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 asynchro-nous asynchronous
nous 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 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 interpreter may already be in the middle of evaluating a script; it may not even be safe to allo-cate allocate
cate 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 sensitive. 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_Async-Mark(token) Tcl_AsyncMark(token)
Mark(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_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 comple-tion completion
tion code returned by that command. The command'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 exam-ple, example,
ple, 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_Async-Invoke Tcl_AsyncInvoke
Invoke at interesting times for that application. For example, Tcl's event handler calls Tcl_Asyn-cReady Tcl_AsyncReady
cReady 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 han-dler handler
dler 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 asyn-chronous asynchronous
chronous 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, errorInfo, and errorCode, and return the code argument.
KEYWORDS
asynchronous event, handler, signal
Tcl 7.0 Tcl_AsyncCreate(3)
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