I have the following code, which creates the defined number of child threads using fork():
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <sys/wait.h>
#define NUM_THREADS 4
int main()
{
int i;
pid_t pid;
for(i = 0; i < NUM_THREADS; i++){ //Do this equal to the number of threads
pid = fork();//Use this to avoid multiple forking
if(pid == 0){ //If it's a child process
printf("Hello World! Greetings from PID: %ld! :D\n", (long)getpid()); //getpid returns the pid of the process
exit(0); //Exit the process
}else if(pid == -1){
printf("Oh no! Could not fork! :( Exiting!\n");
return 0;
}
}
int status;
for(i = 0; i < NUM_THREADS; i++){
wait(&status);//wait until all the children processes have finished.
}
printf("All done! I am the parent process! My PID is: %ld if you were curious! \n", (long)getpid());
return 0;
}
This was provided to us as an example. Its output is like:
Hello World! Greetings from PID: 118358! :D
Hello World! Greetings from PID: 118359! :D
Hello World! Greetings from PID: 118360! :D
Hello World! Greetings from PID: 118362! :D
What I'd like to do, instead of having 1 parent process with many children, is to have the parent create a child process, which creates a child process, and so on for the defined number of threads. How can I do that?
First, the term "thread" you used is inappropriate in this situation. Fork means "creating process" not "threads".
You can use a recursive solution but you have to remember that each process creating a process should wait for the return status in order to avoid Zombie Process. Without using any shared variable between process you can use a single variable counter that gets incremented for each forking. But once the counter variable reached the max value, the "youngest" process, i mean the last process created should exit, and one by one other process will exit.
Here is a simple code that works fine:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define NUM_PROCESS 5
int counter = 0;
void child_func()
{
pid_t pid;
if (counter < NUM_PROCESS) {
counter++;
pid = fork();
if (pid < 0) {
printf("fork failed counter = %d\n", counter);
}
else if (pid == 0) {
printf("Hello world ! Greetings from pid %ld\n", (long)getpid());
if (counter == NUM_PROCESS) {
exit(0);
}
else {
child_func();
}
}
else {
long var = pid;
wait(&pid);
printf("My pid is %ld and i am the parent of %ld child, i exit\n", (long)getpid(), var);
exit(0);
}
}
}
int main(void)
{
pid_t pid;
pid = fork();
if (pid < 0) {
printf("Fork failed %d\n", counter);
return -1;
}
if (pid == 0) {
child_func();
}
else {
wait(&pid);
}
return 0;
}
