source: downloads/tcl8.5.2/doc/CrtFileHdlr.3 @ 44

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.so man.macros
.TH Tcl_CreateFileHandler 3 8.0 Tcl "Tcl Library Procedures"
.BS
.SH NAME
Tcl_CreateFileHandler, Tcl_DeleteFileHandler \- associate procedure callbacks with files or devices (Unix only)
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
\fBTcl_CreateFileHandler\fR(\fIfd, mask, proc, clientData\fR)
.sp
\fBTcl_DeleteFileHandler\fR(\fIfd\fR)
.SH ARGUMENTS
.AS Tcl_FileProc clientData
.AP int fd in
Unix file descriptor for an open file or device.
.AP int mask in
Conditions under which \fIproc\fR should be called:
OR-ed combination of \fBTCL_READABLE\fR, \fBTCL_WRITABLE\fR,
and \fBTCL_EXCEPTION\fR.  May be set to 0 to temporarily disable
a handler.
.AP Tcl_FileProc *proc in
Procedure to invoke whenever the file or device indicated
by \fIfile\fR meets the conditions specified by \fImask\fR.
.AP ClientData clientData in
Arbitrary one-word value to pass to \fIproc\fR.
.BE

.SH DESCRIPTION
.PP
\fBTcl_CreateFileHandler\fR arranges for \fIproc\fR to be
invoked in the future whenever I/O becomes possible on a file
or an exceptional condition exists for the file.  The file
is indicated by \fIfd\fR, and the conditions of interest
are indicated by \fImask\fR.  For example, if \fImask\fR
is \fBTCL_READABLE\fR, \fIproc\fR will be called when
the file is readable.
The callback to \fIproc\fR is made by \fBTcl_DoOneEvent\fR, so
\fBTcl_CreateFileHandler\fR is only useful in programs that dispatch
events through \fBTcl_DoOneEvent\fR or through Tcl commands such
as \fBvwait\fR.
.PP
\fIProc\fR should have arguments and result that match the
type \fBTcl_FileProc\fR:
.CS
typedef void Tcl_FileProc(
        ClientData \fIclientData\fR,
        int \fImask\fR);
.CE
The \fIclientData\fR parameter to \fIproc\fR is a copy
of the \fIclientData\fR
argument given to \fBTcl_CreateFileHandler\fR when the callback
was created.  Typically, \fIclientData\fR points to a data
structure containing application-specific information about
the file.  \fIMask\fR is an integer mask indicating which
of the requested conditions actually exists for the file;  it
will contain a subset of the bits in the \fImask\fR argument
to \fBTcl_CreateFileHandler\fR.
.PP
.PP
There may exist only one handler for a given file at a given time.
If \fBTcl_CreateFileHandler\fR is called when a handler already
exists for \fIfd\fR, then the new callback replaces the information
that was previously recorded.
.PP
\fBTcl_DeleteFileHandler\fR may be called to delete the
file handler for \fIfd\fR;  if no handler exists for the
file given by \fIfd\fR then the procedure has no effect.
.PP
The purpose of file handlers is to enable an application to respond to
events while waiting for files to become ready for I/O.  For this to work
correctly, the application may need to use non-blocking I/O operations on
the files for which handlers are declared.  Otherwise the application may
block if it reads or writes too much data; while waiting for the I/O to
complete the application will not be able to service other events. Use
\fBTcl_SetChannelOption\fR with \fB\-blocking\fR to set the channel into
blocking or nonblocking mode as required.
.PP
Note that these interfaces are only supported by the Unix
implementation of the Tcl notifier.   

.SH KEYWORDS
callback, file, handler