fmShareDenyRead

fmShareDenyRead = 48

Example

function OpenWhileAllowingWriters(const FileName: String): TFileStream;
begin
  Result := TFileStream.Create(FileName, fmOpenReadWrite or fmShareDenyRead);
end;

Usage

fmShareDenyRead allows compatible writers but denies compatible readers, creating a deliberately unstable and platform-sensitive concurrency policy.

Additional Technical Info

fmShareDenyRead denies compatible competing read access while permitting compatible write access. The current Windows RTL passes FILE_SHARE_WRITE as the handle's sharing mask.

This counterintuitive policy does not protect the current handle from concurrent mutation: another process can open for writing if its own share request is compatible. Reads can observe changing content and writes can interleave or overwrite. Use it only when that precise coordination policy is required; it is not a general “lock for update.”

Embarcadero marks this value nonportable, and Free Pascal documents that deny-read locking is not supported uniformly outside Windows. Deployment behavior must therefore be treated as operating-system specific.

OR the value with exactly one existing-file access mode and free the stream promptly. Combining multiple share values is invalid reasoning: bitwise OR can accidentally select a different defined nibble rather than combining independent permissions.

The example returns an owned stream and is source-reviewed only; no file was opened.

External references

• Embarcadero TFileStream.Create
• Free Pascal FileOpen

Created 2026-07-15