Ch20.7 – Signal Types & Default Actions
Linux System Programming · EmbeddedPathashala
📡 Topic 7 of 19
📚 Reference Table
💻 1 Code Example
❓ Interview Q&A
🔑 Key Terms
SIGABRTSIGALRMSIGBUS SIGCHLDSIGFPESIGHUP SIGILLSIGINTSIGKILL SIGPIPESIGQUITSIGSEGV SIGTERMSIGUSR1/2
Quick Guide to Badge Colors
term = process terminated core = core dump + terminate ignore = signal discarded stop = process suspended cont = resume stopped process
📊 Complete Linux Signal Reference Table (Standard Signals)
| Signal | No. | Default | Description & When Generated |
|---|---|---|---|
| SIGABRT | 6 | core | Process calls abort(). Useful for debugging — produces core dump. |
| SIGALRM | 14 | term | Real-time timer set by alarm() or setitimer() has expired. |
| SIGBUS | 7 | core | Bus error — certain memory access errors, e.g. mmap beyond file end. |
| SIGCHLD | 17 | ignore | A child process terminated, stopped, or resumed. Sent to parent. |
| SIGCONT | 18 | cont | Resume a stopped process. Ignored by non-stopped processes. |
| SIGFPE | 8 | core | Arithmetic exception — integer divide-by-zero, floating point fault. |
| SIGHUP | 1 | term | Terminal disconnect (hangup). Also used to reload daemons. |
| SIGILL | 4 | core | Illegal (malformed) machine-language instruction executed. |
| SIGINT | 2 | term | Terminal interrupt — user pressed Ctrl+C. |
| SIGIO | 29 | term | I/O event available on certain file descriptors (fcntl async I/O). |
| SIGKILL | 9 | term | Sure kill — cannot be blocked, ignored, or caught. Always kills. |
| SIGPIPE | 13 | term | Write to pipe/socket with no reader. Normally means reader closed. |
| SIGPROF | 27 | term | Profiling timer (user + kernel CPU time) expired. |
| SIGPWR | 30 | term | UPS power failure detected — sent to init to shut down cleanly. |
| SIGQUIT | 3 | core | Terminal quit — user pressed Ctrl+\. Generates core dump. |
| SIGSEGV | 11 | core | Invalid memory reference — bad pointer, accessing unmapped memory. |
| SIGSTOP | 19 | stop | Sure stop — cannot be blocked, ignored, or caught. Always stops. |
| SIGSYS | 31 | core | Invalid system call number used. |
| SIGTERM | 15 | term | Standard termination signal. Default sent by kill command. |
| SIGTRAP | 5 | core | Debugger breakpoint or system call tracing (used by gdb, strace). |
| SIGTSTP | 20 | stop | Job-control stop — user pressed Ctrl+Z. Stops foreground process. |
| SIGTTIN | 21 | stop | Background process tried to read from terminal. |
| SIGTTOU | 22 | stop | Background process tried to write to terminal (when TOSTOP set). |
| SIGURG | 23 | ignore | Out-of-band (urgent) data arrived on a socket. |
| SIGUSR1 | 10 | term | User-defined signal 1 — available for application use. |
| SIGUSR2 | 12 | term | User-defined signal 2 — available for application use. |
| SIGVTALRM | 26 | term | Virtual timer (user-mode CPU time only) expired. |
| SIGWINCH | 28 | ignore | Terminal window size changed. Used by vi, less to redraw. |
| SIGXCPU | 24 | core | Process exceeded CPU time resource limit. |
| SIGXFSZ | 25 | core | Process tried to exceed file size resource limit. |
💻 Code Example – Print All Signal Names and Numbers
/* Print all signal names and numbers on this system
Compile: gcc -o list_sigs list_sigs.c
Run: ./list_sigs */
#define _GNU_SOURCE
#include <stdio.h>
#include <signal.h>
#include <string.h>
int main(void)
{
int sig;
printf("%-5s %-12s %s\n", "Num", "Name", "Description");
printf("%-5s %-12s %s\n", "---", "----", "-----------");
for (sig = 1; sig < NSIG; sig++) {
/* strsignal() returns human-readable description */
const char *desc = strsignal(sig);
if (desc == NULL)
desc = "(unknown)";
printf("%-5d %-12s %s\n", sig, "SIG?", desc);
}
printf("\nNSIG = %d (total signal slots)\n", NSIG);
return 0;
}
Run it! This prints all signal descriptions for your system using strsignal(). On Linux x86-64 NSIG is typically 65.
❓ Interview Questions
Q1. What is the difference between SIGTERM and SIGKILL?
SIGTERM is the polite termination request — a process can catch it, run cleanup code, and exit gracefully. SIGKILL is the forceful terminator — it cannot be caught, blocked, or ignored; the kernel kills the process immediately.
Q2. What is the difference between SIGINT and SIGQUIT?
Both are generated by the terminal. SIGINT (Ctrl+C) terminates the process. SIGQUIT (Ctrl+\) terminates the process AND produces a core dump, which is useful for debugging hung programs.
Q3. What is the purpose of SIGUSR1 and SIGUSR2?
They are reserved for application-defined purposes. The kernel never generates them. Applications use them to send custom notifications between processes — for example, telling a daemon to rotate its log file.
Q4. What is SIGHUP and how do daemons use it?
SIGHUP originally meant a terminal hangup (modem disconnected). Daemons repurpose it as a “reload configuration” signal — the system administrator sends SIGHUP to make a daemon re-read its config file without restarting.
Q5. What is the difference between SIGSTOP and SIGTSTP?
SIGSTOP is the sure-stop signal — cannot be caught or ignored. SIGTSTP is the terminal stop signal (Ctrl+Z) which CAN be caught, allowing a program to do cleanup before stopping or to refuse to stop.
Q6. When is SIGCHLD sent and what is its default action?
SIGCHLD is sent to a parent process when one of its children terminates, stops, or resumes. The default action is to ignore it. Parents that want to collect child exit status install a SIGCHLD handler or call wait().
Next Topic →
The signal() API — How to install handlers using the original UNIX API