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CopyFrom

function CopyFrom(Source: TStream; Count: Int64; BufferSize: Integer): Int64;

Example​

procedure CopyCompleteStream(Source, Dest: TStream);
var
  Copied: Int64;
begin
  if (Source = nil) or (Dest = nil) then Exit;

  Copied := Dest.CopyFrom(Source, 0, 65536);
  Log('Copied bytes: ' + IntToStr(Copied));
end;

Usage​

CopyFrom copies bytes from a borrowed source using an explicitly sized buffer, with rewind-on-nonpositive Count and partial-output failure semantics.

Additional Technical Info​

CopyFrom reads from borrowed Source and writes into the receiving stream at its current Position. It returns bytes copied. Neither stream is freed.

Unlike the native Delphi declaration, Velox requires all three arguments because PascalScript registration does not carry default parameter values. BufferSize must be greater than zero; otherwise ERangeError is raised before copying. It controls temporary memory/chunk size, not the amount copied.

Count behavior in the installed runtime​

CountBehavior
> 0Copy exactly Count bytes from the source's current Position.
= 0Set source Position to 0, obtain source Size and copy the whole source.
< 0Also set source Position to 0 and replace Count with source Size; if that Size is negative/unknown, read chunks until Read returns 0.

The destination Position is never reset automatically. In whole-source mode only the source is rewound. A positive Count larger than remaining input calls ReadBuffer and raises after any earlier chunks were already written.

Writes use WriteBuffer, so a destination short write is retried and then raises if progress stops. Copying is not transactional: on any read/write exception, both cursors and destination Size/content can already be partially advanced. The current Delphi implementation can preallocate capable destinations and shrinks that preallocation to the reached Position if copying fails, but already written bytes remain.

Source and destination should be distinct. Passing the same stream shares one cursor and can overwrite, extend or otherwise corrupt content rather than create a snapshot. Also do not concurrently use either stream during the copy.

The example is source-reviewed only; no bytes were copied.

External references

Created 2026-07-15