Lecture 17-18 – UNIX Signals Vocabulary

Vocabulary flashcards covering core UNIX signal concepts, common signals, handling APIs, and best-practice terminology from Lectures 17-18.

kill (command)

Shell utility that ends, pauses, or continues a process by sending it a signal.

SIGTERM

Default signal sent by kill; politely asks a process to terminate.

SIGSTOP

Signal that pauses (stops) a process; cannot be caught or ignored.

SIGCONT

Signal that resumes a process previously stopped by SIGSTOP or SIGTSTP.

Ctrl-C

Terminal shortcut that sends the SIGINT signal to the foreground process.

SIGINT

Interrupt signal (Ctrl-C) that by default terminates the process.

Signal

Asynchronous notification sent by the OS/CPU to a process to indicate an event.

Pending Signals

32-bit bitmap in the kernel marking signals generated but not yet delivered.

Blocked Signals

32-bit bitmap specifying signals a process is temporarily ignoring.

Signal Handler

User-defined function executed when a specified signal is delivered to a process.

Default Handler

Built-in action the kernel performs for a signal when no custom handler exists.

SIGKILL

Non-catchable, non-ignorable signal that forcefully terminates a process.

SIGHUP

"Hang up" signal sent when a controlling terminal or remote connection is lost.

SIGPIPE

Signal raised when writing to a pipe whose reading end has closed.

SIGTSTP / SIGSTP

Signal sent by Ctrl-Z that suspends (pauses) the foreground process.

SIGQUIT

Signal sent by Ctrl-\ that terminates a process and creates a core dump.

Async-Signal-Safe Function

Library routine guaranteed to be safely callable from inside a signal handler (e.g., write).

SIG_DFL

Constant passed to signal()/sigaction() to restore a signal's default behavior.

SIG_IGN

Constant passed to signal()/sigaction() to make a process ignore a signal.

SIGUSR1

User-defined signal 1 with no predefined action; available for application use.

SIGUSR2

User-defined signal 2 with no predefined action; available for application use.

Hardware Interrupt

CPU-level event (e.g., key press, device ready) that suspends current execution to run an interrupt service routine.

Exception

CPU-detected error (e.g., divide by zero) that triggers a kernel handler and often results in a signal to the process.

SIGFPE

Signal sent to a process after a floating-point exception such as divide-by-zero.

SIGSEGV

Signal sent when a process performs an invalid memory access (segmentation fault).

sigprocmask()

System call used to block or unblock specified signals for the calling process.

Last-In First-Out Signal Handling

Strategy where a newly arrived signal preempts the handler currently running (stack-style).

Block-and-Queue Strategy

Approach that defers additional signals until the current handler finishes, then delivers them in order.

sigaction()

Advanced API for installing signal handlers with extra control and reliability.

sa_handler

Field in struct sigaction specifying the function (or SIGDFL/SIGIGN) to run when the signal occurs.

sa_sigaction

Alternate field giving a handler that receives extra context information (siginfo_t *).

sa_flags

Bitmask in struct sigaction controlling options such as whether to use sa_sigaction.

sa_mask

Signal set in struct sigaction listing additional signals to block while the handler runs.

Mousetrap Problem

Historical issue where a handler had to reinstall itself, risking missed signals if another arrived before reinstallation.