path: root/doc/SetChanErr.3

'\"
'\" Copyright (c) 2005 Andreas Kupries <andreas_kupries@users.sourceforge.net>
'\"
'\" See the file "license.terms" for information on usage and redistribution
'\" of this file, and for a DISCLAIMER OF ALL WARRANTIES.
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.TH Tcl_SetChannelError 3 8.5 Tcl "Tcl Library Procedures"
.so man.macros
.BS
'\" Note:  do not modify the .SH NAME line immediately below!
.SH NAME
Tcl_SetChannelError, Tcl_SetChannelErrorInterp, Tcl_GetChannelError, Tcl_GetChannelErrorInterp \- functions to create/intercept Tcl errors by channel drivers.
.SH SYNOPSIS
.nf
\fB#include <tcl.h>\fR
.sp
void
\fBTcl_SetChannelError\fR(\fIchan, msg\fR)
.sp
void
\fBTcl_SetChannelErrorInterp\fR(\fIinterp, msg\fR)
.sp
void
\fBTcl_GetChannelError\fR(\fIchan, msgPtr\fR)
.sp
void
\fBTcl_GetChannelErrorInterp\fR(\fIinterp, msgPtr\fR)
.sp
.SH ARGUMENTS
.AS Tcl_Channel chan
.AP Tcl_Channel chan in
Refers to the Tcl channel whose bypass area is accessed.
.AP Tcl_Interp* interp in
Refers to the Tcl interpreter whose bypass area is accessed.
.AP Tcl_Obj* msg in
Error message put into a bypass area. A list of return options and values,
followed by a string message. Both message and the option/value information
are optional. This \fImust\fR be a well-formed list.
.AP Tcl_Obj** msgPtr out
Reference to a place where the message stored in the accessed bypass area can
be stored in.
.BE
.SH DESCRIPTION
.PP
The standard definition of a Tcl channel driver does not permit the direct
return of arbitrary error messages, except for the setting and retrieval of
channel options. All other functions are restricted to POSIX error codes.
.PP
The functions described here overcome this limitation. Channel drivers are
allowed to use \fBTcl_SetChannelError\fR and \fBTcl_SetChannelErrorInterp\fR
to place arbitrary error messages in \fIbypass areas\fR defined for channels
and interpreters. And the generic I/O layer uses \fBTcl_GetChannelError\fR and
\fBTcl_GetChannelErrorInterp\fR to look for messages in the bypass areas and
arrange for their return as errors. The POSIX error codes set by a driver are
used now if and only if no messages are present.
.PP
\fBTcl_SetChannelError\fR stores error information in the bypass area of the
specified channel. The number of references to the \fBmsg\fR value goes up by
one. Previously stored information will be discarded, by releasing the
reference held by the channel. The channel reference must not be NULL.
.PP
\fBTcl_SetChannelErrorInterp\fR stores error information in the bypass area of
the specified interpreter. The number of references to the \fBmsg\fR value
goes up by one. Previously stored information will be discarded, by releasing
the reference held by the interpreter. The interpreter reference must not be
NULL.
.PP
\fBTcl_GetChannelError\fR places either the error message held in the bypass
area of the specified channel into \fImsgPtr\fR, or NULL; and resets the
bypass, that is, after an invocation all following invocations will return
NULL, until an intervening invocation of \fBTcl_SetChannelError\fR with a
non-NULL message. The \fImsgPtr\fR must not be NULL. The reference count of
the message is not touched.  The reference previously held by the channel is
now held by the caller of the function and it is its responsibility to release
that reference when it is done with the value.
.PP
\fBTcl_GetChannelErrorInterp\fR places either the error message held in the
bypass area of the specified interpreter into \fImsgPtr\fR, or NULL; and
resets the bypass, that is, after an invocation all following invocations will
return NULL, until an intervening invocation of
\fBTcl_SetChannelErrorInterp\fR with a non-NULL message. The \fImsgPtr\fR must
not be NULL. The reference count of the message is not touched.  The reference
previously held by the interpreter is now held by the caller of the function
and it is its responsibility to release that reference when it is done with
the value.
.PP
Which functions of a channel driver are allowed to use which bypass function
is listed below, as is which functions of the public channel API may leave a
messages in the bypass areas.
.IP \fBTcl_DriverInputProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverOutputProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverWideSeekProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverSetOptionProc\fR
Has already the ability to pass arbitrary error messages. Must \fInot\fR use
any of the new functions.
.IP \fBTcl_DriverGetOptionProc\fR
Has already the ability to pass arbitrary error messages. Must
\fInot\fR use any of the new functions.
.IP \fBTcl_DriverWatchProc\fR
Must \fInot\fR use any of the new functions. Is internally called and has no
ability to return any type of error whatsoever.
.IP \fBTcl_DriverBlockModeProc\fR
May use \fBTcl_SetChannelError\fR, and only this function.
.IP \fBTcl_DriverGetHandleProc\fR
Must \fInot\fR use any of the new functions. It is only a low-level function,
and not used by Tcl commands.
.IP \fBTcl_DriverHandlerProc\fR
Must \fInot\fR use any of the new functions. Is internally called and has no
ability to return any type of error whatsoever.
.PP
Given the information above the following public functions of the Tcl C API
are affected by these changes; when these functions are called, the channel
may now contain a stored arbitrary error message requiring processing by the
caller.
.DS
.ta 1.9i 4i
\fBTcl_Flush\fR	\fBTcl_GetsObj\fR	\fBTcl_Gets\fR
\fBTcl_ReadChars\fR	\fBTcl_ReadRaw\fR	\fBTcl_Read\fR
\fBTcl_Seek\fR	\fBTcl_StackChannel\fR	\fBTcl_Tell\fR
\fBTcl_WriteChars\fR	\fBTcl_WriteObj\fR	\fBTcl_WriteRaw\fR
\fBTcl_Write\fR
.DE
.PP
All other API functions are unchanged. In particular, the functions below
leave all their error information in the interpreter result.
.DS
.ta 1.9i 4i
\fBTcl_Close\fR	\fBTcl_UnstackChannel\fR	\fBTcl_UnregisterChannel\fR
.DE
.SH "REFERENCE COUNT MANAGEMENT"
.PP
The \fImsg\fR argument to \fBTcl_SetChannelError\fR and
\fBTcl_SetChannelErrorInterp\fR, if not NULL, may have any reference count;
these functions will copy.
.PP
\fBTcl_GetChannelError\fR and \fBTcl_GetChannelErrorInterp\fR write a value
reference into their \fImsgPtr\fR, but do not manipulate its reference count.
The reference count will be at least 1 (unless the reference is NULL).
.SH "SEE ALSO"
Tcl_Close(3), Tcl_OpenFileChannel(3), Tcl_SetErrno(3)
.SH KEYWORDS
channel driver, error messages, channel type