QSharedMemory & QSystemSemaphore in Qt 7

Session Summary

What to do with QSharedMemory and QSystemSemaphore

Session Owners

Thiago Macieira

Notes

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• 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