Is there relyable way in UNIX to know how many child processes has my certain child process? For example, main process forks child process which exec login program, can i know whether login forked or not?
UPD i'm using C
Children are linked via their ppid (parent pid) to the parent, so it's just a matter of following those links, depending on what language you use to implement that. The pstree command uses this to display the process tree.
Regarding "reliable", you have to handle processes appearing and disappearing all the time, best is to snapshot the running processes as fast as possible, and only then analyze the data.
You have to go through all processes, checking their PPID (Parent Process ID) and compare that to the PID of the process you want the children of.
It's crude, but you could just use a popen version of what this page is doing, and parse the returned values.
Instead of grep-ing for httpsd you grep for your process. Or you could use the --ppid switch on the ps command and just get all the child processes of this parent process.
Related
I write a program in C. I do fork() in the main process in order to do execve() in the forked child process to execute an unknown app (given by a user in the command line). I know a PID of the process of the executed app - it is returned by fork(), but this unknown app can possibly fork() many times and I do not know PIDs of all its children (they are grandchildren of the main parent process). How can I check in the main parent process WHEN its child process (it is the unknown app) and ALL children of the unknown app exit? (I do not know even how many children it can have and I do not know PIDs of these children).
This can be done by making your parent process a subreaper. A subreaper gets all children orphaned by its descendants, which would traditionally always go to init (process ID 1). The subreaper status needs to be enabled before forking the interesting child process. Once this is done, a waitpid() or similar call for any process will return the child process and all orphaned descendants until it returns error [ECHILD] when the entire tree is gone.
On Linux, this is enabled using prctl()'s PR_SET_CHILD_SUBREAPER option and on FreeBSD this is enabled using procctl() PROC_REAP_ACQUIRE command (see man pages for details).
On Linux you will be able to monitor only one child process individually this way, since the orphans do not remember from which original fork call they came. On FreeBSD, PROC_REAP_GETPIDS allows distinguishing individual subtrees, although this is less efficient if the tree contains many processes.
You can use waitpid(-1,NULL, WNOHANG) to tell if one child has exited. If you receive a positive number (a pid) then one child has exited. In your parent process you have a line that checks if the amount of child processes you have, here called x, is more than 0. if it is use this command to see if any child process has ended. If you have x items then when you add an item increment x and when one exits decrement x. When x, the amount of children you have, is zero all you children have been killed.
If in a simple C program I am writing 4 calls to the fork() API and when I execute this program then the total processes created will be 16 on Linux.
Using getpid() can get the process id of that current process.
Using getppid() can get the parent process id of the calling process.
Question: How can the last child know the process id of the first process
(the ancestor's parent id) from which I call the fork() API four times?
NOTE: The assumption is that all processes are running; nobody died.
Just use
first = getpid()
before all the forks, and use that variable in the children.
1) You can use process tree command called ps l (for ppid seperate table is there.)
2) Using getppid() you can check recursively and reach to main process. Here you need some smart logic for this.
I have a PID of process that may contain childs. How can I get the PID of all child processes? I make my own PTY handler, so when user run a shell in this handler he may run anymore programs ( directly from shell ), every ran program becomes a child of shell. So, when I press Ctrl+C I need to send signal to the most new process, so need to know PID of that last one.
You should keep explicitly all the pids (result of fork(2)...) of your child processes (and remove a pid once you waited it successfully with wait(2) etc...)
It is up to you to choose the data structures to keep these pids.
Any other approach (e.g. using proc(5)... which is what ps and pstree are doing.) is not very portable and inefficient.
So the basic rule is that every time you call fork you should explicitly keep its result (and test for the 3 cases: 0 if in child process, >0 if in parent process, <0 on error) and use that at wait time.
Read Advanced Linux Programming; it has many pages relevant to that subject.
You might also be interested by process groups and sessions. See setpgrp(2), setsid(2), daemon(3), credentials(7) etc. Notice that with a negative or zero pid kill(2) can send a signal to a process group, and that you could also use killpg(2) for that purpose.
i am required to produce something similar to ptree in solaris for my assignment.
to do something like that i would need the child PID of the processes.
i already have the list of processes running on the machine but the only information i have are the parent PID of each processes. is there any way to fetch the child PID of each process in the system?
the program will be ran on a solaris machine.
You said you already have the list of processes and their parents. To find the child PIDs of any process, simply loop over the processes looking for ones whose parent PID is the PID of the process you're investigating.
i have a c problem can any one help me . i wrote a code of process creation in c language , it uses pid & fork() call. the fork call is use for making child process now can any body tell me how to make parent process? i know that creating a child in process make the process parent automatically but i want to make a parent process right from the start so can any one tell me how to do this,
Secondly i just create the process i don't know how to use it i cant assign any work(computation) to it.so can any one tell me how to use the process for work?
Third one i want to give a name to my process how can i do it & how i can control their execution?
please if anyone can enlighten me than please help me to understand all this.
i shall be thank full for this forever
thanks in advance
The fork call creates a new process that is identical to the existing process except for a few minor differences such as its pid, parent pid. The original process carries on from exactly the same place and this is the parent process. Which means your question is basically meaningless. You don't create the parent process, the original process becomes the parent process once it creates a child process.
It's a bit like asking "I created a child by getting pregnant and giving birth, but how do I create the parents?" You are a parent automatically.
Back to computers...
When you fork, the system call returns the pid of the child to the parent and 0 to the child, so you should have code something like:
int pid = fork();
if (pid == 0)
{
// in child, do child processing - normally exec an executable
}
else if (pid > 0)
{
// in parent, do some processing - often wait for child to complete
}
else
{
// fork failed - handle the error
}
When you fork a process, one process becomes two processes. Each continues running at exactly the same place. The only difference is that the fork returns the PID of the child process to the parent process and returns the value 0 to the child process.
Without any help, the child process does not know its parent. If the two processes need to communicate with one another then they will need to use some sort of IPC mechanism.
A common form of IPC is pipe. If one opens pipes before forking, then both the child and the parent keep the open file descriptors. This will allow both processes to communicate with one another. The parent is now free to communicate its PID to the child process if so desired.
Secondly i just create the process i don't know how to use it i cant assign any
work(computation) to it
You need to use fork and exec in pair make it run the program you want to execute.
Here is the wiki link for more information.
Fork-exec is a commonly used technique in Unix whereby an executing process spawns a new program. fork() is the name of the system call that the parent process uses to "divide" itself ("fork") into two identical processes. After calling fork(), the created child process is actually an exact copy of the parent - which would probably be of limited use - so it replaces itself with another process using the system call exec().
To create a parent process, take your code for creating a child and reverse the roles of parent and child. Presto change-o, the new process is the parent and the old process is the child.
For communications between the processes, use a pipe(2), or several. Also, there is shared memory.
To control execution, use kill(2) and wait(2). I'm not sure about assigning names, we might need to know what platform you're on.
About renaming (I assume you mean the name displayed by ps), to "rename" a process, just copy your new name into argv[0]