How can I create three child processes by a child process? - c

Question

I want to create three child processes from a child process of the main process (P0). So something like -->
P0 -> P1 ->P2
->P3
->P4
However, whenever I run it I get (for the processes P2,P3,P4) the ppid of the main process (ppid = 1).
I am using fork() system call in order to create the children and the implementation of the program is in C.
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
int i, pid1, pid2;
pid1 = fork();
printf("Pid1 pid -> %d ppid -> %d\n",getpid(),getppid());
if(pid1 != 0)
{
pid2 = fork();
printf("Pid2 pid -> %d ppid -> %d\n",getpid(),getppid());
if(pid2 == 0)
{
for(i=2; i<5; i++)
{
//if(fork()==0)
//{
printf("Child %d pid -> %d Parent -> %d\n",i+1,getpid(),getppid());
//}
exit(0);
}
}
else
{
printf("Pid2 father process \n");
}
}
else
{
printf("Pid1 child process\n");
}
}

Answer 1

You want something like the following:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
int i, pid1, pid2;
pid1 = fork();
printf("Pid1 pid -> %d ppid -> %d\n",getpid(),getppid());
// parent
if(pid1 != 0)
{
pid2 = fork();
printf("Pid2 pid -> %d ppid -> %d\n",getpid(),getppid());
if(pid2 == 0)
{
for(i=2; i<5; i++)
{
if(fork()==0)
{
printf("Child %d pid -> %d Parent -> %d\n",i+1,getpid(),getppid());
// some functionality here
exit(0);
}
}
exit(0);
}
else
{
printf("Pid2 father process \n");
}
}
else
{
printf("Pid1 child process\n");
}
}
Giving the following output in my machine:
Pid1 pid -> 17764 ppid -> 32242
Pid1 pid -> 17765 ppid -> 17764
Pid1 child process
Pid2 pid -> 17764 ppid -> 32242
Pid2 father process
Pid2 pid -> 17766 ppid -> 17764
Child 3 pid -> 17767 Parent -> 17766
Child 4 pid -> 17768 Parent -> 17766
Child 5 pid -> 17769 Parent -> 17766
Thus, with the following hierarchy:
Parent (pid: 17764) -> C1 (17765) -> C2 -> 17666
C3-> 17767
C4-> 17768
C5-> 17769

Answer 2

Warning, in
pid1 = fork();
printf("Pid1 pid -> %d ppid -> %d\n",getpid(),getppid());
the printf is done both in P0 and P1, this is unclear at all, you have first to check pid1 before to print where you are.
Warning, in
pid1 = fork();
...
if(pid1 != 0)
{
pid2 = fork();
you supposed to be in P1 when you fork again, but you are still in P0 because pid1 is not 0
It is also recommended to check fork success (return value is not -1), and wait for the child(ren) termination rather than to exit the parent process before
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
int main()
{
int pid1 = fork();
if (pid1 == -1)
perror("cannot fork p1");
else if(pid1 != 0)
{
int pid2;
printf("In P1, Pid1 pid -> %d ppid -> %d\n", getpid(), getppid());
pid2 = fork();
if (pid2 == -1)
perror("cannot fork p2");
else if (pid2 == 0) {
int pid3[2];
int i;
printf("in P2, Pid2 pid -> %d ppid -> %d\n", getpid(), getppid());
for (i = 0; i != 2; ++i) {
pid3[i] = fork();
if (pid3[i] == -1)
perror("cannot fork P2 child");
else if (pid3[i] == 0) {
printf("in Child %d pid -> %d Parent -> %d\n",i+1,getpid(),getppid());
return 0;
}
else
puts("in p2");
}
waitpid(pid3[0], 0, 0); /* erroned if fork failed, whatever */
waitpid(pid3[1], 0, 0); /* erroned if fork failed, whatever */
}
else {
puts("still in p1");
waitpid(pid2, 0, 0);
}
}
else
{
puts("in P0");
waitpid(pid1, 0, 0);
}
return 0;
}
Compilation and execution :
/tmp % ./a.out
In P1, Pid1 pid -> 68995 ppid -> 54669
in P0
still in p1
in P2, Pid2 pid -> 68997 ppid -> 68995
in p2
Child 1 pid -> 68998 Parent -> 68997
in p2
Child 2 pid -> 68999 Parent -> 68997
/tmp %
Note the written pid are wrong, from the manual of getppid :
then a call to getpid() in the child will return the wrong value (to be precise: it will return the PID of the parent process)