Difference between Events and Signals wrt Interprocess Communication in RTOS

A signal is according to the unix terminology a notification sent to a process that will interrupt that process. In order to catch a signal a process must have a signal handler.

An event is an inter task signalling mechanism that is used by some RTOS. You can send, receive pend for or check an event. Some RTOS have the concept of event flag groups where individual bits can be signalled.

In article , "Lanarcam" writes: |> |> > I am not aware of the exact difference between an event and a signal.I |> > was under impression what was available as events in other RTOS is |> > available as Signals with Vxworks.Can you please elaborate me the |> > difference between the two or point me to some links where I can get |> > the difference between the two? |> |> A signal is according to the unix terminology a notification sent to |> a process that will interrupt that process. In order to catch a signal |> a process must have a signal handler.

I wish :-(

The Unix signal concept is a hodge-podge. It includes all of:

Notifications from other processes or the kernel Exceptions involving interrupts of the current thread Control operations sent to the process (SIGCONT, SIGSTOP) Elimination of the process without further ado (SIGKILL) And, heaven help us, signal zero

|> An event is an inter task signalling mechanism that is used by some |> RTOS. You can send, receive pend for or check an event. Some RTOS |> have the concept of event flag groups where individual bits can be |> signalled.

There are many other meanings of the term event, I am afraid.

Regards, Nick Maclaren.

Based on the very limited experience I've obtained from implementations especially in using VxWorks events, to me, in VxWorks, a signal("soft-interrupt") looks like a system-defined facility while a VxWorks event (,ported from pSos and only supported in the versions of

5.5 and higher?) looks like a user-defined signaling facility.

For example, to realise a VxWorks event, one needs to assoicated with some triggering resources such as a semaphore, or a message queue, or a task or an interrupt. The receiver may pend on these resources.

A VxWorks event may be used for asynchronous inter-task communication. It may also be used in intra-task. An ISR can send an event but can not receive a one.

Some real expert may be able to elaborate in-depth what's behind.

I think a signal is just one of the several ways an event can manifest, and can be understood by VxWorks. May be you can say, for VxWorks the events are intangible but their manifestations (signal, interrupt..whatever) are tangible.

Hello folks,

Frank schrieb:

The important difference is the common chracteristic, which is: signals and events are sent to a specific task. So an event is handled by the receiving task, so it is addressed to a specified task.

This is not the case for message queues semaphores et. al. .

So in pSOS+ an event is one of 32 bits in a long word in the TCB of the task and events can only be received by the owning task. So one or more events can be received at a time.

Similar story like a signal handler, where a signal is an int, but received one after another.

--
BaSystem Martin Raabe
E: Martin.RaabeB-a-S-y-s-t-e-mde

First, I should correct myself that events are based on a synchronous model, NOT an asynchronous one. Are signals based on an asynchornous model?

So the event field (VXEVENT type) is task-specific. For inter-task communication, the event field is included as an argument when an event is sent to the receiving task. I guess the receiving task should update the event field of its TCB. Is this guess true?

Events are accumulated, i.e., the same event received several times is regarded as one. I don't know if this is just the pSos/VxWorks specific feature. It looks like signls are not accumulated as you explained. At least, one can queue the signals and dispatches them to the destination tasks at a later time. I am afraid if one can do the same with events.

Martin and others,

generally like flags and a task who is sending is just setting a flag.A task who wishes to recieve it is going to pend checking for this flag and if the condition check is satisfied,its going to take necessary action.You can pend on more then one event using AND/OR combination.

Where as signals are software interrupts.If this is true,is it the case that the recieving task is not pending for this signal?Incase the reciever is not pending,how will the reciever get notification that a signal has been raised for him?I beleive there should be some flags present for checking the status of signals just like hardware interrupts.

Now another question comes to mind is Are events synchronous and signals asynchronous? Incase we conclude signals are asynchronous,how does the reciever task know when it should jump to the signal handler?

Incase of hardware interrupts we have special registers to check status of interrupt.Is similar concept applicable to a signal?For eg to track whether a signal is in service,raised already etc?

It would be helpful if you could clarify this.

Regards, s.subbarayan

A process as they are called in unix is interrupted by the OS and a signal handler if it exists receives the thread of control when a signal is raised. The process itself does not pend for or check a signal. It is best to think of signals as soft interrupts.

By nature, yes, as they say.

It does not jump to the signal handler, the OS interrupts the process and the signal handler is executed or the process is killed if there is no signal handler.

You are comparing two different systems unix and rtos which have characteristics of their own, this is interesting but somewhat artificial. In reality you are dealing with one or the other for a project not with both.

Hello Lanarcam and others,

Lanarcam schrieb:

The signalhandler for signals is what the Interrupt Service Routine is for HW-Interrupts.

