fmShareDenyWrite
fmShareDenyWrite = 32
Example
function OpenForStableSharedRead(const FileName: String): TFileStream;
begin
Result := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
end;
Usage
fmShareDenyWrite allows compatible readers while denying compatible writers, the common Velox mode for a stable shared file read.
Additional Technical Info
fmShareDenyWrite permits compatible competing readers and denies compatible competing writers for the lifetime of the handle. The current Windows RTL passes FILE_SHARE_READ to CreateFile.
fmOpenRead or fmShareDenyWrite is the common Delphi and Velox stable-read pattern: multiple readers can coexist, while a process that requests write access cannot open compatibly until the readers close. Velox uses this pattern for script loading, file-engine input, XML/dataset loading and several transport/file readers.
This is sharing at open-handle level, not a transaction or content signature. It cannot undo a mutation completed before open, guarantee remote/network filesystem semantics, prevent a process using incompatible low-level behavior, or ensure that a path is still the same object after the handle closes. Always free the stream in finally so writers are not blocked longer than necessary.
OR this share value with one access-mode alternative. Do not OR it with another share constant; the high-nibble encodings are mutually exclusive choices.
The example returns an owned stream and is source-reviewed only; no file was opened.
External references
Created 2026-07-15