The Linux Signals Handling Model

Communication is the key to healthy relationships between threads and the kernel; these are the signals they use to communicate.
Signals and Interrupts, A Perfect Couple

Signals can be sent from system calls, interrupts and bottom-half handlers (see sidebar) alike; there is no difference. In other words, the Linux signal queue is interrupt-safe, as strange and recursive as that sounds, so it's fairly flexible.

Bottom-Half Handlers

An interesting signal-delivery case, however, is on SMP. Imagine a thread is executing on one processor, and it gets an asynchronous event (e.g., synchronous socket I/O signal) from an IRQ handler (or another process) on another CPU. In that case, we send a cross-CPU message to the running process, so there is no latency in signal delivery. (The speed of cross-CPU delivery is about five microseconds on a Pentium II 350MHz.)


Once again, we notice how Linux is actually the technology leader in important kernel aspects such as scheduling, interrupt handling and signals handling. This also proves the conjecture that the Linux developer community is collectively more capable and more resourceful than any private corporation's R&D department could ever be.


Moshe Bar ( is an Israeli system administrator and OS researcher, who started learning UNIX on a PDP-11 with AT&T UNIX Release 6 back in 1981. He holds an M.Sc. in computer science. Visit Moshe's web site at



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I have One Question about the

Zafar's picture

I have One Question about the Usage of SIGNAL() and SIGACTION() so which function shall I Use. Is the use of traditional SIGNAL() is a disadvantage.

"Asynchronous signals are

Ling's picture

"Asynchronous signals are delivered to the first thread found not blocking the signal." The only solution I found so far is to create a thread to process all signals, and mask the rest of the threads.

This makes the signal pretty useless on Android, as we don't have control of how threads are created, and the system might use other signals.

I have read some suggestion and working solution in the kernel to deliver the signal to the originated thread, instead of random thread.

I wonder if the latest kernel is supporting this, or if anyone has solution for this.


Signal Handling

Anonymous's picture

Is there a pictorial representation of the signal handling process in Linux to help visualize the switch between the user space and kernel space?

What does 'reliable' mean? What happens if a signal is unreliabl

Ming's picture

"Implementation of correct and reliable signals has been in place for many years now ... reliable signals require the use of the newer sigaction interface" - What is the definition of 'reliable signals'? Are there some signals reliable and some not, in the same implementation? Or are there reliable implementations in which all signals are reliable, and unreliable implementations where all signals are unreliable?
And I have a question that is not answered in all the papers and books I have read so far: what happens if a signal arrives when a previous one (of the same type) is being handled in a signal catching function?

RE: What does 'reliable' mean?

Anonymous's picture

I trust reliable is referred to in the sense that some functions makes the entire signaling handling reliable, e.g. 'sigaction' may block signals and set a new signal handler in one atomic swoop, as to eliminate the potential race condition.
As to your last question, if the signal is not blocked and a new instance is generated in the midst of the processing of the last one, then the signal handler must be re-entrant. IMHO, the best method is to block the signal on entering the signal handler. It will be automatically restored on exit. On old UNIX versions, once a signal handler is invoked, the signal handling is reverted to default. If a new signal instance comes along, it will be treated the default way, most often terminating the process.

Table 1 HTML lacks tag

Anonymous's picture

Table 1 HTMl page lacks start tag for the table, making the page unrenderable, even if the article is dated in year 2000.

start tag.

Keith Daniels's picture

Thanks for catching that. I will fix it ASAP

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