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ZLibCompressFile

Function ZLibCompressFile( const aFileName : string) : TBytes

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := Base64EncodeBytes(ZLibCompressFile('C:\Example\input.dat'));
end;

Usage​

ZLibCompressFile reads a file and returns its contents as zlib-compressed bytes.

Parameters

NameTypeDescription
aFileNameString, constSource path resolved under the executing Velox process identity.

Returns

The file content as one zlib-wrapped deflate stream in memory.

Behaviour

The returned payload has zlib framing and carries no source filename, timestamps, attributes or content type. Because sharing denies neither reads nor writes, another process can change the source while it is being compressed.

Errors

File-open, access, read, allocation, stream-write and EZCompressionError exceptions propagate. No partial result or Boolean error indication is returned.

Usage notes

Use an allow-listed stable input path. If a physical compressed artefact is needed, store the returned bytes separately and use a temporary-file/atomic-move policy appropriate to the flow.

Additional Technical Info​

ZLibCompressFile opens a source file, streams it through Delphi's zlib compressor at the default level and returns the complete compressed stream as TBytes. It does not write a compressed file or modify the source.

The example path is fictional. The example is source-reviewed and is not executed by the documentation workflow.

Implementation

The direct registration calls vxCompression.ZLibCompressFile. It opens TFileStream with fmOpenRead or fmShareDenyNone, creates an empty TBytesStream, and passes both to System.ZLib.ZCompressStream(..., zcDefault). The installed RTL reads and writes through 32 KiB buffers, finalises deflate, and returns. Velox then allocates an exact-size result and copies only lStream.Size bytes rather than the capacity of the byte stream.

Edge cases and quirks

  • Input is streamed, but the complete compressed output remains in TBytesStream and is then copied to the result.
  • A zero-byte file is still represented by a non-empty, valid zlib stream after the compressor is finalised.
  • The source file is not locked against writers, so the result is not a guaranteed point-in-time snapshot.
  • The exact-result copy uses element-zero expressions. A normal compressed stream is non-empty, but the expression remains dependent on compiler range behaviour if upstream output were ever empty.
  • The file stream is constructed before the byte stream and before the try/finally. If creating the byte stream fails, the already-open file stream is not freed by this routine.
  • No maximum source size, output size, timeout or cancellation mechanism is exposed to the script.

Side effects

Opens and reads the file and allocates output memory. It performs no destination write.

Performance and concurrency

The file is processed in 32 KiB chunks, while the complete compressed output is retained and copied. Compression is synchronous. Product compression state is local, but concurrent source writers are an external race.

Related entries

External references

Created 2026-07-15