Linux Signalfd Is Useless

15 hours ago

UNIX signals are problematic due to their ability to interrupt a program at any point, imposing strict restrictions on signal handlers.
Signal handlers can't use many common functions like stdio or malloc because they might be holding locks when interrupted.
The self-pipe trick is a common workaround to defer signal handling work until outside the handler.
System calls interrupted by signals return EINTR, causing issues as few libraries handle restarting these calls properly.
Linux's signalfd API was introduced to simplify signal handling by delivering signals via a file descriptor, but it doesn't change signal disposition mechanics.
Masking signals for signalfd inheritance causes issues in child processes, which inherit the mask and may not reset it.
Signal coalescing is another issue, where multiple instances of the same signal are reported as one, losing information.
A proposed better solution involves using SA_NODEFER with a dedicated signal-handling thread to avoid EINTR issues, though signal coalescing can still occur.
Signalfd could be improved by integrating it more deeply with signal disposition, ensuring signals don't need to be masked and preventing inheritance issues.

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