So here is the situation, we have a C++ datafeed client program which we run ~30 instances of with different parameters, and there are 3 scripts written to run/stop them: start.sh stop.sh and restart.sh (which runs stop.sh and then start.sh).
When there is a high volume of data the client "falls behind" real time. We test this by comparing the system time to the most recent data entry times listed. If any of the clients falls behind more than 10 minutes or so, I want to call the restart script to start all the binaries fresh so our data is as close to real time as possible.
Normally I call a script using System(script.sh), however the restart script looks up and kills the process using kill, BUT calling System() also makes the current program execution ignore SIGQUIT and SIGINT until system() returns.
On top of this if there are two concurrent executions with the same arguments they will conflict and the program will hang (this stems from establishing database connections), so I can not start the new instance until the old one is killed and I can not kill the current one if it ignores SIGQUIT.
Is there any way around this? The current state of the binary and missing some data does not matter at all if it has reached the threshold, I also can not just have the program restart itself, since if one of the instances falls behind, we want to restart all 30 of the instances (so gaps in the data are at uniform times). Is there a clean way to call a script from within C++ which hands over control and allows the script to restart the program from scratch?
FYI we are running on CentOS 6.3
Use exec() instead of system(). It will replace your process with the new one. Note there is a significant different in how exec() is called and how it behaves: system() passes its string argument to the system shell to run. exec() actually executes an executable file, and you need to supply the arguments to the process one at a time, instead of letting the shell parse them apart for you.
Here's my two cents.
Temporary solution: Use SIGKILL.
Long-term solution: Optimize your code or the general logic of your service tree, using other system calls like exec or by rewritting it to use threads.
If you want better answers maybe you should post some code and or degeneralize the issue.
Related
I am trying to run multiple command in ubuntu using c++ code at the same time.
I used system() call to run multiple command but the problem with system() call is it invoke only one command at a time and rest commands are in waiting.
below I wrote my sample code, may this help you to get what I am trying to do.
major thing is I want to run all these command at a time not one by one. Please help me.
Thanks in advance.
main()
{
string command[3];
command[0]= "ls -l";
command[1]="ls";
command[2]="cat main.cpp";
for(int i=0;i<3;i++){
system(command[i].c_str());
}
}
You should read Advanced Linux Programming (a bit old, but freely available). You probably want (in the traditional way, like most shells do):
perhaps catch SIGCHLD (set the signal handler before fork, see signal(7) & signal-safety(7)...)
call fork(2) to create a new process. Be sure to check all three cases (failure with a negative returned pid_t, child with a 0 pid_t, parent with a positive pid_t). If you want to communicate with that process, use pipe(2) (read about pipe(7)...) before the fork.
in the child process, close some useless file descriptors, then run some exec function (or the underlying execve(2)) to run the needed program (e.g. /bin/ls)
call (in the parent, perhaps after having got a SIGCHLD) wait(2) or waitpid(2) or related functions.
This is very usual. Several chapters of Advanced Linux Programming are explaining it better.
There is no need to use threads in your case.
However, notice that the role of ls and cat could be accomplished with various system calls (listed in syscalls(2)...), notably read(2) & stat(2). You might not even need to run other processes. See also opendir(3) & readdir(3)
Perhaps (notably if you communicate with several processes thru several pipe(7)-s) you might want to have some event loop using poll(2) (or the older select(2)). Some libraries provide an event loop (notably all GUI widget libraries).
You have a few options (as always):
Use threads (C++ standard library implementation is good) to spawn multiple threads which each perform a system call then terminate. join on the thread list to wait for them all to terminate.
Use the *NIX fork command to spawn a new process, then within each child process use exec to execute the desired command (see here for an example of "getting the right string to the right child"). Parent process can use waitpid to determine when all children have finished running, in order to move on with the program.
Append "&" to each of your commands, which'll tell the shell to run each one in the background (specifically, system will start the process in the background then return, without waiting for the result). Not tried this, don't know if it'll work. You can't then wait for the call to terminate though (thanks PSkocik).
Just pointing out - if you run those 3 specific commands at the same time, you're unlikely to be able to read the output as they'll all print text to the terminal at the same time.
If you do require reading the output from within the program (though not mentioned in your question), this is relevant (although it doesn't use system).
I am using named pipes on windows (C++). I was able to send data from one unrelated process to another process.
But for this I have to start the server first. and use "CreateNamedPipe" before I run the client. Client connects to the server using "CreateFile".
Is there a way I could run the client first before starting the server? (without trying to use the "CreateFile" inside a loop till it succeed)
Thank you.
IMO, it depends on your use case. My answer will be based on a case your software doesn't require the named pipe to work. For example, let's say a software which use a named pipe to log activities. This way we can understand your software can work perfectly without logging.
It should be possible if you start up your program without requiring the named pip to exist. Then, once everything is loaded up and functional, you could have sub-routine periodically checking for the named pipe existence (let's say every 5 seconds in order to not overload your CPU) and once created, you start using it.
