Tcl_AsyncDelete

Tcl_AsyncCreate(3)          Tcl Library Procedures          Tcl_AsyncCreate(3)



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NAME
       Tcl_AsyncCreate, Tcl_AsyncMark, Tcl_AsyncInvoke, Tcl_AsyncDelete -
       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
                                                 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 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 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 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



Tcl                                   7.0                   Tcl_AsyncCreate(3)