The signal received is what the Interrupt status register is. So the fact that the signal handler has been called is like Interrupt pending implicitly. Leaving the signal handler implicitly removes the IRQ pending "flag". The signal handler mechanism works under the assumption, that for each signal the signalhandler gets called again. in pSOSystem you can configure, if a signal received

The simplest and easiest difference is, handling events is explicitly task's responsibilty (like waiting for event as a part of code flow )while handling signals is not part of code flow. As already mentioned, signals are software interrupts which are basically exceptional conditions while events are not. You should not use signals as interprocess communication primitive. When you write signal handlers, they get registered with the other software interrupt handlers by operating system. But in case of events, operating system maintains separate data structure like circular doubly linked list for events.

As Discussed , reiterating the point, signals are software interrupts.And they always run with higher priority than the task! The interrupt handlers identify that the signal is generated for which process and it will set a particulr bit in the PCB(process control block) for that process.This basically happens when the process is scheduled out to serve the ISR. While coming from the ISR, schedule function will be invoked and it will first look for any ready for run process with higher priority than the current one which was interrupted. If the current process is the highest priority process then it looks for any pending signal in the register. If it locates that perticular bit is set, it looks whether the signal handler is loaded by that task or not. If not then default signal handler will be invoked and action will be taken.

Both are asynchronous but events can be made as sunchronous. Though signals can be , I won't suggest as it is poor programming practice which will reduce the performance!

I answered this above!

I answered this above!

This is from VxWorks API reference about its events: "Events are a means of communication between tasks and interrupt routines, based on a synchronous model. ..."

I understand this by thinking that the receiving tasks are pending on the events. Also, events are not accumulated. If the current event is not handled in time, the same event coming next will overwrite the current one. However, unlike signals, an events may not have a default handler (no receiving task assigned, i.e., one can define an event without assigning a task to handle it).

Can you elaborate more why events are also asynchronous? Do you think the synchronous model stating in the VxWorks reference manual is just an implementation recommendation? Does anyone have the experience of (or can anyone think of a case of) implementing the events in an asynchronous way?

Frank wrote:

I think, I said they can be used as synchronous way of communication. It is important how you use it!! Let me elaborate this. Consider a thread taking an action based on the event. It doesn't bother who fires this event. What is important is that it is waiting in a loop for the event. Now assume that this event is being fired when data is receieved from I/O device! And the ISR handling this device will fire the event to indicate that the data is in the buffer. What will you call this? Asynchronous or synchronous? (though generally this way of design is not considered as pertinent design!)The task will be in suspended state till it doesn't receive the event!! Which the device only knows for how much time it will in suspended state!! What VxWorks manual enforces that events should be used as inter process communication like to intimate the another task that it has done something interesting for it and it is free to take the action. So the task which was waiting for the event can come from suspended state to ready queue and take the action.(This also is basically done by the scheduler. In case of rate monotonic scheduler, the implementation is different!) This is generally done as a periodic action! Means one thread will generate the event and other thread will take some action based on that event. Now if I design my thead to fire the event in aperiodic manner, do you call it as synchronous? So that's why I said, it is basically how you use it!

just an implementation recommendation?

Exaclty!!

implementing the events in an asynchronous way?

I think I gave one example above for this!

----------- Sagar

Frank wrote:

I think, I said they can be used as synchronous way of communication. It is important how you use it!! Let me elaborate this. Consider a thread taking an action based on the event. It doesn't bother who fires this event. What is important is that it is waiting in a loop for the event. Now assume that this event is being fired when data is receieved from I/O device! And the ISR handling this device will fire the event to indicate that the data is in the buffer. What will you call this? Asynchronous or synchronous? (though generally this way of design is not considered as pertinent design!)The task will be in suspended state till it doesn't receive the event!! Which the device only knows for how much time it will in suspended state!! What VxWorks manual enforces that events should be used as inter process communication like to intimate the another task that it has done something interesting for it and it is free to take the action. So the task which was waiting for the event can come from suspended state to ready queue and take the action.(This also is basically done by the scheduler. In case of rate monotonic scheduler, the implementation is different!) This is generally done as a periodic action! Means one thread will generate the event and other thread will take some action based on that event. Now if I design my thead to fire the event in aperiodic manner, do you call it as synchronous? So that's why I said, it is basically how you use it!

just an implementation recommendation?

Exaclty!!

implementing the events in an asynchronous way?

I think I gave one example above for this!

----------- Sagar

sorry for the same post again!!

-----------

Sagar and others, How can you say that signals are asynchronous?I can see that from a sending tasks perspective you can raise a signal to any reciever task.That means you know very well when you are going to send it and and when the reciever is going to recieve it.Only way I can see it is as asynchronous is from the recieving tasks perspective that ,it will jump to signal handler rather then the tasks code.So If you know already where its going to be sent and since you are not bothered about the status of the recieving task when you are sending the signal,I believe you are saying it as asynchronous from a recieving tasks context.

