The mutex_lock_interruptible() function in the linux kernel basically tries to lock a mutex and will continue waiting until a task is interrupted. Well how do I actually interrupt a task?
Suffix _interruptible in Linux kernel means that waiting by the function will be interrupted if thread(process) receives the signal.
It can be signal sent by kill() user-space function, or signals generated by specific functions when condition met, e.g. by the timer (create_timer() when time is expired, or by asinchronous IO when pending operation has been completed.
Note, that uninterruptible wait cannot be interrupted even by SIGKILL, that is process cannot be finished until such wait ends.
Related
When using a globally named mutex to synchronize across two processes, and one of the two processes are killed (say in Task Manager, or due to a fault), the other process returns from WaitForSingleObject() with the appropriate error code and can continue.
When using a globally name semaphore, it does not release the waiting process if the other process is killed / terminated. WaitForSingleObject() will wait until it times out (which may be INFINITE or hours).
How do I stop WaitForSingleObject() from waiting when the other process is killed or terminated?
In this case, there is a single count on the semaphore used to control read/write requests of a shared buffer. The Requester signals the Provider to provide certain data, the Provider updates the buffer and signals back to the Requester that it can now read the buffer.
I suggest that you switch to using WaitForMultipleObjects and wait for the handle of the process that might get terminated (or thread if you want to do this within a single process) in addition to your semaphore handle. That way you can continue to use INFINITE timeouts. Just have to check the return value to see which object was signalled.
Also, I would consider a process terminating while holding a semaphore somewhat of a bug, particularly a semaphore used for actual inter-process communication.
Adding to the accepted answer.
I added logic if the waitms was going to be longer than some value maxwaitms then the requester/provider exchange the providers process id (GetCurrentProcessId()) before the long process. The requester opens a handle (OpenHandle()) to the provider process and waits on both the semaphore and the process handle to know when writing is done (or process terminated).
There are two processes (P-A and P-B) running on the same CPU core. Process B is multithreaded. I want process A to be preempted after every 2 microseconds by a thread of Process B.
Is it possible to write a timer interrupt (or anything else) to preempt process A after a fixed interval of time (in microseconds)?
alarm system call is what you need. According to its manpage, alarm() arranges for a SIGALRM signal to be delivered to the calling process in seconds seconds. You can register your signal handler for SIGALRM by signal/sigaction interface.
I have a scenario, where one process should wait for a signal from another process, and this wait should be blocking wait, and as soon as it gets a signal, it should wake up.
However, with mechanisms like kill() or raise(), the first process goes to wait state, but periodically checks after a specified amount of time, whether the even/signal occurred or not, and decides to wait or go on.
My requirement is a bit stringent, I want that process should wake up at the same instant as signal is received.
Please suggest something.
This can be achieved using semaphore,mutex or conditional variable. Or You can write wait and signal function by your own and you can control the behavior of these as per need. For reference see here: IPC examples
IPC concept and Examples Mutex and Conditional Variables
http://msdn.microsoft.com/en-us/library/windows/desktop/ms686289%28v=vs.85%29.aspx
According to msdn, in the remarks sections, it states:
"If the thread that set the timer terminates and there is an associated completion routine, the timer is canceled. However, the state of the timer remains unchanged. If there is no completion routine, then terminating the thread has no effect on the timer."
Then further down, it states:
"If the thread that called SetWaitableTimer exits, the timer is canceled. This stops the timer before it can be set to the signaled state and cancels outstanding APCs; it does not change the signaled state of the timer."
Hence my question,
if I have one thread calling SetWaitableTimer without an associated completion routine and another thread calling WaitOnMultipleObjects(passing in the timer object handle) and the thread that calls SetWaitiableTmer exits shortly thereafter, would the timer object be cancelled or would it still become signaled when the period expires?
To give more information directly from the implementation of waitable timers: if you use a CompletionRoutine, the timer is placed on a linked list chained off the thread which called SetWaitableTimer. When the thread is terminated, the kernel walks the dying thread's linked list and cancels are timers which are still queued.
If you're not using a completion routine, the timer is never added to any thread's linked list and thus isn't cancelled when any particular thread dies.
The documentation is somewhat unclear. I think the best you can do is test it yourself. I believe however that the timer cancels automatically only if the I/O completion routine is used.
I can give some "theoretical" background about windows APCs, to justify my (educated) guess.
APC = "asynchronous procedure call". In windows every user-mode thread is equipped with a so-called APC queue, a system-managed queue of procedures that must be called on this thread. A thread may enter a so-called "alertable wait" state (on purpose), during which it may execute one or more of the procedures in this queue. You may either put the procedure call in the APC queue manually, or issue an I/O, which on completion will "put" the procedure call there.
In simple words the scenario is the following: you issue several I/Os, and then you wait for either of them to complete (or fail), and, perhaps, some other events. You then call one of the alertable-waiting functions: SleepEx, WaitForMultipleObjectsEx or similar.
