Use of C.Strings.Value Function Considered Harmful

[Anton Gibbs <agibbs@dera.gov.uk>]

Dear Ada Community,

Now here is a funny thing - though I doubt whether I am the first to
experience it.

I have been passed a C structure one of whose fields purports to be a
pointer to a standard C null-terminated string. I want to copy the
string into the Ada world so I thought I would use the Value function in
B.3.1 (37):

   function Value( Item : in chars_ptr ) return String;

which is defined as being equivalent to:

   To_Ada( Value( Item ), Trim_Nul => False );

This works fine except that I want to guard against the C pointer being
bad and serving me up with a potentially limitless string of rubbish. So
I thought I would use the Value function from B.3.1 (35):

   function Value( Item : in chars_ptr; Length : in size_t )
            return String;

with Length set to some maximum string size that I would be willing to
handle (say 20).

The trouble is that if the pointer designates a string containing 20 or
more characters before its first Nul, then, far from truncating the
result as I had expected, all Value does is raise Terminator_Error. What
is the use of that ?

Presumably I am missing something here.

Thanks for whatever help and advice you can give.

Best -- Anton Gibbs

-- 
Civil Air Traffic Management Group
Defence Evaluation and Research Agency
Bedford, UK

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