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.so man.macros

.TH Tcl_AsyncCreate 3 7.0 Tcl "Tcl Library Procedures"

.BS

.SH NAME

Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke, Tcl_AsyncDelete \- handle asynchronous events

.SH SYNOPSIS

.nf

\fB#include \fR

.sp

Tcl_AsyncHandler

\fBTcl_AsyncCreate\fR(\fIproc, clientData\fR)

.sp

\fBTcl_AsyncMark\fR(\fIasync\fR)

.sp

int

\fBTcl_AsyncInvoke\fR(\fIinterp, code\fR)

.sp

\fBTcl_AsyncDelete\fR(\fIasync\fR)

.sp

int

\fBTcl_AsyncReady\fR()

.SH ARGUMENTS

.AS Tcl_AsyncHandler clientData

.AP Tcl_AsyncProc *proc in

Procedure to invoke to handle an asynchronous event.

.AP ClientData clientData in

One-word value to pass to \fIproc\fR.

.AP Tcl_AsyncHandler async in

Token for asynchronous event handler.

.AP 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.

.AP int code in

Completion code from command that just completed in \fIinterp\fR,

or 0 if \fIinterp\fR is NULL.

.BE

.SH DESCRIPTION

.PP

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

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.

.PP

\fBTcl_AsyncCreate\fR 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 \fBTcl_AsyncMark\fR with the

token for the handler.

\fBTcl_AsyncMark\fR will mark the handler as ready to execute, but it

will not invoke the handler immediately.

Tcl will call the \fIproc\fR associated with the handler later, when

the world is in a safe state, and \fIproc\fR can then carry out

the actions associated with the asynchronous event.

\fIProc\fR should have arguments and result that match the

type \fBTcl_AsyncProc\fR:

.CS

typedef int Tcl_AsyncProc(

        ClientData \fIclientData\fR,

        Tcl_Interp *\fIinterp\fR,

        int \fIcode\fR);

.CE

The \fIclientData\fR will be the same as the \fIclientData\fR

argument passed to \fBTcl_AsyncCreate\fR when the handler was

created.

If \fIproc\fR is invoked just after a command has completed

execution in an interpreter, then \fIinterp\fR will identify

the interpreter in which the command was evaluated and

\fIcode\fR will be the completion code returned by that

command.

The command's result will be present in \fIinterp->result\fR.

When \fIproc\fR returns, whatever it leaves in \fIinterp->result\fR

will be returned as the result of the command and the integer

value returned by \fIproc\fR will be used as the new completion

code for the command.

.PP

It is also possible for \fIproc\fR 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 \fIinterp\fR will be NULL and \fIcode\fR will be

0, and the return value from \fIproc\fR will be ignored.

.PP

The procedure \fBTcl_AsyncInvoke\fR is called to invoke all of the

handlers that are ready.

The procedure \fBTcl_AsyncReady\fR will return non-zero whenever any

asynchronous handlers are ready;  it can be checked to avoid calls

to \fBTcl_AsyncInvoke\fR when there are no ready handlers.

Tcl calls \fBTcl_AsyncReady\fR after each command is evaluated

and calls \fBTcl_AsyncInvoke\fR if needed.

Applications may also call \fBTcl_AsyncInvoke\fR at interesting

times for that application.

For example, Tcl's event handler calls \fBTcl_AsyncReady\fR

after each event and calls \fBTcl_AsyncInvoke\fR if needed.

The \fIinterp\fR and \fIcode\fR arguments to \fBTcl_AsyncInvoke\fR

have the same meaning as for \fIproc\fR:  they identify the active

interpreter, if any, and the completion code from the command

that just completed.

.PP

\fBTcl_AsyncDelete\fR removes an asynchronous handler so that

its \fIproc\fR will never be invoked again.

A handler can be deleted even when ready, and it will still

not be invoked.

.PP

If multiple handlers become active at the same time, the

handlers are invoked in the order they were created (oldest

handler first).

The \fIcode\fR and \fIinterp->result\fR 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,

\fBTcl_AsyncInvoke\fR 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.

.SH WARNING

.PP

It is almost always a bad idea for an asynchronous event

handler to modify \fIinterp->result\fR or return a code different

from its \fIcode\fR 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 \fIinterp->result\fR plus the values

of the variables \fBerrorInfo\fR and \fBerrorCode\fR (this can

be done, for example, by storing them in dynamic strings).

When the asynchronous handler is finished it should restore

\fIinterp->result\fR, \fBerrorInfo\fR, and \fBerrorCode\fR,

and return the \fIcode\fR argument.

.SH KEYWORDS

asynchronous event, handler, signal