[Info-vax] Whither VMS?

[Tim E. Sneddon]

glen herrmannsfeldt wrote:
> JF Mezei <jfmezei.spamnot at vaxination.ca> wrote:
> < Re: C and buffer overflows.
>  
> < Is there a language which keeps 2 values for each string: amount
> < allocated and amount currently in use ?

As Glen points out PL/I does support this as part of the language.
VMS also has the descriptor type of DSC$K_DTYPE_VT.  This is a
descriptor that keeps the maximum length in DSC$W_LENGTH while
maintaining the actual length as the first word of the buffer
pointed at by DSC$A_POINTER.

>  
> < Without such 2 values, you can't have totally save "read" and "write"
> < into that variable. When you write, you need to know how many bytes you
> < can write, and when you read, you need to know how many bytes were
> < written to it.
> 
> I believe that PL/I does it.  In the implementations that I know of,
> as well as I remember, the allocated length is known in the procedure
> and passed in a descriptor (dope vector) on calls.  The current
> length is at the beginning of the string.  Note, for example, that
> for an array (in most languages) all elements have the same 
> allocated length but possibly different current length.   

Correct.  For OpenVMS the format of a varying string matches
that used by other languages, like Pascal.  The layout is
detailed here:

     http://www.kednos.com/pli/docs/REFERENCE_MANUAL/6291pro_009.html#vary_data

The built-in function LENGTH(string) provides the current length
of string.  Whereas MAXLENGTH(string) returns the maximum.  To
get this functionality the string needs to be declared with
the VARYING attribute.

Tim.