TLPI: Linux Signals


0 Introduction

I’ve started reading The Linux Programming Interface! And throughout the book I’ll be writing some cool articles regarding new things I learned. The book is being read in no particular order, so expect the articles to follow that non-orderness (if that’s even a word). In this article specifically, however, I’ll be writing about signals. What are signals? How do they work? How to implement them inside my own program? and much more.

Note: If you want to follow along, this article is based on the 20th chapter of the book (Signals: Fundamental Concepts). I may add my own tips and lessons from time to time though.

1 Signals

In short, signals are software interrupts. During the program’s execution, if the program receives a signal, it will stop the program’s execution and finish some new and more important job. That job can depend on which signal was fired and what the program is set to do after receiving that specific signal.

“Ok, in what context is that useful?” you might ask. Well, a simple example might be killing a program through the shell. If you’ve worked with Linux for long enough, you might know that pressing CTRL+C kills any currently pending program that was executed from the shell. All the shell is really doing is waiting for keyboard input, and sending a specific kill signal to the program.

There’s more signals than just the kill signal though (i.e. SIGINT or SIGKILL in many cases)

Usually, every signal has it’s own default action that should be taken after the program receives it. But guess what, we can change it! That is called Signal Disposition. And we can change it to: *func(int) SIG_IGN or SIG_DFL

  • *func(int): could be a custom function that we want to execute when the signal is received.
  • SIG_IGN: just ignore the signal (i.e. no action for a specific signal)
  • SIG_DFL: execute default action of the signal (whatever it may be)

And we can do so, using either:

void (signal(int sig, void (*func)(int)))(int);

or a more flexible/complex alternative:

int sigaction(int sig, const struct sigaction *act, struct sigaction *oldact);

both of which are defined in <signal.h>

2 Sending signals

Signals can be sent using the following function.

int kill(pid_t pid, int sig);  // send signal to a process 
int killpg(int pgrp, int sig); // send signal to a process group

Or an alternative (in case you want to send the signal to the same program)

int raise(int sig); // literally the same as the line below 
kill(getpid(), sig);

You might be already familiar with the program kill from the command line. And in case you’re wondering.. well, yeah, they serve practically the same purpose.

Note: Just because they’re named kill, it does not mean that they specifically have to kill a process. It’s just called that way because originally you could in fact, only kill processes. But now you can send various signals with it (and also check if a PID is in use - more on that later).

3 Signal mask

What if we want to block a signal? Or just temporarily let the program know that we don’t want to accept certain types of signals? In that case we use something called a Signal Mask.

You can use the following function to change/edit the process signal mask.

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

Note: sigset_t is a type of data structure that can contain all types of signals. Imagine it more like a dictionary than a Queue or array where you can find multiple elements of the same signal type.

Hint: More information can be found here

If a signal of a specific type is being sent to a process which blocked that type, it will be marked as pending and will be delivered once the signal mask unblocks it (if ever).

If you want to view all the currently pending signals you can do so, using the following function.

int sigpending (sigset_t *set);

Note: Yes, it does return a signal set, and so no matter how many pending signals of a blocked type you’ll send, they’ll still be counted as one and sent only once after they’re unblocked. As I’ve said, this is not a queue, but more like a dictionary.. or.. queue where duplicates do not exist.

4 Pause

If you want to suspend the execution of the program and wait for signals, you can do so using the following function

int pause(void);

Note: from what the book said, yes, it is more optimal to use pause than to use a loop. Although I didn’t verify this in practice, I’m giving it the benefit of the doubt. You can verify it yourself if you care that much about program timings.