I am writing a shell where I need to launch several child processes at once and record the system time and user time.
So far I am able to do it. The only problem is that I am using wait4 to grab the system resources used by the child program and put it in my rusage structure called usage.
How can I launch all the processes at the same time and keep track of the user and system times? I can remove the wait4() system call and use it outside to loop so I can make the parent wait, but if I do that then I can only record the times for the last process and not all of them.
Do you have any idea how I can fix this?
execute(commandPipev,"STANDARD",0);
wait4(pid,&status,0,&usage);
printf("Child process: %s\t PID:%d\n", commandPipev[0], pid);
printf("System time: %ld.%06ld sec\n",usage.ru_stime.tv_sec, usage.ru_stime.tv_usec);
printf("User time: %ld.%06ld sec\n\n",usage.ru_utime.tv_sec, usage.ru_utime.tv_usec);
A convoluted answer.
In a POSIX environment, launch the children, then use waitid() with the WNOWAIT option to tell you that some child has exited. The option leaves the child in a waitable state - that is, you can use another wait-family call to garner the information you need. You can then use the non-POSIX wait4() system call to garner the usage information for the just exited child, and deal with the accounting you need to do. Note that you might find a different process has terminated between the waitid() and wait4() calls; you need to use a loop and appropriate flags and tests to collect all the available corpses (dead child processes) before going back to the waitid() call to find out about the other previously incomplete child processes. You also have to worry about any of the wait-family of functions returning the information for a process that was previously started in the background and has now finished.
The Linux man page for wait4(2) suggests that WNOWAIT might work directly with wait4(2), so you may be able to do it all more cleanly - if, indeed, you need the option at all.
Consider whether you can use process groups to group the child processes together, to make waiting for the members of the process group easier.
Related
The scenario:
There are several processes running on a machine. Names and handles unknown, but they all have a piece of code running in them that's under our control.
A command line process is run. It signals to the other processes that they need to end (SetEvent), which our code picks up and handles within the other processes.
The goal:
The command line process needs to wait until the other processes have ended. How can this be achieved?
All that's coming to mind is to set up some shared memory or something and have each process write its handle into it so the command line process can wait on them, but this seems like so much effort for what it is. There must be some kernel level reference count that can be waited on?
Edit 1:
I'm thinking maybe assigning the processes to a job object, then the command line processes can wait on that? Not ideal though...
Edit 2:
Can't use job objects as it would interfere with other things using jobs. So now I'm thinking that the processes would obtain a handle to some/any sync object (semaphore, event, etc), and the command line process would poll for its existance. It would have to poll as if it waited it would keep the object alive. The sync object gets cleaned up by windows when the processes die, so the next poll would indicate that there are no processes. Not the niceset, cleanest method, but simple enough for the job it needs to do. Any advance on that?
You can do either of following ways.
Shared Memory (memory mapped object) : CreateFileMapping, then MapViewOfFile --> Proceed the request. UnmapViewFile. Close the file,
Named Pipe : Create a nameed pipe for each application. And keep running a thread to read the file. So, You can write end protocol from your application by connecting to that named pipe. ( U can implement a small database as like same )
WinSock : (Dont use if you have more number of processes. Since you need to send end request to the other process. Either the process should bind to your application or it should be listening in a port.)
Create a file/DB : Share the file between the processes. ( You can have multiple files if u needed ). Make locking before reading or writing.
I would consider a solution using two objects:
a shared semaphore object, created by the main (controller?) app, with an initial count of 0, just before requesting the other processes to terminate (calling SetEvent()) - I assume that the other processes don't create this event object, neither they fail if it has not been created yet.
a mutex object, created by the other (child?) processes, used not for waiting on it, but for allowing the main process to check for its existence (if all child processes terminate it should be destroyed). Mutex objects have the distinction that can be "created" by more than one processes (according to the documentation).
Synchronization would be as follows:
The child processes on initialization should create the Mutex object (set initial ownership to FALSE).
The child processes upon receiving the termination request should increase the semaphore count by one (ReleaseSemaphore()) and then exit normally.
The main process would enter a loop calling WaitForSingleObject() on the semaphore with a reasonably small timeout (eg some 250 msec), and then check not whether the object was granted or a timeout has occurred, but whether the mutex still exists - if not, this means that all child processes terminated.
This setup avoids making an interprocess communication scheme (eg having the child processes communicating their handles back - the number of which is unknown anyway), while it's not strictly speaking "polling" either. Well, there is some timeout involved (and some may argue that this alone is polling), but the check is also performed after each process has reported that it's terminating (you can employ some tracing to see how many times the timeout has actually elapsed).
The simple approach: you already have an event object that every subordinate process has open, so you can use that. After setting the event in the master process, close the handle, and then poll until you discover that the event object no longer exists.
The better approach: named pipes as a synchronization object, as already suggested. That sounds complicated, but it isn't.
The idea is that each of the subordinate processes creates an instance of the named pipe (i.e., all with the same name) when starting up. There's no need for a listening thread, or indeed any I/O logic at all; you just need to create the instance using CreateNamedPipe, then throw away the handle without closing it. When the process exits, the handle is closed automatically, and that's all we need.
To see whether there are any subordinate processes, the master process would attempt to connect to that named pipe using CreateFile. If it gets a file not found error, there are no subordinate processes, so we're done.
If the connection succeeded, there's at least one subordinate process that we need to wait for. (When you attempt to connect to a named pipe with more than one available instance, Windows chooses which instance to connect you to. It doesn't matter to us which one it is.)
The master process would then call ReadFile (just a simple synchronous read, one byte will do) and wait for it to fail. Once you've confirmed that the error code is ERROR_BROKEN_PIPE (it will be, unless something has gone seriously wrong) you know that the subordinate process in question has exited. You can then loop around and attempt another connection, until no more subordinate processes remain.
(I'm assuming here that the user will have to intervene if one or more subordinates have hung. It isn't impossible to keep track of the process IDs and do something programmatically if that is desirable, but it's not entirely trivial and should probably be a separate question.)
I have a program written in C++ intended to run on a Linux OS. Ignoring much of the program, it boils down to this - it starts X number of executables after some amount of time (for simplicity sake, let's use 5 seconds).
Currently, I'm using system(path/to/executable/executable_name) to do the actual starting of the executable(s) and that works just fine for getting the executable(s) to start.
I'm also trying to maintain a status for each executable (for simplicity sake again, let's just say the status is either "UP" or "DOWN" (running or not running)). I have been able to accomplish this...somewhat...
Backing up just a tad, when my program is told to start the executable(s), the logic looks something like this:
pid = fork()
if (pid < 0) exit 0; //fork failed
if (pid == 0) {
system(path/to/executable/executable_name)
set executable's status to DOWN
} else {
verify executable started
set executable's status to UP
}
Herein lies my problem. fork() causes a child process to be spawned, which is what I thought I needed in order for the original process to continue starting additional executables. I don't want to wait for an executable to stop in order to start another.
However, the executable starts in another child process...which is separate from the parent process... and if I try to set the executable's status to DOWN in the child process when system returns, the parent process does not know about it...
I have a few ideas of what I might need to do:
use threads instead of fork: create a new thread to call system, but would the parent/main thread know about the new thread changing the status of the executable?
use fork and exec: but I'm not sure that would be any better than what I already have (I've read the man pages for fork and exec but I guess I'm still a little fuzzy on how to best utilize exec)
Any suggestions?
EDIT 1
I thought I'd better give a little more context for the logic:
void startAll() {
for each 'executable'
call startExecutable(executable_name)
}
...
void startExecutable (executable_name) {
pid = fork()
if (pid < 0) exit 0; //fork failed
if (pid == 0) {
system(path/to/executable/executable_name)
set executable's status to DOWN
exit (1); <-- this is because once the child process's system returns, I don't want it to return to the above loop and start starting executables
} else {
verify executable started
set executable's status to UP
}
}
EDIT 2
As mentioned at the beginning, this is assuming a simplified setup (a first run if you will). The plan is to handle not just an "UP" or "DOWN" state, but also a third state to handle sending a message to the executables my program has started - "STANDBY." I initially left this piece out to avoid complicating the explanation but I now see that it is imperitive to include.
You need to understand what exactly is happening when you fork. What you're doing is creating a subprocess that's an exact clone of the forking process. All variables currently in memory are copied exactly, and the subprocess has access to all of those copies of all of those variables.
But they're copies, so as you've noticed, fork and exec/system does not on its own handle inter-process communication (IPC). Setting a memory value in one of the processes doesn't alter that variable in any other process, including its parent, because the memory spaces are different.
Also, system is very similar to exec, but gives you much less control over the file descriptors and execution environment. You're effectively already doing a fork and exec, which is what you should be doing.
When you fork properly (as you do in your example), you now have two processes, and neither one is waiting for the other - they just run in completely different codepaths. What you basically want is to have the parent do nothing but sit around waiting for new programs to open, and occassionally check the status of the kids, while the kids run and play as long as they want.
There are IPC solutions such as pipes and message FIFO queues, but that's excessive in your case. In your case, you're just looking for process management. The parent is given the pid of the children. Save it and use it. You can call waitpid to wait for the child to end, but you don't want that. You just want the parent to check the status of the child. One way to do that is check if kill(childPid,0) == 0. If not, then the pid has exited, i.e. it's no longer running. You can also check /proc/childPid for all sorts of information.
If your status is less simple than your question implied, you'll want to look into piping after forking and execing. Otherwise, all you need is process monitoring.
Based on your EDIT 2, you're still within the domain of process management, instead of IPC. The kill command sends a signal to a process (if the command is non-0). What you're looking for is to have the parent kill(childPid, SIGTSTP). On the child side, you just need to make a signal handler, using the signal command. Among many other references, see http://www.yolinux.com/TUTORIALS/C++Signals.html. Basically, you want:
void sigTempStopHandler(int signum) { /* ... */ }
signal(SIGTSTP, sigTempStopHandler);
to be executed in the child code. The parent, of course, would know when this state is sent, so can change the status. You can use other signals for resuming when necessary.
When to pipe vs. signal:
Piping is the most robust IPC you could use - it lets you send any amount of data from one process to another, and can be in whichever direction you want. If you want your parent to send "You've been a very bad boy" to the child, it can, and the child can send "But I'll choose your nursing home one day" to the parent. (Less flippantly, you can pass any data, whether text or binary from one process to another - including objects that you serialize, or just the raw data for objects if it doesn't depend on memory, e.g. an int.)
So far, what you've described is sending simple command structures from the parent to the child, and kill is perfect for that. The child could send signals almost as easily - except that it would need to know the parent's pid to do that. (Not hard to do - before forking, save the pid: int pid = getPid();, now the child knows the parent.) Signals have no data, they're just very raw events, but so far, that sounds like all you're looking for.
In C++, I have a resource that is tied to a pid. Sometimes the process associated with that pid exits abnormally and leaks the resource.
Therefore, I'm thinking of putting the pid in the file that records the resource as being in use. Then when I go to get a resource, if I see an item as registered as being in use, I would search to see whether a process matching the pid is currently running, and if not, clean up the leaked resource.
I realize there is a very small probability that a new unrealated pid is now sharing the same number, but this is better than leaking with no clean up I have now.
Alternatively, perhaps there is a better solution for this, if so, please suggest, otherwise, I'll pursue the pid recording.
Further details: The resource is a port number for communication between a client and a server over tcp. Only one instance of the client may use a given port number on a machine. The port numbers are taken from a range of available port numbers to use. While the client is running, it notes the port number it is using in a special file on disk and then cleans this entry up on exit. For abnormal exit, this does not always get cleaned up and the port number is left annotated as being in use, when it is no longer being used.
To check for existence of process with a given id, use kill(pid,0) (I assume you are on POSIX system). See man 2 kill for details.
Also, you can use waitpid call to be notified when the process finishes.
I would recommend you use some kind of OS resource, not a PID. Mutexes, semaphores, delete-on-close files. All of these are cleaned up by the OS when a process exits.
On Windows, I would recommend a named mutex.
On Linux, I would recommend using flock on a file.
How about a master process that starts your process (the one which terminates abnormally) waits for your process to crash (waitpid) and spawns it again when waitpid returns.
while(1) {
fork exec
waitpid
}
The problem domain isn't clear, unfortunately, you could try re-explaining it in some other way.
But if I understand you correctly, you could create a map like
std::map< ProcessId, boost::shared_ptr<Resource> > map;
// `Resource` here references to some abstract resource type
// and `ProcessId` on Windows system would be basically a DWORD
and in this case you simply have to list every running process (this can be done via EnumProcesses call on Windows) and remove every entry with inappropriate id from your map. After doing this you would have only valid process-resource pairs left. This action can be repeated every YY seconds depending on your needs.
