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ExtractFileDrive

Function ExtractFileDrive( const FileName : string) : string

Example​

procedure ScriptEvent(var Value: variant);
begin
  Value := ExtractFileDrive('\\fictional-server\inbound\order.csv');
  // \\fictional-server\inbound
end;

Usage​

ExtractFileDrive returns the drive or UNC share prefix parsed from a file-name string.

Parameters

NameTypeDescription
FileNamestring, constPath text whose leading drive or UNC prefix is required.

Returns

For a drive-letter path, the two-character prefix such as 'C:'; for a UNC path, the server/share prefix such as '\\server\share'; otherwise an empty string.

Behaviour

  • Both 'C:\Folder\file.txt' and 'C:file.txt' return 'C:'; the result alone does not distinguish rooted from drive-relative input.
  • '\\server\share\folder\file.txt' returns '\\server\share'.
  • A relative path such as 'folder\file.txt' returns ''.
  • Letter case and original prefix spelling are preserved.

Errors

No normal content-dependent exception is raised. Argument conversion and allocation failures can propagate.

Usage notes

Do not use a non-empty result as proof that storage is available. Combine it with an approved absolute-path policy and the appropriate existence or access operation.

Additional Technical Info​

ExtractFileDrive parses and returns the drive designator or UNC share prefix at the start of FileName. It is a string parser: it does not query drives, servers or shares.

The example uses a fictional UNC path. It is source-reviewed and was not executed by the documentation workflow.

Implementation

The Velox wrapper delegates to installed System.SysUtils.ExtractFileDrive. The current Windows helper recognises a colon in the second character, a conventional \\server\share prefix, and extended/device forms beginning \\?\ or \\.\. It calculates the recognised prefix length and copies that prefix unchanged.

Edge cases and quirks

  • The function recognises syntax, not validity. A returned drive/share can be absent, inaccessible or incomplete for the intended operation.
  • Extended-length and device prefixes have specialised scanning rules. Preserve the original full path when later APIs must retain those semantics.
  • A colon outside the drive position is not a drive designator.
  • On the Windows build, backslashes define UNC structure; mixed or forward-slash paths are not normalised first.
  • Empty and malformed/incomplete UNC text can return an empty or partial lexical result; it does not raise a validation error.

Side effects

None.

Performance and concurrency

Parsing is linear only across the short leading prefix and then allocates the result. It has no mutable shared state.

Related entries

External references

Created 2026-07-15