Important note: this mechanism is designed to support a single-threaded concurrency. That is, the same thread issues several I/Os, waits for something to happen, and responds appropriately. All the APC routines are guaranteed to be called in the same thread. Hence - if this thread exits - there's no way to call them. Hence - all the outstanding I/Os are also cancelled.
There are several Windows API functions that deal with asynchronous I/O, whereas they allow a choice of several completion mechanisms (such as ReadFileEx): APC, setting an event, or putting a completion in the I/O completion port. If those functions are used with APC - they automatically cancel the I/O if the issuing thread exits.
Hence, I guess that waitable timer auto-cancels only if used with APC.
How can I handle CTRL+C in PHP on the command line? Pcntl_* functions do not work in Windows.
The following works on unix systems.
We can catch keys using stream_get_contents(), but it does not catch the CTRL key. Also filtering ^C does not works.
What we need to do is to catch the SIGINT posix signal.
To supress CTRL + c default behavior.
Program won't quit, you need then to implement another way of exiting!:
function shutdown(){};
pcntl_signal(SIGINT,"shutdown");
To handle CTRL + c, and run some code before exiting:
function shutdown(){
echo "\033c"; // Clear terminal
system("tput cnorm && tput cup 0 0 && stty echo"); // Restore cursor default
echo PHP_EOL; // New line
exit; // Clean quit
}
register_shutdown_function("shutdown"); // Handle END of script
declare(ticks = 1); // Allow posix signal handling
pcntl_signal(SIGINT,"shutdown"); // Catch SIGINT, run shutdown()
List of POSIX signals:
Php won't catch SIGKILL, can't be.
SIGABRT and SIGIOT
The SIGABRT and SIGIOT signal is sent to a process to tell it to abort, i.e. to terminate. The signal is usually initiated by the process itself when it calls abort() function of the C Standard Library, but it can be sent to the process from outside like any other signal.
SIGALRM, SIGVTALRM and SIGPROF
The SIGALRM, SIGVTALRM and SIGPROF signal is sent to a process when the time limit specified in a call to a preceding alarm setting function (such as setitimer) elapses. SIGALRM is sent when real or clock time elapses. SIGVTALRM is sent when CPU time used by the process elapses. SIGPROF is sent when CPU time used by the process and by the system on behalf of the process elapses.
SIGBUS
The SIGBUS signal is sent to a process when it causes a bus error. The conditions that lead to the signal being sent are, for example, incorrect memory access alignment or non-existent physical address.
SIGCHLD
The SIGCHLD signal is sent to a process when a child process terminates, is interrupted, or resumes after being interrupted. One common usage of the signal is to instruct the operating system to clean up the resources used by a child process after its termination without an explicit call to the wait system call.
SIGCONT
The SIGCONT signal instructs the operating system to continue (restart) a process previously paused by the SIGSTOP or SIGTSTP signal. One important use of this signal is in job control in the Unix shell.
SIGFPE
The SIGFPE signal is sent to a process when it executes an erroneous arithmetic operation, such as division by zero. This may include integer division by zero, and integer overflow in the result of a divide (only INT_MIN/-1, INT64_MIN/-1 and %-1 accessible from C).[2][3].
SIGHUP
The SIGHUP signal is sent to a process when its controlling terminal is closed. It was originally designed to notify the process of a serial line drop (a hangup). In modern systems, this signal usually means that the controlling pseudo or virtual terminal has been closed.[4] Many daemons will reload their configuration files and reopen their logfiles instead of exiting when receiving this signal.[5] nohup is a command to make a command ignore the signal.
SIGILL
The SIGILL signal is sent to a process when it attempts to execute an illegal, malformed, unknown, or privileged instruction.
SIGINT
The SIGINT signal is sent to a process by its controlling terminal when a user wishes to interrupt the process. This is typically initiated by pressing Ctrl+C, but on some systems, the "delete" character or "break" key can be used.[6]
SIGKILL
The SIGKILL signal is sent to a process to cause it to terminate immediately (kill). In contrast to SIGTERM and SIGINT, this signal cannot be caught or ignored, and the receiving process cannot perform any clean-up upon receiving this signal. The following exceptions apply:
Zombie processes cannot be killed since they are already dead and waiting for their parent processes to reap them.
Processes that are in the blocked state will not die until they wake up again.
The init process is special: It does not get signals that it does not want to handle, and thus it can ignore SIGKILL.[7] An exception from this exception is while init is ptraced on Linux.[8][9]
An uninterruptibly sleeping process may not terminate (and free its resources) even when sent SIGKILL. This is one of the few cases in which a UNIX system may have to be rebooted to solve a temporary software problem.
SIGKILL is used as a last resort when terminating processes in most system shutdown procedures if it does not voluntarily exit in response to SIGTERM. To speed the computer shutdown procedure, Mac OS X 10.6, aka Snow Leopard, will send SIGKILL to applications that have marked themselves "clean" resulting in faster shutdown times with, presumably, no ill effects.[10] The command killall -9 has a similar, while dangerous effect, when executed e.g. in Linux; it doesn't let programs save unsaved data. It has other options, and with none, uses the safer SIGTERM signal.
