more externals questions

[Alex Rice]

> On Thursday, June 19, 2003, at 01:57  PM, Yennie at aol.com wrote:
>> This is because you want to explicity allocate memory for the string 
>> which MetaCard can use and dispose of. A string constant is allocated 
>> locally in your external and could potentially make bad things happen 
>> when MetaCard tries to read from that spot in memory (or dispose of 
>> the memory).
>>
>> It may be the case that the memory is dealt with so quickly that the 
>> risk is small with such a small return value, but you could 
>> potentially get into trouble by way of bad luck, platform 
>> differences, and/or the size of the return value.

>> Most likely MetaCard checks for NULL, in which case this is ok- but 
>> if Metacard is expecting a pointer, it's more "correct" to give it 
>> one. If the engine were to try to read from that pointer after you 
>> set it to NULL, it could crash your app.
>>
>> All this is doing is creating an empty string: strings are terminated 
>> with a NULL byte, so the 1 byte is just the "end-of-string" marker 
>> with no string data before it

Yennie, & List:

I have the Hypercard Script Language Guide, and Appendix A is about 
writing Externals.

At first I was put off because the call interface seems to be different 
in RR externals than inHypercard XCMD/XFCNs: there used to be an 
XCmdPtr structure, but now it's a more explicit interface (args, nargs, 
retstring, pass, error).

Upon looking closer, the rest of the Appendix seems to more or less 
match up with what I'm seeing in the external.c demo stack. Lo and 
behold, I guess this is the oft referred to API for externals!!

Here is a note about the question above. (p.510) "Storing a result 
string is optional for an XCMD; it is expected of an XFCN, but it's not 
required." If that still holds true then it's OK to return NULL from 
xcommand/functions?

Question:  How literally should I take the stuff in Appendix A of 
Hypercard Script Language Guide? Any areas where RR is known to have 
made a big departure from this original specification of how externals 
are meant to be written?

Alex Rice, Software Developer
Architectural Research Consultants, Inc.
http://ARCplanning.com