the
handlers that are ready. The global variable tcl_AsyncReady
will be non-zero whenever any asynchronous handlers are
ready; it can be checked to avoid calls to Tcl_AsyncInvoke
when there are no ready handlers. Tcl checks tcl_AsyncReady
after each command is evaluated and calls Tcl_AsyncInvoke if
needed. Applications may also call Tcl_AsyncInvoke at
interesting times for that application. For example, Tk's
event handler checks tcl_AsyncReady after each event and
calls Tcl_AsyncInvoke if needed. The interp and code
arguments to Tcl_AsyncInvoke have the same meaning as for
proc: they identify the active intepreter, if any, and the
completion code from the command that just completed.
Tcl_AsyncDelete removes an asynchronous handler so that its
proc will never be invoked again. A handler can be deleted
even when ready, and it will still not be invoked.
If multiple handlers become active at the same time, the
handlers are invoked in the order they were created (oldest
handler first). The code and interp->result for later
handlers reflect the values returned by earlier handlers, so
that the most recently created handler has last say about
the interpreter's result and completion code. If new
handlers become ready while handlers are executing,
Tcl_AsyncInvoke will invoke them all; at each point it
invokes the highest-priority (oldest) ready handler,
repeating this over and over until there are no longer any
ready handlers.
WARNING
It is almost always a bad idea for an asynchronous event
handler to modify interp->result or return a code different
from its code argument. This sort of behavior can disrupt
the execution of scripts in subtle ways and result in bugs
that are extremely difficult to track down. If an
asynchronous event handler needs to evaluate Tcl scripts
then it should first save interp->result plus the values of
the variables errorInfo and errorCode (this can be done, for
example, by storing them in dynamic strings). When the
asynchronous handler is finished it should restore interp-
>result, errorInfo, and errorCode, and return the code
argument.
KEYWORDS
asynchronous event, handler, signal