SIGPIPE
The SIGPIPE signal is sent to a process when it attempts to write to a pipe without a process connected to the other end.
SIGPOLL
The SIGPOLL signal is sent when an event occurred on an explicitly watched file descriptor.[11] Using it effectively leads to making asynchronous I/O requests since the kernel will poll the descriptor in place of the caller. It provides an alternative to active polling.
SIGRTMIN to SIGRTMAX
The SIGRTMIN to SIGRTMAX signals are intended to be used for user-defined purposes. They are real-time signals.
SIGQUIT
The SIGQUIT signal is sent to a process by its controlling terminal when the user requests that the process quit and perform a core dump.
SIGSEGV
The SIGSEGV signal is sent to a process when it makes an invalid virtual memory reference, or segmentation fault, i.e. when it performs a segmentation violation.[12]
SIGSTOP
The SIGSTOP signal instructs the operating system to stop a process for later resumption.
SIGSYS
The SIGSYS signal is sent to a process when it passes a bad argument to a system call. In practice, this kind of signal is rarely encountered since applications rely on libraries (e.g. libc) to make the call for them. SIGSYS can be received by applications violating the Linux Seccomp security rules configured to restrict them.
SIGTERM
The SIGTERM signal is sent to a process to request its termination. Unlike the SIGKILL signal, it can be caught and interpreted or ignored by the process. This allows the process to perform nice termination releasing resources and saving state if appropriate. SIGINT is nearly identical to SIGTERM.
SIGTSTP
The SIGTSTP signal is sent to a process by its controlling terminal to request it to stop (terminal stop). It is commonly initiated by the user pressing Ctrl+Z. Unlike SIGSTOP, the process can register a signal handler for, or ignore, the signal.
SIGTTIN and SIGTTOU
The SIGTTIN and SIGTTOU signals are sent to a process when it attempts to read in or write out respectively from the tty while in the background. Typically, these signals are received only by processes under job control; daemons do not have controlling terminals and, therefore, should never receive these signals.
SIGTRAP
The SIGTRAP signal is sent to a process when an exception (or trap) occurs: a condition that a debugger has requested to be informed of – for example, when a particular function is executed, or when a particular variable changes value.
SIGURG
The SIGURG signal is sent to a process when a socket has urgent or out-of-band data available to read.
SIGUSR1 and SIGUSR2
The SIGUSR1 and SIGUSR2 signals are sent to a process to indicate user-defined conditions.
SIGXCPU
The SIGXCPU signal is sent to a process when it has used up the CPU for a duration that exceeds a certain predetermined user-settable value.[13] The arrival of a SIGXCPU signal provides the receiving process a chance to quickly save any intermediate results and to exit gracefully, before it is terminated by the operating system using the SIGKILL signal.
SIGXFSZ
The SIGXFSZ signal is sent to a process when it grows a file that exceeds the maximum allowed size.
SIGWINCH
The SIGWINCH signal is sent to a process when its controlling terminal changes its size (a window change).[14]
As of PHP 7.4, this is now possible by registering a handler callback with the sapi_windows_set_ctrl_handler() function.
This is complemented by sapi_windows_generate_ctrl_event(), which can be used to dispatch signals to other processes attached to the same console as the caller.
Only the CTRL-C and CTRL-BREAK events can be handled in user space, the close/log-off/shutdown events cannot be implented safely as the operating system will likely be in an unpredictable state of partial shutdown by the time the handler function is invoked, so there is a risk that any code executed at this point will do more harm than good.
You can find more information about the underlying mechanism on MSDN:
SetConsoleCtrlHandler()
GenerateConsoleCtrlEvent()
The PHP API is almost identical to the underlying C API, the only notable difference being that PHP only permits a single callback to be registered, and consequently the handler does does not have a meaningful return value, the engine simply marks the events as handled. This is in order to keep the implementation simple, as a stack of functions can easily be implemented in userland, and likewise if you don't want to handle an event you can simply call exit.
If you want to run a task in PHP via command line that takes a very long time, I would try to organize it in badges and keep track of what is already done.
Now you can completely process each badge (ex: process and then store it in an xml file) and not only after the whole list is processed. So a crash/stop in between will only cancel one badge and not all of them.
If you store your current position after each badge somewhere, you can easily resume when your script crashes or is stopped.
Now if you check the OS process-list to see if your script is running, you can write a cron job that starts your script every X minutes if it had crashed and was not already running.
So, TL;DR
Process job in small badges
Store position of last successfully processed badge
Check for already running process at start
Continually start script until all are happy!
That aside, I like PHP for small command line jobs but if you have such a large task, something else might be better suited. Check for something that can run stable for a long time and has a means of showing it's progress. Maybe a small C# app with a minimalistic gui.