[Info-vax] VMS Basic strings class D vs class S

[Stephen Hoffman]

On 2024-02-25 21:33:33 +0000, Arne Vajhøj said:

> If I have understood it correctly then VMS Basic strings use class D 
> descriptors.
> 
> That is very nice.
> 
> But what happens if non-Basic code call Basic code with a string using 
> a class S descriptor? For input/read? For output/write?
> 
> Does the Basic runtime call some STR$ function that understands the 
> difference between S and D and handle A properly? Or will I get a 
> runtime error due to invalid string?

"It depends."

Most everything in most of the traditional languages and in the RTLs 
does the right thing with both dynamic and static text strings, though 
the app code involved might not. BASIC app code works pretty well here, 
absent "heroic" efforts by the app developer.

If the app code assumes a dynamic arriving and gets handed static, the 
RTL will either copy it, or space-pad the results into the static, or 
the RTL will return a string truncation error. BASIC space-pads into 
static string buffers if and as needed. Or truncates with an error.

Apps written in C, C++, BLISS, MACRO32, and probably some others may or 
may not do the right thing with descriptors, as these languages can 
need to handle descriptors in app code due to the (~lack of) descriptor 
support in those languages. (Yes, I well know about dscdef.h, 
descrip.h, et al., thanks.) Some devs will use RTL calls, and some use 
explicit code.

As for home-grown descriptor code, few apps (nobody?) implements all of 
the different sorts of descriptors available. Not outside of the RTL 
itself, that is.

Apps expecting to work with dynamic descriptors might fail with the 
truncation error as mentioned, and apps expecting to massage static 
descriptors directly and not coded sufficiently cautiously around any 
arriving dynamic strings can fail with heap and other errors.

Of the common languages, Pascal utilizes a wide variety of descriptors. 
Calling into or getting called from Pascal tends to teach much about 
descriptors and descriptor usage.

it wouldn't surprise me to learn that BASIC will fail to work correctly 
with 64-bit string descriptors, though. Lots of home-grown app code 
also won't. I also wouldn't expect the RTLs to work with encodings 
other than ASCII and DEC MCS, either. And UTF-8 will fail in the 
expected places, and most searching and sorting tends not to be 
sensitive to the (written) language used within the text string.



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