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    <p>Essentially the way this is done is to do a partial read, enough to get the number of bytes, then read the rest of the data into a discriminated record.</p> <p>Something like the following in pseudo-Ada:</p> <pre><code>type Payloads is array (Payload_Sizes range &lt;&gt;) of Your_One_Byte_Payload_Type; type Data (Payload_Length : Payload_Sizes) is record Tag : Tag_Type; Payload : Payloads(1 .. Payload_Length); end record; for Data use record Tag at 0 range 0 .. 15; Payload_Length at 2 range 0 .. 15; -- Omit a rep spec for Payload end record; </code></pre> <p>Typically the compiler will locate the Payload data immediately following the last rep-spec'ed field, but you'll need to verify this with the vendor, or do some test programs. (There may be a more explicit way to specify this, and I'm open to having this answer updated if someone provides a workable approach.)</p> <p>And do <em>not</em> provide a default for the Payload_Length discriminant, that would cause instances of the record to always reserve the maximum amount of storage needed for the largest value.</p> <p>Then, in your data reading code, something along the lines of:</p> <pre><code>loop Get(Data_File, Tag); Get(Data_File, Payload_Length) declare Data_Item : Data(Payload_Length); begin Data_Item.Tag := Tag; Get(Data_File, Data_Item.Payload); Process_Data(Data_Item); end; ... exit when Whatever; end loop; </code></pre> <p>(You'll need to work out your exit criteria.)</p> <p>Data_Item will then be dynamically sized for the Payload_Length. Beware, though, if that length is odd, as padding may occur...</p>
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    1. POHow can I size/match a record component while reading in its data from a Stream in Ada?
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    1. COHi, Yes thanks for that. That is how I'm moving now, I was hoping perhaps I could have the discriminant re-evaluated at the time read dumps data where I put in a large enough default and create a blank record object and go ahead take the initial mem hit then do a first read like read(file, record) fresh from the starting of the TAG position and then the object would then have a new value for its discriminant and re-init the entire object to the correct values. I assume the array or String size would "shrink" to the correct one. Is something like this possible?
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    2. CO@Josh - The only way to reevaluate discriminiants after an object is declared is if you supply a default value. As Marc mentioned, this causes the declaration to actually allocate the maximum possible value (Payload_Sizes - which may be 4Gig if you weren't careful). Another possibility would be to dynamically allocate the Data object after you know what its size is.
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    3. COHi, Yes, as I mentioned above I was aware of the default discriminant value memory issue. However for practical reasons the string is not likely to be more than several hundred characters so I'd likely at least constrain the payload type to a reasonable max. But again, thank you for the refresher.
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