The explanation below is not exact, and several aspects of how this works differ between different systems (and maybe even the same OS on different hardware for some portions), but I think that it is generally good enough for you to satisfy your curiosity enough to use them. Most people start using signals in programming without even this level of understanding, but before I got comfortable using them I wanted to understand them.

signal delivery

The OS kernel has a data structure called a process control block for each process running which has data about that process. This can be looked up by the process id (PID) and included a table of signal actions and pending signals.

When a signal is sent to a process the OS kernel will look up that process's process control block and examines the signal action table to locate the action for the particular signal being sent. If the signal action value is SIG_IGN then the new signal is forgotten about by the kernel. If the signal action value is SIG_DFL then the kernel looks up the default signal handling action for that signal in another table and preforms that action. If the values are anything else then that is assumed to be a function address within the process that the signal is being sent to which should be called. The values for SIG_IGN and SIG_DFL are numbers cast to function pointers whose values are not valid addresses within a process's address space (such as 0 and 1, which are both in page 0, which is never mapped into a process).

If a signal handling function were registered by the process (the signal action value was neither SIG_IGN or SIG_DFL) then an entry in the pending signal table is made for that signal and that process is marked as ready to RUN (it may have been waiting on something, like data to become available for a call to read, waiting for a signal, or several other things).

Now the next time that the process is run the OS kernel will first add some data to the stack and changes the instruction pointer for that process so that it looks almost like the process itself has just called the signal handler. This is not entirely correct and actually deviates enough from what actually happens that I'll talk about it more in a little bit.

The signal handler function can do whatever it does (it is part of the process that it was called on behalf of, so it was written with knowledge about what that program should do with that signal). When the signal handler returns then the regular code for the process begins executing again. (again, not accurate, but more on that next)

Ok, the above should have given you a pretty good idea of how signals are delivered to a process. I think that this pretty good idea version is needed before you can grasp the full idea, which includes some more complicated stuff.

Very often the OS kernel needs to know when a signal handler returns. This is because signal handlers take an argument (which may require stack space), you can block the same signal from being delivered twice during the execution of the signal handler, and/or have system calls restarted after a signal is delivered. To accomplish this a little bit more than stack and instruction pointer changes.

What has to happen is that the kernel needs to make the process tell it that it has finished executing the signal handler function. This may be done by mapping a section of RAM into the process's address space which contains code to make this system call and making the return address for the signal handler function (the top value on the stack when this function started running) be the address of this code. I think that this is how it is done in Linux (at least newer versions). Another way to accomplish this (I don't know if this is done, but it could be) would be do make the return address for the signal handler function be an invalid address (such as NULL) which would cause an interrupt on most systems, which would give the OS kernel control again. It doesn't matter a whole lot how this happens, but the kernel has to get control again to fix up the stack and know that the signal handler has completed.

WHILE LOOKING INTO ANOTHER QUESTION I LEARNED

that the Linux kernel does map a page into the process for this, but that the actual system call for registering signal handlers (what sigaction calls ) takes a parameter sa_restore parameter, which is an address that should be used as the return address from the signal handler, and the kernel just makes sure that it is put there. The code at this address issues the I'm done system call (sigreturn)and the kernel knows that the signal handler has finished.

signal generation

I'm mostly assuming that you know how signals are generated in the first place. The OS can generate them on behalf of a process due to something happening, like a timer expiring, a child process dying, accessing memory that it should not be accessing, or issuing an instruction that it should not (either an instruction that does not exist or one that is privileged), or many other things. The timer case is functionally a little different from the others because it may occur when the process is not running, and so is more like the signals sent with the kill system call. For the non-timer related signals sent on behalf of the current process these are generated when an interrupt occurs because the current process is doing something wrong. This interrupt gives the kernel control (just like a system call) and the kernel generates the signal to be delivered to the current process.

Some issues that are not addressed in all of the above statements are multi core, running in kernel space while receiving a signal, sleeping in kernel space while receiving a signal, system call restarting and signal handler latency.

Here are a couple of issues to consider:

• What if the kernel knows that a signal needs to be delivered to process X which is running on CPU_X, but the kernel learns about it while running on CPU_Y (CPU_X!=CPU_Y). So the kernel needs to stop the process from running on a different core.
• What if the process is running in kernel space while receiving a signal? Every time a process makes a system call it enters kernel space and tinkers with data structures and memory allocations in kernel space. Does all of this hacking take place in kernel space too?
• What if the process is sleeping in kernel space waiting for some other event? (read, write, signal, poll, mutex are just some options).

