New Process Creation in Operating System (Unix)

Question

I am sorry if my question seems silly. I have a query regarding new process creation in operating system. Consider the following simple C code:

//hello.c
#include <stdio.h>

int main()

    {
        printf("Hello World\n");
        return 0;
    }

When is Compiled with gcc.

gcc hello.c

now executing the executable a.out

./a.out

Now I don't understand how in this case New Process is created, who calls the fork() and exec system calls and which process is duplicated to have a.out as child process? In this example, parent process explicitly calls fork system call to create child process but in above hello.c code there is no fork call.

The shell calls `fork()`. It duplicates itself, then calls `execv()` to execute the command in the child process. — Barmar, Jun 15 '17 at 16:05
the shell calls `fork` to create a process for your program to run inside — Chris Turner, Jun 15 '17 at 16:05
If that would be necessary, how would **that** code be started? Tortoise all the way down? — too honest for this site, Jun 15 '17 at 16:17
@Olaf the kernel starts the first process called `init` (in linux), that has the pid = 0. The kernel is loaded by the kernel loader that is loaded by the bios. The bios looks for this at a fixed address. The bios is started by the hardware. The hardware is started by you when you press the power button. :) — Ajay Brahmakshatriya, Jun 15 '17 at 16:39
You could use `getppid()` to get the `PID` of the Parrent (if it is still running). — 12431234123412341234123, Jun 15 '17 at 16:41
What happens if the parents process size (in terms of page frames) is much more than the child process like in above case!! Will the unnecessary pages from the parent page frames (duplicate one) will be removed to load the entire different process pages??? — Aimal, Jun 15 '17 at 17:01
@AjayBrahmakshatriya: You very much missed the point. Did you understand the citation ("Tortoise all the way down?")? — too honest for this site, Jun 15 '17 at 17:01
@Aimal Linux use COW (Copy On Write), so `fork()` copy only a few pages, if not only a single page. — 12431234123412341234123, Jun 15 '17 at 17:08
Read this https://stackoverflow.com/questions/39877495/time-waste-of-execv-and-fork/39877580 — Tony Tannous, Jun 15 '17 at 17:46
Thanks @TonyTannous, let say parent process contains 10 page frames and the child process (to be executed a.out) only contains 3 page frames so OS will change only three page frames via COW operation. what about the reaming 7 page frames, will they be removed or what?? — Aimal, Jun 15 '17 at 18:04
@Aimal, the parent has 10 pages, child will only copy the page table, they both will point to the same frames. When one of them tries to change by writing we duplicate. If the child process issues `execv()` then it doesn't copy anything and a new program is executed and its pages are copied to memory when needed. — Tony Tannous, Jun 15 '17 at 18:08
@TonyTannous, what about if executable is double clicked. in that case which process is the parent process? — Aimal, Jun 15 '17 at 22:12
Open a new question as I am not sure what to answer :) I can only guess. But it is worth being asked. Don't forget to link me with the new question :) — Tony Tannous, Jun 15 '17 at 22:42

score 3 · Accepted Answer · answered Jun 15 '17 at 16:33

3

Typically, the parent process issues the fork() system call, creating a duplicate process having (mostly) the same properties as the original process (different process ID, for example). From there, the child process issues one of the exec family system calls to replace its own process image with a new one. This is explained quite well on the Unix SE.

In your case, the shell is the parent process, and the "new program" you are running is the child process which eventually calls exec.

answered Jun 15 '17 at 16:33

Cloud

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what if executable is double clicked, and not invoked through bash shell? – Aimal Jun 15 '17 at 22:15
1

@Aimal Then the parent application would be the file explorer or desktop manager. It still does the same thing: forks, and execs. – Cloud Jun 15 '17 at 22:59
@Aimal No sweat. If this answers your question adequately, please mark it as "accepted" via the check mark icon. – Cloud Jun 16 '17 at 01:39
1

got it @DevNull. – Aimal Jun 16 '17 at 06:08

score 0 · Answer 2 · answered Jun 15 '17 at 18:39

Your shell will create new process for your program and it will do exec. It means parent process for your program is your shell.

echo $$ This command will give shell process ID.

Program :

printf("PPID : %d, PID : %d\n", getppid(), getpid() );

When you are run the program you will get below output.

PPID : 19172, PID : 26388

Note :

  Parent process and shell process ID's are same.

New Process Creation in Operating System (Unix)

2 Answers2