Creating a process tree with 4 children - c

Question

I am having trouble creating this process tree.
This is my code:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/wait.h>
int main ()
{
int i;
printf("\n [ID = %d] I am the root parent \n", getpid());
for(i=0;i<4;i++)
{
pid_t ret=fork();
if(ret <0) //eror occured
{
printf("Fork Failed \n");
return 1;
}
else if (ret == 0){
printf("\n [ID =%d] My parent is [%d]\n", getpid(), getppid());
}
else
{
wait(NULL);
exit(0);
}
}
return 0;
}
and this is my output
[ID = 4478] I am the root parent
[ID =4479] My parent is [4478]
[ID =4480] My parent is [4479]
[ID =4481] My parent is [4480]
[ID =4482] My parent is [4481]
When I sketch this out, it's just a chain process tree.
p
|
C1
|
C2
|
C3
|
C4
I tried other methods of writing this but this was as close to getting four children. I've been getting 6 on my other tries.

Answer 1

Your flaw is in the logic of the program itself. First you think of making a loop with 4 iterations, when in reality you only want 2 children on the parent process. But then, when fork is successful (ret > 0) you are calling wait(NULL); exit(0); which will stop the for loop and exit the process once wait stops hanging. When the children are forked, they will be inside the for loop themselfs as well. If the loop wasn't getting stuck in the wait(NULL); exit(0); statements you'd have an even bigger mess, with the parent process having 4 children, and the childrens having between 3 and 0 childrens, and the grandchildrens having between 2 and 0 childrens, and so on..
What you need is something like this:
for(i = 0; i < 2; ++i){
pid_t ret = fork();
if(ret < 0){
printf("Fork Failed!\n");
return 1;
} else if(ret == 0) { //Children process
pid_t children_ret = fork();
if(children_ret < 0){
printf("Fork Failed!\n");
return 1;
} else if(children_ret == 0) { //Grandchildren process
//... Do whatever you want on the grandchildren process
exit(0);
}
//... Do whatever you want on the children process
wait(NULL);
exit(0);
}
}
//... Do whatever you want on the parent process
wait(NULL);
exit(0);
Note that the exit calls on the children and grandchildren processes are very important. That's because all the processes share the same code, so if they don't exit on those points they will keep running whatever rest of code you have (i.e.: the grandchildren will run the children code and the parent code).