Running 3 child processes - c++

Question

I have three child process and one parent.
I want the program runs in that order [ Child1 then child2 then child3 then parent ].
I have been trying to do with the follwing code but it dose not give me right sequence!
Code:
#include<iostream>
#include<string.h>
#include<fstream>
#include<cstdlib>
#include <sys/types.h>
#include <sys/wait.h>
using namespace std;
int main()
{
pid_t ch11;
pid_t ch22;
pid_t ch33;
int ch1 = fork();
int ch2 = fork();
int ch3 = fork();
if (ch1==0) //child1
{ cout<<"this is child 1\nGoing from 1 to child 2 ...\n\n";
exit(0);
}
else if ( ch2==0)
{ waitpid(ch11,0,0);
cout<<"This is child 2\nGoing from 2 To Child 3 ...\n\n";
exit(0);
}
else if (ch3==0)
{ waitpid(ch22,0,0);
cout<<"This is child 3\nFineshed !! going from 3 to parent\n\n";
exit(0);
}
else
{ waitpid(ch33,0,0);
cout<<"This is parent , waited the whole childes to finish !!\n\n";
exit(0);
}
return 0 ;
}
Output:
ubuntu#ubuntu-desktop:~$ c++ a.cpp -o p1
ubuntu#ubuntu-desktop:~$ ./p1
This is child 2
Going from 2 To Child 3 ...
this is child 1
Going from 1 to child 2 ...
this is child 1
Going from 1 to child 2 ...
this is child 1
Going from 1 to child 2 ...
This is child 2
Going from 2 To Child 3 ...
This is child 3
Fineshed !! going from 3 to parent
this is child 1
Going from 1 to child 2 ...
This is parent , waited the whole childs to finish.
I know that it will be solved by using waitpid() function , I think im using the waitpid() wrong.

Answer 1

Here's a better instrumented version of your code; it includes the PID in the output, and the outputs are one line each.
#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>
using namespace std;
int main()
{
int ch1 = fork();
int ch2 = fork();
int ch3 = fork();
if (ch1 == 0) // child1
{
cout << (int)getpid() << ": This is child 1 - Finished\n";
exit(0);
}
else if (ch2 == 0)
{
waitpid(ch1, 0, 0);
cout << (int)getpid() << ": This is child 2 - Finished\n";
exit(0);
}
else if (ch3 == 0)
{
waitpid(ch2, 0, 0);
cout << (int)getpid() << ": This is child 3 - Finished!\n";
exit(0);
}
else
{
waitpid(ch3, 0, 0);
cout << (int)getpid() << ": This is parent - waited for all children to finish!\n";
exit(0);
}
return 0;
}
Sample output:
$ ./3kids
40287: This is child 3 - Finished!
40285: This is child 1 - Finished
40286: This is child 2 - Finished
40290: This is child 1 - Finished
40289: This is child 2 - Finished
40288: This is child 1 - Finished
40284: This is parent - waited for all children to finish!
40291: This is child 1 - Finished
$
As you can see, there is one process that considers itself to be child 3, two processes that consider themselves to be child 2, and four processes that consider themselves to be child 1 and one that considers itself to be the parent. This is consistent with the unconstrained forking which creates 8 processes.
To have 3 children only, and to wait for each in turn, you need code more like:
#include <iostream>
#include <cstdlib>
#include <sys/wait.h>
#include <unistd.h>
using namespace std;
void child(int n)
{
flush(cout); // No pending output
int pid = fork();
if (pid < 0)
cerr << (int)getpid() << ": failed to fork\n";
else if (pid == 0)
{
cout << (int)getpid() << ": This is child " << n << " - Finished\n";
exit(0);
}
else
{
int corpse;
int status;
while ((corpse = wait(&status)) != -1)
cout << (int)getpid() << ": PID " << corpse << " exited with status "
<< status << "\n";
}
}
int main()
{
child(1);
child(2);
child(3);
cout << (int)getpid() << ": This is parent - waited for all children to finish!\n";
return 0;
}
Sample output:
$ ./3kids
40336: This is child 1 - Finished
40335: PID 40336 exited with status 0
40337: This is child 2 - Finished
40335: PID 40337 exited with status 0
40338: This is child 3 - Finished
40335: PID 40338 exited with status 0
40335: This is parent - waited for all children to finish!
$

Answer 2

Despite your comment "the code is working but it gives me the wrong sequence of executing" the whole thing is still completely incorrect and it is obvious you did not understand what other people tried to say, hence I'll make an attempt here.
int main()
{
pid_t ch11;
pid_t ch22;
pid_t ch33;
What is the purpose of these variables?
int ch1 = fork();
Have you read fork manpage? It clearly states when a child process is created it returns 0 in the child and pid in the parent. It can also fail, so you should have checked for that.
With this in mind....
int ch2 = fork();
Both the child and the parent reach this line. Hence previously created child forks as well.
int ch3 = fork();
Guess what.
if (ch1==0) //child1
{ cout<<"this is child 1\nGoing from 1 to child 2 ...\n\n";
Except you are not (see below).
exit(0);
Should be _Exit.
}
else if ( ch2==0)
{ waitpid(ch11,0,0);
What? ch11 is not even initialized, so what was this supposed to accomplish?
Do you compile your code with warnings enabled?
In general I don't know if you were trying to make the children fork and thus create a chain of for processes with parent<->child relationship or you wanted 3 children of the same parent process. If the latter this is just bad, if the former this is even more wrong since you would not be able to just wait for such a process.
[snip the rest]
Finally, let's address
the wrong sequence of executing
What?
There are no guarantees whatsoever as to the order of execution of your processes. In fact any number of them can execute for any amount of time and be scheduled out multiple times before any other number of them even gets a chance to run.
Given what was shown up to this point I'm inclined to guess you either misunderstood your homework assignment or are approaching it quite "unconventionally". Either way, I strongly recommend you state the actual problem which was supposed to be addressed with your code.