Introduction#

Throwing Numbers At Processes#

Signal: integer number with predefined meaning
Termination request, invalid memory access, …
Sent to a process (or a process group, or a thread inside a process)
Either by another process
Or by the kernel

How Are Signals Received?#

Default: asynchronous signal delivery
- In history, asynchronous delivery was implemented first (right at epoch)
- Process may establish a signal handler
- Interrupts process flow ⟶ race conditions
- Hard to get right - much like writing interrupt service routines
Synchronous delivery
- Wait synchronously until signal is delivered
- Immediate question: what if the program wants to do something else?
- ⟶ Dedicate a separate thread to signal handling
Event driven programs can use signalfd()

A Little Terminology#

Generate: to send a signal. Processes can send a signal by using dedicated system calls like kill(), pthread_kill(), sigqueue()
Pending: a signal is said to be pending until the process reacts on it. Reasons may be:
- Signal is blocked by the process (more)
- The process is scheduled out, either because another process is running on the CPU, or because it is not runnable at all
“Traditional” signals are not queued against the process ⟶ multiple arrivals go unnoticed
Realtime signals are queued (see here for more
- No predefined meanings like traditional signals
- SIGRTMIN up to SIGRTMAX (32 on Linux)

Disposition: determines how the process reacts on the signal. From (man -s 7 signal):

Term	Default action is to terminate the process.
Ign	Default action is to ignore the signal.
Core	Default action is to terminate the process and dump core (see core(5)).
Stop	Default action is to stop the process.
Cont	Default action is to continue the process if it is currently stopped.

List Of Traditional Signals#

From man -s 7 signal:

Note

The signals SIGKILL and SIGSTOP cannot be caught, blocked, or ignored.

Signal      Standard   Action   Comment
────────────────────────────────────────────────────────────────────────
SIGABRT      P1990      Core    Abort signal from abort(3)
SIGALRM      P1990      Term    Timer signal from alarm(2)
SIGBUS       P2001      Core    Bus error (bad memory access)
SIGCHLD      P1990      Ign     Child stopped or terminated
SIGCLD         -        Ign     A synonym for SIGCHLD
SIGCONT      P1990      Cont    Continue if stopped
SIGEMT         -        Term    Emulator trap
SIGFPE       P1990      Core    Floating-point exception
SIGHUP       P1990      Term    Hangup detected on controlling terminal
                                or death of controlling process
SIGILL       P1990      Core    Illegal Instruction
SIGINFO        -                A synonym for SIGPWR
SIGINT       P1990      Term    Interrupt from keyboard
SIGIO          -        Term    I/O now possible (4.2BSD)
SIGIOT         -        Core    IOT trap. A synonym for SIGABRT
SIGKILL      P1990      Term    Kill signal
SIGLOST        -        Term    File lock lost (unused)
SIGPIPE      P1990      Term    Broken pipe: write to pipe with no
                                readers; see pipe(7)
SIGPOLL      P2001      Term    Pollable event (Sys V);
                                synonym for SIGIO
SIGPROF      P2001      Term    Profiling timer expired
SIGPWR         -        Term    Power failure (System V)
SIGQUIT      P1990      Core    Quit from keyboard
SIGSEGV      P1990      Core    Invalid memory reference
SIGSTKFLT      -        Term    Stack fault on coprocessor (unused)
SIGSTOP      P1990      Stop    Stop process
SIGTSTP      P1990      Stop    Stop typed at terminal
SIGSYS       P2001      Core    Bad system call (SVr4);
                                see also seccomp(2)
SIGTERM      P1990      Term    Termination signal
SIGTRAP      P2001      Core    Trace/breakpoint trap
SIGTTIN      P1990      Stop    Terminal input for background process
SIGTTOU      P1990      Stop    Terminal output for background process
SIGUNUSED      -        Core    Synonymous with SIGSYS
SIGURG       P2001      Ign     Urgent condition on socket (4.2BSD)
SIGUSR1      P1990      Term    User-defined signal 1
SIGUSR2      P1990      Term    User-defined signal 2
SIGVTALRM    P2001      Term    Virtual alarm clock (4.2BSD)
SIGXCPU      P2001      Core    CPU time limit exceeded (4.2BSD);
                                see setrlimit(2)
SIGXFSZ      P2001      Core    File size limit exceeded (4.2BSD);
                                see setrlimit(2)
SIGWINCH       -        Ign     Window resize signal (4.3BSD, Sun)

Examples: “Term” And “Core”#

code/stupid.cpp#

#include <unistd.h>
#include <print>

int main()
{
    std::println("PID={}", getpid());
    for(;;);
    return 0;
}

Send SIGTERM
Press Ctrl-C ⟶ SIGINT

code/segv.cpp#

#include <unistd.h>
#include <print>

int main()
{
    std::println("PID={}", getpid());
    int* bad = nullptr;
    *bad = 666;
    return 0;
}

code/fpe.cpp#

#include <unistd.h>
#include <print>

int main()
{
    int bad = 1/0;
    std::println("{}", bad);
    return 0;
}

SIGSEGV and SIGFPE (target_compile_options(prog PRIVATE -Wno-div-by-zero)
Better to not handle (denial of reality)
Talk about core dumps
Show in gdb

$ cat /proc/sys/kernel/core_pattern
$ sudo sh -c 'echo core > /proc/sys/kernel/core_pattern'