**NRE enabled C commands**

***Rationale***
[HaO] 2014-01-29: (to be removed) I started this page due to lack of information how to handle NRE for commands implemented in C.
I would be glad, if this page could be populated by people having more knowledge about the issue.
The following information comes from this [https://groups.google.com/forum/#!topic/comp.lang.tcl/eu0_3dpViug%|%clt%|%] communication from [Miguel Sofer] and [Donal Fellows]:

***Intro***

I have some commands in a dll which are typically created in the Dllname_Init using:
======
Tcl_CreateObjCommand(Interp, "cmd", cmdProc, NULL, NULL);
======

I want to use them with tcl8.6 and I am thinking if it is wise/necessary to modify them to be NRE enabled.

Thus, the upper has to be replaced by:
======
Tcl_NRCreateCommand(Interp, "cmd", cmdProc, cmdNREProc, NULL, NULL);
======

Then there is an example on: [http://wiki.tcl.tk/21771] but I am really lost there.

Thus my questions:


***Q1) Is NRE-enableing for C commands a good idea?***

If yes, under which circumstances ?

****Answer by Miguel Sofer****

It is only a good idea if:

   1. the command is likely to cause a script (or proc, or lambda, or oo method) evaluation (except via traces, those are "innocuous")
   2. you want to allow the evaluated scripts to [yield]

If either of those two conditions are not met, you are much better off NOT NRE-enabling your commands - you'd just be paying the cost with zero benefits.

Note that 8.6 properly runs all 8.5 C-extensions (assuming they do not use internal headers, in which case it's a "maybe"). 


***Q2) Compatibility***

I suppose a NRE enabled dll is not usable with tcl 8.5.x.
Is there a way to make a dll working on tcl 8.5 and 8.6 nre enabled
 
****Answer by Miguel Sofer****

Yes. Compile using stubs against 8.6's tcl.h, and switch at runtime between 8.5 or 8.6 code depending on (for instance) the existence of the command [yield]. 


***Q3) Is there a programming example***

I am sorry, the wiki example did not help me understanding what to do.
Is there any way.

****Answer by Miguel Sofer****

Plenty of those in the 8.6 sources.
I suggest starting with something simple, for instance [catch].
Compare the 8.5 and 8.6 implementations in tclCmdAH.c 

***Q4) Is it worthwile to NRE enable Event handler***

Answer moved on [http://wiki.tcl.tk/17195].

2015-03-23: [APN] Although not documented as such, Tcl_NREvalObjv
require the objv parameter to be dynamically allocated,
*not* on the stack. The following comes from Miguel on the chat:

Indeed, dyn alloc is required. One good strategy is to use a tcl list
(unshared, so that it can't shimmer!) to store the objv, push a
callback to decrRefCount it, then call Tcl_NREvalObjv with the
list elements as objv. Need better docs for that ... or even a new 
api to automate that. In the core most of those live in the Tcl
stack ... which would also be the case if your twapi command is
called from normal Tcl code... but if YOU are in charge, you
have to take care of the important detail: not only must objv be
accessible until the command finishes runnind, the contents must
have a live refCount for the duration too