COMP 321 Lab 3

# Signals

## Alan Cox

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<footer><em>COMP 321 Spring 2026 • Alan Cox</em></footer>

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# What Are Signals?

- Software interrupts for asynchronous event notification
- Sent by the kernel or other processes
- Common signals:
  - `SIGINT` - Interrupt from keyboard (Ctrl+C)
  - `SIGALRM` - Timer expired
  - `SIGUSR1`, `SIGUSR2` - User-defined signals
  - `SIGKILL`, `SIGTERM` - Termination requests

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# Signal Handlers

- Functions that run when a signal is delivered
- Installed with `sigaction(2)`
- Signals can interrupt system calls, causing them to fail with `EINTR`; use `SA_RESTART` to avoid this

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# The Problem: Race Conditions

void handler(int sig)
{
    if (counter % 2 == 0) {
        // Error: counter is even!
    }
}

int main(void)
{
    // Install handler, set timer...
    while (...) {
        counter++;
        // Signal can arrive here!
        counter++;
    }
}
```
]

.col[
- Signal can arrive at any time
- Handler may see inconsistent state
- Need to control when signals are delivered
]
]

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# Signal Masking with `sigprocmask(2)`

```
int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);
```

- Block signals to defer their delivery
- `how` values:
  - `SIG_BLOCK` - Add signals to blocked set
  - `SIG_UNBLOCK` - Remove signals from blocked set
  - `SIG_SETMASK` - Replace blocked set entirely
- Blocked signals are held pending, not lost

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# Waiting for Signals with `sigsuspend(2)`

```
int sigsuspend(const sigset_t *mask);
```

- Atomically unblocks signals and suspends until one arrives
- Replaces signal mask with `mask` temporarily
- Returns after a signal handler executes (returns -1, `errno` = `EINTR`)

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# Blocking Signals During Handlers

- `sa_mask` specifies signals to block while handler runs
- Prevents one handler from interrupting another
- Signal being handled is automatically blocked

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# Sending Signals with `fork(2)` and `kill(2)`

- `fork(2)` creates a child process
  - Returns 0 in child
  - Returns child's PID in parent
- `kill(2)` sends a signal
  - `kill(pid, sig)`
  - `getppid()` returns parent's PID

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# Discussion Questions

- Why can `write()`'s return value be ignored in the signal handler?

- Why must `sigprocmask` be called before `setitimer`?

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# Get Started!