How parent process wait all child processes terminate

Question

I am new to C , So i only know simple functions.(ex: wait(NULL) ).

Here is my main question :

Modify the program so that only the parent process creates 3 child processes, and each new created process calls a function CPU(). In addition, make the parent process wait for each child’s termination.

I know this one is the correct answer

#include<stdio.h>

int main(void)
{
    int i ;

    for( i = 0; i < 3; i++)
    {
        int ret = fork();
        if(ret == 0){
            printf("My process ID is %d\n", getpid());
            return 0;
        }

    }

    for (i = 0; i < 3; i++) //Line B
        wait(NULL);
}

but my problems are

1. why below code is wrong when wait is executed by the parent in the loop

   #include <stdio.h>
   
   int main(void)
   {
       int i ;
   
       for( i = 0; i < 3; i++)
       {
           int ret = fork();
   
           if(ret == 0) {
               printf("My process ID is %d\n", getpid());
               return 0;
           }
           else
               wait(NULL);
       }
   }
   

2. in very first code why do we write wait(NULL) inside a for loop? can't we write it without a for loop

3. if there is no "return 0" in child process, should the for loop be changed to

   for (i = 0; i <7; i++)
       wait(NULL);
   

4. I don't know how to write the CPU function

Answer 1

1. This code would not execute the child processes in parallel but execute each one sequentially. The difference does not show until the child processes do something beyond printing one message and exiting (you could add sleep(1)).

2. wait function will wait for the termination of one child process (whichever finishes first). If you've got 3 child processes, you must call wait successfully at least that 3 times to ensure that each one of them has terminated. Perhaps easier would be to call wait until it sets errno to ECHILD, meaning that there are no children left:

   while (1) {
       errno = 0;
       if (wait(NULL) == -1) {
           if (errno != ECHILD) {
               perror("Unexpected error from wait");
           }
           break;
       }       
   }
   

3. Each parent process should wait for its own children (or set the SIGCHLD disposition to SIG_IGN). Instead of counting the number of waits, use the code above

4. This means just: "call a function by the name CPU", i.e.

   void CPU(void) {
   }
   

   in the child process.

---

BTW, your code is missing the necessary headers for a couple functions -

#include <sys/types.h>
#include <sys/wait.h>

for wait,

#include <unistd.h>

for fork and additionally #include <errno.h> for my errno addition.