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ReadBlobValue

Function ReadBlobValue(aField: TBlobField): String

Example​

procedure ScriptEvent(var Value: variant);
begin
  // Do not use this helper in the current build; see the defect below.
  Value := Null;
end;

Usage​

ReadBlobValue attempts to read a blob field into a string but currently returns uninitialized character data for non-empty blobs.

Parameters

NameTypeDescription
aFieldTBlobFieldBlob field on the current dataset record. Must be non-nil and valid for the active record.

Returns

The declaration says String, but non-empty blobs are not copied into it in the current build. Do not interpret the returned length or characters as blob content.

Usage notes

Use a supported encoding-aware text field API or binary stream/byte-array path. A correct future Velox must define whether the blob is binary, UTF-8, UTF-16 or another encoding; rewind before reading; size the destination in the proper units; and verify the read count.

Additional Technical Info​

ReadBlobValue is intended to save a TBlobField into memory and return its bytes as a Delphi string. The current implementation never rewinds the memory stream before reading, ignores the zero-byte read, and returns a string whose payload was not populated from the blob. Do not use it.

The example intentionally does not invoke the function. It is source-reviewed and no blob/runtime test bed was executed.

Implementation trace and defect

Velox creates an empty TMemoryStream and calls aField.SaveToStream(Stream). Installed Delphi TBlobField.SaveToStream creates a read blob stream and calls destination Stream.CopyFrom(BlobStream, 0). CopyFrom writes at the destination's current position and advances it, so the memory stream is positioned at its end.

Velox then calls:

SetLength(Result, Stream.Size);
Stream.Read(Result[1], Stream.Size);

Because Position = Size, TCustomMemoryStream.Read returns 0. The return count is ignored and no blob byte reaches Result. Installed Delphi string allocation sets the length and terminator but does not initialize the new character payload, so a non-empty result can contain stale/uninitialized memory rather than predictable NUL characters. Returning or logging it creates a potential information-disclosure and correctness risk.

There is a second unit error: Stream.Size is bytes, while SetLength(UnicodeString, N) allocates N two-byte characters. Even after adding a rewind, raw bytes cannot safely be read into a Unicode string without a defined encoding and correct byte-to-character conversion.

Edge cases

  • A null/empty blob yields stream size 0, then the code still evaluates Result[1]. The current Velox project enables Delphi range checking, so indexing the zero-length string raises ERangeError before the zero-count Read can complete.
  • aField = nil, an inactive dataset or an invalid current record can raise.
  • Arbitrary binary data is not text and may contain invalid UTF-16, embedded NULs or odd byte counts.
  • The code uses Read, not ReadBuffer, and ignores the number actually read.

Side effects and errors

A temporary stream is allocated and always freed. There is no exception handler, so field/stream/allocation errors propagate into the script. The uninitialized return can expose process-memory remnants even when no exception occurs.

Performance and concurrency

The entire blob is copied into a memory stream and a second, incorrectly sized string allocation is made, so memory cost is at least proportional to the blob and can be substantially higher. Dataset field state remains context-owned and must not be read concurrently.

Related entries

External references

Created 2026-07-15