((Bit-Mage) 'buffer) ::

process

1. Signals

  • interrupts, used to announce asynchronous events to a process
  • all start with "SIG" and around 64 of them

1.1. how does a process handle a signal?

three cases:

  • ignores it
  • catch and handle the exception
  • let the default action apply

1.2. standard signals

Signal # Name Default Action Comment POSIX
1 SIGHUP Terminate Hang up detected on controlling terminal or process. Yes
2 SIGINT Terminate Interrupt from keyboard (Ctrl+C). Yes
3 SIGQUIT Dump Quit from keyboard (Ctrl+\). Yes
4 SIGILL Dump Illegal instruction. Yes
5 SIGTRAP Dump Breakpoint for debugging. No
6 SIGABRT Dump Abnormal termination. Yes
6 SIGIOT Dump Equivalent to `SIGABRT`. No
7 SIGBUS Dump Bus error (hardware fault). No
8 SIGFPE Dump Floating-point exception. Yes
9 SIGKILL Terminate Forced process termination (cannot be caught/ignored). Yes
10 SIGUSR1 Terminate User-defined signal 1. Yes
11 SIGSEGV Dump Invalid memory reference (segmentation fault). Yes
12 SIGUSR2 Terminate User-defined signal 2. Yes
13 SIGPIPE Terminate Writing to a pipe with no readers. Yes
14 SIGALRM Terminate Real-time timer expired. Yes
15 SIGTERM Terminate Termination request (default signal for `kill`). Yes
16 SIGSTKFLT Terminate Stack fault on coprocessor. No
17 SIGCHLD Ignore Child process stopped, terminated, or received signal. Yes
18 SIGCONT Continue Resume execution, if stopped. Yes
19 SIGSTOP Stop Stop process execution (cannot be caught/ignored). Yes
20 SIGTSTP Stop Stop signal from terminal (Ctrl+Z). Yes
21 SIGTTIN Stop Background process trying to read from terminal. Yes
22 SIGTTOU Stop Background process trying to write to terminal. Yes
23 SIGURG Ignore Urgent condition on socket. No
24 SIGXCPU Dump CPU time limit exceeded. No
25 SIGXFSZ Dump File size limit exceeded. No
26 SIGVTALRM Terminate Virtual timer expired. No
27 SIGPROF Terminate Profiling timer expired. No
28 SIGWINCH Ignore Window size change. No
29 SIGIO Terminate I/O now possible. No
29 SIGPOLL Terminate Equivalent to `SIGIO`. No
30 SIGPWR Terminate Power failure. No
31 SIGSYS Dump Bad system call. No
31 SIGUNUSED Dump Equivalent to `SIGSYS`. No

1.3. sending signals

  • checkout man kill
  • can send all signals irrespective of what the name indicates
  • default is to terminate if no number provided

1.4. raising signals

  • can raise signals within a process using the `raise()` or `kill()` functions, both declared in the `signal.h` header file in C.
  • man 2 signal and check out the "SEE ALSO" section
  • man 3 raise
  • man 2 kill
  • and more …

1.5. masking signals

  • fetch and/or change the signal mask of the calling thread
  • set of signals whose delivery is currently blocked for the caller
  • man 2 sigprocmask

1.6. catching signals

SIGUSR1 and SIGUSR2 are user-defined signals. Here's a basic example using SIGUSR1 to toggle a flag in a C program: 

#include <stdio.h>
#include <unistd.h>
#include <signal.h>

volatile int flag = 0;

void signal_handler(int signum) {
  if (signum == SIGUSR1) {
    flag = !flag; // Toggle the flag
    printf("Signal received, flag toggled to: %d\n", flag);
  }
}

int main() {
  signal(SIGUSR1, signal_handler); // Register signal handler

  while (1) {
    printf("Waiting for signal (flag: %d)...\n", flag);
    sleep(1); // Check every second
  }
  return 0;
}

gcc signal_example.c -o signal_example
./signal_example

ps aux | grep signal_example
kill -USR1 <process_id>

2. Resources

Tags::linux:programming: