Thread: Switch process

I have this code

Code:

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

#define   MAX_COUNT 200

void  ChildProcess();
void  ParentProcess();

int  main()
{
    pid_t  pid;
    pid = fork();
    if (pid == 0)
         ChildProcess();
    else
         ParentProcess();
    return 0;
}

void  ChildProcess() {
    int   i;
    for (i = 1; i <= MAX_COUNT; i++)
         printf("   This line is from child, value = %d\n", i);
    printf("   *** Child Process is done ***\n");
}

void  ParentProcess() {
    int   i;
    //wait();
    for (i = 1; i <= MAX_COUNT; i++)
         printf("This line is from parent, value = %d\n", i);
    printf("*** Parent Process is done ***\n");
}

I have three queries:

1. why child process starts first?
2. Why shometime it prints upto 70(i's value) from child then 200 from parent and then switch again to parent, is it due to the service time from the processor handeled by OS?
3. How I can print alternatively from both process:
like first 1 from child, then 1 from parent then two from child and then 2 from parents and so on?
Is there any way to switch between the processes? Child and Parents.

1. why child process starts first?

when you use fork, the child process is always started first...

2. Why shometime it prints upto 70(i's value) from child then 200 from parent and then switch again to parent, is it due to the service time from the processor handeled by OS?

after the fork() call the child process becomes an independent process and thus, is scheduled interdependently with other processes for CPU time. the exact time alloted to each process and their relative scheduling is platform dependent. you cannot directly control that...

3. How I can print alternatively from both process:

use semaphores or mutexes...

Originally Posted by grumpy
It doesn't. The parent process starts the child process. The visible output seen will depend on how the operating system allocates processor time to each process.

yes, the parent process starts the child, but just after the call to fork() the child executes first. the reason is to enable safe copy-on-write.