Note: it will still looks like an infinite "a loop till it succeed" but I don't see anything wrong with that since you do it properly, says, you run it with non-blocking mechanism.
Note: it doesn't necessarily implies multi-process techniques. You can imagine a single main loop with a periodic checking (not every iteration).
Is it possible to create a windows service to create and maintain another process? Like I'm writing a program, and say a virus killed the process, could I have my window service running and basically 'watching' it? I already have the code for a regular application that stays running and executes a program if it's not currently running, to keep it running.
I've never written a service before, but would it be that hard to just write this simple program, which basically runs a check to see if the process is running, if not, it executes it and sleeps for a few minutes?
Thanks.
Yes, it is possible. It is not uncommon to see third-party apps have watchdog services to keep them running in case of crashes. A service can enumerate running processes using EnumProcesses(), and if the desired executable is not running then start a new copy of it using CreateProcessAsUser().
If the service is the one starting the executable process in the first place, or can find it after an enumeration, one optimization would be to keep an open handle to the process (returned by CreateProcess...(), or use OpenProcess() on the process ID an enumeration returns), and then use a wait function, like WaitForSingleObject(), to detect when the process stops running. That way, you don't have to enumerate processes to find out if the intended process is still running or not.
I'm trying to test a Bash script which copies files individually and does some stuff to each file. It is meant to be resumable, so I'd like to make sure to test this properly. What is an elegant solution to kill or otherwise abort the script which does the copying from the test script, making sure it does not have time to copy and process all the files?
I have the PID of the child process, I can change the source code of both scripts, and I can create arbitrarily large files to test on.
Clarification: I start the script in the background with &, get the PID as $!, then I have a loop which checks that there is at least one file in the target directory (the test script copies three files). At that point I run kill -9 $PID, but the process is not interrupted - The files are copied successfully. This happens even if the files are big enough that creating them (with dd and /dev/urandom) takes a couple seconds.
Could it be that the files are only visible to the shell when cp has finished? It would be a bit strange, but it would explain why the kill command is too late.
Also, the idea is not to test resuming the same process, but cutting off the first process (simulate a system crash) and resuming with another invocation.
Send a KILL signal to the child process:
kill -KILL $childpid
You can try an play the timing game by using large files and sleeps. You may have an issue with the repeatability of the test.
You can add throttling code to the script your testing and then just throttle it all the way down. You can do throttling code by passing in a value which is:
a sleep value for sleeping in the loop
the number of files to process
the number of seconds after which the script will die
a nice value to execute the script at
Some of these may work better or worse from a testing point of view. nice'ing may get you variable results, as will setting up a background process to kill your script after N seconds. You can also try more than one of these at the same time which may give you the control you want. For example, accepting both a sleep value and the kill seconds could give you fine grained throttling control.
I need to execute some commands via "/bin/sh" from a daemon. Some times these commands takes too long to execute, and I need to somehow interrupt them. The daemon is written in C++, and the commands are executed with std::system(). I need the stack cleaned up so that destructors are called when the thread dies. (Catching the event in a C++ exception-handler would be perfect).
The threads are created using boost:thread. Unfortunately, neither boost::thread::interrupt() or pthread_cancel() are useful in this case.
I can imagine several ways to do this, from writing my own version of system(), to finding the child's process-id and signal() it. But there must be a simpler way?
Any command executed using the system command is executed in a new process. Unfortunately system halts the execution of the current process until the new process completes. If the sub process hangs the new process hangs as well.
The way to get round this is to use fork to create a new process and call one of the exec calls to execute the desired command. Your main process can then wait on the child process's Process Id (pid). The timeout can be achieve by generating a SIGALRM using the alarm call before the wait call.
If the sub process times out you can kill it using the kill command. Try first with SIGTERM, if that fails you can try again will SIGKILL, this will certainly kill the child process.
Some more information on fork and exec can be found here
I did not try boost::process, as it is not part of boost. I did however try ACE_Process, which showed some strange behavior (the time-outs sometimes worked and sometimes did not work). So I wrote a simple std::system replacement, that polls for the status of the running process (effectively removing the problems with process-wide signals and alarms on a multi threading process). I also use boost::this_thread::sleep(), so that boost::thread::interrupt() should work as an alternative or in addition to the time-out.
Stackoverflow.com does not work very good with my Firefox under Debian (in fact, I could not reply at all, I had to start Windows in a VM) or Opera (in my VM), so I'm unable to post the code in a readable manner. My prototype (before I moved it to the actual application) is available here: http://www.jgaa.com/files/ExternProcess.cpp
You can try to look at Boost.Process:
Where is Boost.Process?
I have been waiting for a long time for such a class.
If you are willing to use Qt, a nice portable solution is QProcess:
http://doc.trolltech.com/4.1/qprocess.html
Of course, you can also make your own system-specific solution like Let_Me_Be suggests.
Anyway you'd probably have to get rid of the system() function call and replace it by a more powerful alternative.