# TIP 428: Produce Error Dictionary from 'fconfigure -error'
Author: Harald Oehlmann <[email protected]>
Author: Harald Oehlmann <[email protected]>
State: Draft
Type: Project
Vote: Pending
Created: 16-Mar-2014
Post-History:
Keywords: socket,non-blocking,error reporting,option dictionary
Tcl-Version: 8.7
Tcl-Branch: tip-428
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# Abstract
This TIP proposes a new method which allows to return the error message and the error code of a background socket error \(as reported by **fconfigure -error**\), similar to the option dictionaries produced by catch and try and consumed by return.
# Rationale
The error message of a background channel error may be retrieved and
cleared by **fconfigure** _channel_ **-error**, but there is no access to the error code.
In addition, the error may not be handled in the TCL-like way using **catch** or **try** \(or just let fail the program\).
Specially the new **try** syntax \(see example in the man page\) is well suited to handle socket errors.
Example:
try {set h [socket $host $port]}\
trap {POSIX ECONNREFUSED} {} {
# handle not open port
}
Drivers mostly use POSIX errors to report issues where the error code is more portable than the error message \(AFAIK\).
To handle an error by **try**, the error must be thrown.
We are limited to an option to the command **fconfigure**, as this is implemented within the driver interface.
Throwing the error would change the semantics of **fconfigure** and thus should not happen \(consensus on the core list\).
Instead, the new **fconfigure** operation should return the error message and the error code.
To finally throw the error, an utility function \(**chan throwerror $h**\) may be defined in TCL.
This is not part of this TIP.
# Proposed Change
The option **fconfigure channel -error** should be extended to take an optional argument as follows:
> **fconfigure** _channel_ **-error** _?errorDictVar?_
If the optional argument _errorDictVar_ is given, the following dict is written in the named variable of the caller environment:
* if there is no error, it should be set to **-code 0**
* if there is an error, it should be set to **-code 1 -errorcode ** _errorCode_
This is executed in addition to the standard action of **fconfigure** _channel_ **-error**.
# Example
Usage example with failing async connect:
% set h [connect -async localhost 30001]
d00000af
% fileevent $h writable {set x 1}
% vwait x
% fconfigure $h -error errorDict
connection refused
% set errorDict
-code 1 -errorcode {POSIX ECONNREFUSED {connection refused}}
% close $h
The following example demonstrates the implementation of **chan throwerror** eg to throw the error from the provided dict.
proc throwerror {h} {
set errorMessage [fconfigure $h -errorDict]
return -options $errorDict $errorMessage
}
# Alternatives
* Revision 1.11 of this TIP proposed to really throw the error.
* Revision 1.21 of this TIP proposed a new option to return an error dict directly.
# Implementation
The tip is implemented in fossil branch **tip-428**.
# Remarks
The idea of this semantics and a feasability study is from Reinhard Max.
# Copyright
This document has been placed in the public domain.