CODEWITHSUNDEEP
linuxsignals
gandalf
gandalf

Reputation: 1

Linux: signal source

How does one find reliably whether a process received a signal due to its own misbehavior or was sent the same by another process? Basically, how does one determine whether si_pid field is valid or not.

Upvotes: 0

Views: 672

Answers (1)

John Zwinck
John Zwinck

Reputation: 249394

If si_pid in the siginfo_t structure matches getpid() then the process signaled itself. Otherwise, another process did. Since process IDs are unique at any point in time, a PID you have now could not possibly have sent you the signal at a time when it had your PID (because then it would have signaled itself and not you).

Edit:

As you have discovered, the si_pid field is not always set; sometimes it contains garbage values. The first thing to check is that you have passed SA_SIGINFO in the sa_flags field of your struct sigaction when registering your handler. Without this, your handler may not receive a siginfo_t at all.

Once that's done, there are rules for when si_pid is set, described here: https://www.mkssoftware.com/docs/man5/siginfo_t.5.asp#Signal_Codes

In brief: si_pid should be set if si_code is one of:

  • SI_USER - includes calls to kill()
  • SI_QUEUE
  • SI_TIMER
  • SI_ASYNCIO
  • SI_MESGQ

It is also set whenever si_signo is SIGCHLD.

Upvotes: 2

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