Incase ,what I said above is true,then the recievers task is not going to pend for it.So OS needs to keep on checking this particulr bit in the tasks context every time a context switch happens.Are you sure this happens?

Further I am seeing a call sigsuspend() which provides the facility of a task to go in for suspension till a signal is raised to it.This I see can be used to mimic the synchronous behaviour.

So Its not clear to me whether they are asynchronous or synchronous. Can some one clarify me this?

Regards, s.subbarayan

I would like to say,the recieving task in this case need not necessarily be suspended,it can be pended! I will do something like this: Sender task { ........ Raise event( event e1); }

Reciever task {

if (e1)

{ ..... } else { .... }

(OR EVEN CHANGE IT LIKE THIS: MAKE THE SENDER TASK PROVIDE A SEMAPHORE WHEN HE SENDS A EVENT AND MAKE RECIEVER TASK PEND FOR HIM,AND WHEN HE RECIEVES IT,HE KNOWS EVENT HAS HAPPENED AND TAKE NECESSARY ACTION.WHILE WHAT I SAID MAY DEFEAT THE VERY PURPOSE OF EVENT,THIS CAN BE PROVING THAT EVENTS ARE SYNCHRONOUS!)

}

This is just a pseudo code and does not reflect real API. If you can see I can check at anypoint of time and take action in the recieving task.Where as in case of signals you cannot check something like this until you are able to gain access to the particular bit stated by you which is present in the tasks context block.I dont know and not sure whether such a thing is possible. And also,even though the firing of event is asynchronous in the sense you dont know when it will fire,the reciever is well aware when to check for it.Where as in case of signals,you cant check it (according to your own words!). This verywell contradicts the fact that events are asynchronous! Can you substantiate further on your statement?

just an implementation recommendation?

implementing the events in an asynchronous way?

I would like to explain the meaning from dictionary for the terms:

Synchronous: Occurring or existing at the same time or having the same period or phase

Asynchronous: Not Synchronous

When we define something, it should not be inclined to a particular thing but to the most general way so that it can be considered as a standard! So we should not define the words from task's perspective.

First of all you can't raise signals to other processes as per my knowledge of Linux / Unix!! Any user process is not allowed to raise the signals for other process. Refer to raise man page! So I don't know which case you are talking about.Only kernel can raise the signals to the user processes!Either you can raise it through your part of code as exception handling or kernel can raise it when we do some obnoxious activity in the code. If you elaborate this with practical example of raising signals for other processes, I will think about it.

100 % sure!! Operating system will always first check for the signals pending in the register before giving control to the application!

sigsuspend() description is

The sigsuspend() function shall replace the current signal mask of the calling thread with the set of signals pointed to by sigmask and then suspend the thread until delivery of a signal whose action is either to execute a signal-catching function or to terminate the process. This shall not cause any other signals that may have been pending on the process to become pending on the thread.

If the action is to terminate the process then sigsuspend() shall never return. If the action is to execute a signal-catching function, then sigsuspend() shall return after the signal-catching function returns, with the signal mask restored to the set that existed prior to the sigsuspend() call.

It is not possible to block signals that cannot be ignored. This is enforced by the system without causing an error to be indicated.

------

I don't know how this makes you to think like synchronous way of signals! Rather it proves that the time for receiving the signal is unknown which is asynchronous!

Cheers,

Sagar

Hello Sagar, Sagar schrieb:

[snip]

Please do me a favor and don't call the task's state "suspended". the task is "blocked" or "blocking". "Suspended" is a totally different task state.

Usually regarding task states events and other inter process communication resources are used to bring tasks in "blocked" state, when they are waiting. This is seen as "synchronous". When a task is in the "ready" or "running" task state and gets "suspended", which is out of the tasks control by the way, this is seen as "asynchronous". The same is true for the signal handling, which is using a task state very close to being suspended temporarly. The task state switch to "executing signal handler" also is (mostly) out of the control of the task it self.

Just my two EURO cents.

--
BaSystem Martin Raabe
E: Martin.RaabeB-a-S-y-s-t-e-mde

It *is* from the recieving task's perspective.

This is *not* true. Signals are analogous to interrupts. The source of an interrupt may or may not be synchronized with the CPU receiving the interrupt, but from the CPU's point-of-view, the interrupt may happen at any time and therefore is asynchronous.

This is simply wrong. Signals may come from other processes. (e.g. the *nix kill command.) Whether or not the sending process has *permission* to send the signal to another process is another issue.

sigsuspend() allows the "task" to block until a signal is received if the task has nothing better to do. In this case the task is synchronizing with the asynchronous signal. Analogous to a CPU going into a low power sleep state until an external (asynchronous) interrupt wakes the CPU.

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