Answers:

• If the process is running on another CPU the kernel, via cross CPU communication, will deliver an interrupt to the other CPU and a message for it. The other CPU will, in hardware, save state and jump to the kernel on the other CPU and then will do the delivery of the signal on the other CPU. This is all a part of trying not to execute the signal handler of the process on another CPU which will break cache locality.
• If the process is running in kernel space it is not interrupted. Instead it is recorded that this process has received a signal. When the process exits kernel space (at the end of each system call), the kernel will setup the trampoline to execute the signal handler.
• If the process, while running in kernel space, after having received a signal, reaches a sleep function, then that sleep function (and this is common to all sleep functions within the kernel) will check if the process has a signal pending. If it is so, it will not put the process to sleep and instead will cancel all that has been done while coming down into the kernel, and will exit to user space while setting up a trampoline to execute the signal handler and then restart the system call. You can actually control which signals you want to interrupt system calls and which you do not using the siginterrupt(2) system call. You can decide if you want system calls restartable for a certain signal when you register the signal using sigaction(2) with the SA_RESTART flag. If a system call is issued and is cut off by a signal and is not restarted automatically you will get an EINTR (interrupted) return value and you must handle that value. You can also look at the restart_syscall(2) system call for more details.
• If the process is already sleeping/waiting in kernel space (actually all sleeping/waiting is always in kernel space) it is woken from the sleep, kernel code cleans up after itself and jump to signal handler on return to user space after which the system call is automatically restarted if the user so desired (very similar to previous explanation of what happens if the process is running in kernel space).

A few notes about why all of this is so complex:

• You cannot just stop a process running in kernel space since the kernel developer allocates memory, does things to data structures and more. If you just take the control away you will corrupt the kernel state and cause a machine hang. The kernel code must be notified in a controlled way that it must stop its running, return to user space and allow user space to handle the signal. This is done via the return value of all (well, almost all) sleeping functions in the kernel. And kernel programmers are expected to treat those return values with respect and act accordingly.
• Signals are asynchronous. This means that they should be delivered as soon as possible. Imagine a process that has only one thread, went to sleep for hour, and is delivered a signal. Sleep is inside the kernel. So you except the kernel code to wake up, clean up after itself, return to user space and execute the signal handler, possibly restarting the system call after the signal handler finished. You certainly do not expect that process to only execute the signal handler an hour later. Then you expect the sleep to resume. Great trouble is taken by the user space and kernel people to allow just that.
• All in all signals are like interrupt handlers but for user space. This is a good analogy but not perfect. While interrupt handlers are generated by hardware some signal handlers originate from hardware but most are just software (signal about a child process dying, signal from another process using the kill(2) syscall and more).

So what is the latency of signal handling?

• If when you get a signal some other process is running then it up to the kernel scheduler to decide if to let the other process finish its time slice and only then deliver the signal or not. If you are on a regular Linux/Unix system this means that you could be delayed by 1 or more time slices before you get the signal (which means milliseconds which are equivalent to eternity).
• When you get a signal, if your process is high-priority or other processes already got their time slice you will get the signal quite fast. If you are running in user space you will get it "immediately", if you are running in kernel space you will shortly reach a sleep function or return from kernel in which case when you return to user space your signal handler will be called. That is usually a short time since not a lot of time is spent in the kernel.
• If you are sleeping in the kernel, and nothing else is above your priority or needs to run, the kernel thread handling your system call is woken up, cleans up after all the stuff it did on the way down into the kernel, goes back to user space and executes your signal. This doesn't take too long (were talking microseconds here).
• If you are running a real time version of Linux and your process has the highest real time priority then you will get the signal very soon after it is triggered. Were talking 50 microseconds or even better (depends on other factors that I cannot go into).

Think of the signal facility as interrupts, implemented by the OS (instead of in hardware).

As your program merrily traverses its locus of execution rooted in main(), these interrupts can occur, cause the program to be dispatched to a vector (handler), run the code there, and then return to the location where it got interrupted.

These interrupts (signals) can originate from a variety of sources e.g. hardware errors like accessing bad or misaligned addresses, death of a child process, user generated signals using the kill command, or from other processes using the kill system call. The way you consume signals is by designating handlers for them, which are dispatched by the OS when the signals occur. Note that some of these signals cannot be handled, and result in the process simply dying.

But those that can be handled, can be quite useful. You can use them for inter process communication i.e. one process sends a signal to another process, which handles it, and in the handler does something useful. Many daemons will do useful things like reread the configuration file if you send them the right signal.