shell

signals

  • sent from the kernel to process
  • info: sig id and the fact that it arrived
    • 2 SIGINT user typed ctrl-c
    • 9 SIGKILL
    • 11 SIGSEGV: seg fault
    • 17 SIGCHLD, child stopped

Sending a Signal

  • Kernel sends a signal to destination process by setting some state in the context of the dest process (nothing happens except some bits changed in the context, no immediate impact)
  • Kernel sends by two reasons:
    • it detects some event happens, seg-fault, divide-by-zero, etc.
    • another process has invoked the kill system call

Receiving a Signal

  • forced by the kernel to react in some way to the delivery of the signal
  • Some possible ways:
    • ignore
    • Terminate
    • Catch by signal handler
sig received -> control passed to sig handler -> sig handler runs -> returns back to the next instruction

like a interrupt

Pending and Blocked Signals

  • signal is pending if sent but not received yet

    • at most one pending sig for any type
    • not queued, subsequent sig of the same type are discarded

  • A process can block the receipt of certain signal

  • use sigprocmask

// Temporarily Blocking Signals
// block SIGINT and save previous blocked set
Sigprocmask(SIG_BLOCK, &mask, &prev_mask);

/* Code region that won't be interrupted by SIGINT */

// restore previous blocked set, unblocking SIGINT
Sigprocmask(SIG_SETMASK, &prev_mask, NULL);

Signal Handling

  • use signal to install signal handler, which changes the default behavior

  • signal handler runs in the same address space as the process, they have shared global state

  • handlers can be interrupted by other handlers

Guidlines for writing safe handlers

  • keep it simple
  • call only async-signal-safe functions, printf, malloc not allowed
  • save and restore errno on entry and exit
  • … refer to the slides

… as I don’t write signal handlers, I skipped the following content.

Summary

To wrap up, signals for a process is just an id, receiving a signal is the event that the kernel force to change the execution path and pass control to the signal handler.

A process can install signal handler by calling the signal function. Blocking signals is done by calling sigprocmask, which will ignore specified signals until set mask again.