Async
Tcl_AsyncCreate(3) Tcl Library Procedures Tcl_AsyncCreate(3)
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NAME
Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke, Tcl_AsyncDelete,
Tcl_AsyncReady - handle asynchronous events
SYNOPSIS
#include <tcl.h>
Tcl_AsyncHandler
Tcl_AsyncCreate(proc, clientData)
Tcl_AsyncMark(async)
int
Tcl_AsyncInvoke(interp, code)
Tcl_AsyncDelete(async)
int
Tcl_AsyncReady()
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 com-
mand 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 substan-
tive action to process the event. For example, it isn't safe to evalu-
ate a Tcl script since the interpreter 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, Tcl_AsyncDelete, and Tcl_AsyncReady are thread sensi-
tive. They access and/or set a thread-specific data structure in the
event of an --enable-thread built core. The token created by
Tcl_AsyncCreate contains the needed thread information it was called
from so that calling Tcl_AsyncMark(token) will only yield the origin
thread into the AsyncProc.
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_Async-
Mark 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 com-
mand's result will be present in the interpreter's result. When proc
returns, whatever it leaves in the interpreter's 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 procedure Tcl_AsyncReady will return 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
calls 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, Tcl's event
handler calls 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 interpreter, 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 the interpreter's 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 the interpreter's 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 the interpreter's 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 the interpreter's result, error-
Info, and errorCode, and return the code argument.
KEYWORDS
asynchronous event, handler, signal
Tcl 7.0 Tcl_AsyncCreate(3)
