fmOpenRead
fmOpenRead = 0
Example
procedure ReadSharedFile(const FileName: String; Dest: TStream);
var
Source: TFileStream;
begin
Source := TFileStream.Create(FileName, fmOpenRead or fmShareDenyWrite);
try
Dest.CopyFrom(Source, 0, 65536);
finally
Source.Free;
end;
end;
Usage
fmOpenRead opens an existing file read-only, with an explicit share mode required when concurrent readers or writers must be allowed.
Additional Technical Info
fmOpenRead selects read-only access to an existing file. TFileStream.Create calls the current Delphi FileOpen path, which opens with OPEN_EXISTING; it does not create a missing file. Reads are allowed, while Write or resizing operations can fail through the read-only handle.
The value is zero because access is encoded in the low two bits. It is normally OR-ed with exactly one share-mode alternative. A subtle consequence is that bare fmOpenRead contains no explicit sharing bits. On the current Windows RTL, zero sharing grants no read or write sharing to other opens, so it behaves like exclusive sharing for the handle lifetime.
The example explicitly adds fmShareDenyWrite, allowing other readers while preventing compatible writers. Use fmShareDenyNone only when concurrent writes are acceptable and the reader can tolerate observing changing content.
The file cursor starts at byte 0. The returned stream must be freed in finally. Opening and reading remain separate non-atomic steps: permissions, replacement, truncation by permitted writers, short reads and storage errors can still matter according to the chosen sharing policy.
The example is source-reviewed only; no file was opened.
External references
Created 2026-07-15