Note that in this case removing an item from your map would basically call the corresponding destructor (because, if your resource is not being used in your code somewhere else, it's reference count would drop to zero).
The API that achieves that on windows are OpenProcess which takes process ID as input, and GetExitCodeProcess which returns STILL_ACTIVE when the process is, well, still active. You could also use any Wait function with zero timeout, but this API seems somewhat cleaner.
As other answers note, however, this doesn't seem a promising road to take. We might be able to give more focused advice if you provide more scenario details. What is your platform? What is the leaked resource exactly? Do you have access to the leaking app code? Can you wrap it in a high-level try-catch with some cleanup? If not, maybe wait on the leaker to finish with a dedicated thread (or dedicated process altogether)? Any detail you provide might help.
I have a program which needs to invoke a process to perform an operation and wait for it to complete the operation. The problem is that the invoked process clones itself and exits, which causes the wait api to return when the process exits. How can I wait for the cloned process to finish execution and return?
I am using the windows JOB object as mentioned in http://www.microsoft.com/msj/0399/jobkernelobj/jobkernelobj.aspx, But I am not sure if this is the best way.
umm, I'm pretty sure you can can the spawner process id from any process. I'd iterate through all the processes, find the one's who's parent id matches the one of the process you spawned, and wait for it to die.
alternatively (I mean, thats pretty hack) what is the child child process doing? is there some other way you could detect when it has finished doing what it is meant to do?
a hack way to get a process's parent id
http://www.codeguru.com/cpp/w-p/win32/article.php/c1437
takes a handle, and using the method in the code above, returns the parent id.
http://msdn.microsoft.com/en-us/library/ms684280(VS.85).aspx
OpenProcess takes an id, gets a handle to it (if you're lucky)
http://msdn.microsoft.com/en-us/library/ms684320(VS.85).aspx
GetProcessId takes a handle, gets it's id.
http://msdn.microsoft.com/en-us/library/ms683215(VS.85).aspx
GetExitCodeProcess takes a handle, returns whether the process is done or not.
http://msdn.microsoft.com/en-us/library/ms683189(VS.85).aspx
so appart from using hidden nt calls that it expressly tells you not to, you would basically have to create your process, get it's id, then spam all the process, opening them and checking their parent ids against the id of the process you created, if you didn't find one, then it's done, if you do, spam it with GetExitCodeProcess until its done.
I haven't tested any of this, but it looks like A way to do it. though if it's THE BEST way to do it I might just have to loose all faith in windows...
+1 for using job objects ;)
Assuming the process that you're running isn't spawning the cloned version of itself in such a way that it breaks out of the job...
You should be able to simply monitor the job events and act on JOB_OBJECT_MSG_ACTIVE_PROCESS_ZERO (see JOBOBJECT_ASSOCIATE_COMPLETION_PORT and SetInformationJobObject()). Monitoring the job in this way will also give you notifications of the processId's of new processes created within the job and details of when they exit.
If you have control over the source of invoked process, one possible solution would be to make it wait for the process it spawns by cloning itself.
How to restrict proccess to create new processes?
You could assign the process to a job object. Use SetInformationJobObject with the JOB_OBJECT_LIMIT_ACTIVE_PROCESS flag to limit the number of processes in that job object to one. Do NOT set the JOB_OBJECT_LIMIT_BREAKAWAY_OK (which would allow the process to create processes that were not part of the job object).
The process could still work around that, such as by starting a new process via the task scheduler or WMI. If you're trying to do something like create a sandbox to run code you really don't trust, this won't adequate. If you have a program that you trust, but just want to place a few limits on what it does, this should be more than adequate.
To put that slightly differently, this is equivalent to locking your car. Somebody can break in (or out, in this case), but at least they have to do a bit more than just walk in unhindered.
On Windows, there isn't a way to stop a processing from spawning other processes. Nor is there on any operating system I know of.
The CreateProcess() system call is available to all processes, thus any process can create a child process.
You could run the process in a sandbox which restricts process creation, but the overhead for this is probably more than you want.
Can I ask why you want to do such a thing?
Use NT Job objects
JOBOBJECT_BASIC_LIMIT_INFORMATION can limit the number of active processes, or use JOBOBJECT_ASSOCIATE_COMPLETION_PORT and kill the new process (If you only need to kill a subset of all new processes)