QSharedMemory & QSystemSemaphore in Qt 7

Session Summary

What to do with QSharedMemory and QSystemSemaphore

Thiago Macieira

(Note: recreated from memory, note taker had a laptop trouble and the file wasn't saved) Add link to list archive

Thiago proposed deprecating now and moving to Qt6CoreCompat in Qt 7
- No replacement class
- Fabian has volunteered to investigate a replacement API before Qt 6.10
We'll add the deprecation markers now, with a trigger time in the future
- Markers can be disabled with a macro (opt-out)
- Current users shouldn't be affected – any issues that you have are ones you've always had
We'll link to a doc page explaining how to replace QSharedMemory
- Use QFile::map for simple sharing of memory
- Update as we learn more about use-cases
- Ask for contact if use-case isn't taken into account
Known uses:
- Biggest user of QSharedMemory is QtSingleApplication
  - Multiple different versions out there
  - Thiago volunteers to fix it
  - Peppe has reimplemented it from scratch, MIT-licensed, uses QLockFile+QtNetwork https://github.com/KDAB/KDSingleApplication
    - Avoids D-Bus as problematic on non-Linux
- Use D-Bus on Linux, figure out what to do elsewhere
- qtwayland has one use of QSharedMemory too
- Sharing of large data sets to avoid serialisation
- Shared images (qtdeclarative)
- Androidtestrunner (single-application exclusion)
WINE implements Windows semantics on Unix; we could investigate
memfd is the "correct" solution on Linux and FreeBSD
- similar enough to Windows semantics (disappears on last close)
- not portable elsewhere